3 Why Measure pH? Final product quality depends on pH. Pharmaceutical PaperMetal platingDrinking waterFood and BeveragesAlternative fuelChemical reaction rates are often a function of pHCorrosionScalingPrecipitation (salts)FermentationEnvironmental Monitoring
4 Acid and Base Basics HCl H+ + Cl- HAc H+ + Ac- NaOH Na+ + OH- Strong Acids dissociate completely in water releasing hydrogen ions.HCl H+ + Cl-Weak Acids only partially dissociate in water releasing hydrogen ions.HAc H+ + Ac-Strong Bases dissociate completely in water releasing hydroxyl ions.NaOH Na+ + OH-Weak Bases only partially dissociate in water releasing hydroxyl ions.NH4OH NH OH-Strong acid completely dissociate to release hydrogen ion, like hydrochloric acids, while weak acids such as acetic acid (vinegar) only partially dissociate.Bases can also strong and weak. Sodium hydroxide (caustic soda) is a strong base, and ammonia is a weak base.Water itself partially dissociates releasing hydrogen and hydroxyl ions. There is an equilibrium in water between hydrogen and hydroxyl ions.H2O H+ + OH-
5 pH = - log [aH+] aH+=10-pH aH+ = g cH+ What is pH? pH is the “unit of measure” for the acidity of a solution. It is defined as the negative logarithm of the hydrogen ion activity,aH+Activity is related to the concentration of Hydrogen Ion, H+ by an activity coefficient.In general, pH is used as the measure of acidity and rarely used as a direct measurement of concentration by users.pH = - log [aH+]oraH+=10-pHpH is strictly defined as the negative logarithm of the hydrogen ion activity, as shown in the first equation. If you raise both sides of the first equation to the power of 10, you get the second equation, which expresses the hydrogen ion activity.So what is hydrogen ion activity? It is the concentration of hydrogen ions multiplied by an activity coefficient. The activity coefficient takes into account the influence of ions and compounds in the solution, besides hydrogen ion, on the chemical potential of hydrogen ions. Chemical potential might be thought of as tendency of hydrogen ion to react.For example, two solutions could have the same hydrogen ion concentrations, but different ionic backgrounds (i.e. contain different concentrations of other ions). Their hydrogen ion activities would be different due to the differing ionic backgrounds, and so would their pH.In practice, the activity coefficient is rarely determined for hydrogen ion, or anything else for that matter. The ionic background is what it is in a process solution, and the activity of hydrogen ions will be determined by the ionic background. But, the desired pH is the value or range of values found, in practice, to be most effective in promoting a chemical reaction, minimizing scaling or corrosion, meeting environmental regulations, or whatever the goal of monitoring or controlling pH is. So in a sense, pH is a measurement unto itself and is not often directly related to hydrogen ion concentration, much less activity.For the rest of this course, pH will be related to hydrogen ion concentration, without the complication of references to activity or the activity coefficient.aH+ = g cH+
6 pH Scale pH scale is based on dissociation constant of water Kw Kw = aH+aOH- = 10-7·10-7 = mol/liter at 25°C (and ONLY at 25°C)Ignoring activity and translating pH into hydrogen ion concentration, shows that pH measurements, due to their logarithmic nature cover a tremendous range of hydrogen ions concentrations. The definition of pH makes it a convenient shorthand for expressing these concentrations. The use of a logarithm avoids having to write out a number with a large number of zeroes behind the decimal point, or even scientific notation. Use of the negative logarithm, avoids having to write negative numbers.It can be seen that acidic solutions have a higher concentration of hydrogen ions than hydroxyl ions and are less than 7 pH at 25 C, while basic solution have a greater concentration of hydroxyl ions and are greater than 7 pH at 25 C.Neutral solutions have an equal concentration of hydrogen and hydroxyl ions and have a pH of 7 pH at 25 C. Later on in the course we will look at the effects of temperature on the pH of solutions, and its implications for pH measurement.
7 pH values of everyday solutions pH 0 Sulfuric Acid (Battery Acid)pH 1 Gastric Juice, 0.5% Sulfuric AcidpH Lemon JuicepH 3 Vinegar, Coca ColapH Beer, Sour MilkpH 4.8 Pure water air equilibratedpH Fresh MilkpH 7 Pure WaterpH 7.36 BloodpH Sea WaterpH % NaOHpH 14 NaOH (Drain Opener)There is no need to go into a laboratory to experience the whole range of pH scale in everyday life. The everyday experience with weak and dilute acid and base solutions, is that acids taste sour (like vinegar) and bases fell slippery (like soap solutions).
8 Key pH Sensor Components Measuring ElectrodeDevelops a millivolt potential directly proportional to pH in an aqueous solutionReference ElectrodeMaintains a stable reference potential regardless of changes in solution pH or other ionic activityReference Electrode Liquid JunctionMaintains electrical contact between the pH measuring electrode and the reference cell via the process solutionTemperature CompensatorCorrects for changes in the millivolt output of the pH sensor due to process temperature changepH is measured using a pH sensor, which has the components listed above. The components of pH sensor will be examined individually in the following slides.
9 Buffered Fill Solution pH-Glass ElectrodeThe purpose of a pH glass electrode is to develop a millivolt potential directly proportional to pH of an aqueous solutionShieldGlass BodyAg/AgCl Internal WireThe basis for the pH glass electrode is a bulb of pH sensitive glass. This type of glass is corroded by water, which causes it to release lithium ions from the glass matrix and leave a surface layer which has sites that can be occupied by hydrogen ions. Depending upon their chemical potential, i.e. their concentration, hydrogen ions will occupy these sites and the surface of the glass will acquire a positive charge. Opposing this charge is electrostatic repulsion, and so a balance is struck between electrostatic repulsion and the chemical potential of the hydrogen ions, producing a electrochemical potential.The outer surface of the glass bulb is exposed to the process and changes potential as the solution pH changes, while the inner surface of the bulb contacts a buffer solution of constant pH. The potential developed by the glass bulb is the difference of outer surface potential and the inner surface potential.Inside the glass electrode is a silver wire with silver chloride deposited on it, which, as will be seen, matches a silver/silver chloride wire in the reference electrode. The overall potential of a glass pH electrode will be a combination of the potential on the outer surface and inner surface of the electrode, and he potential on the silver/silver chloride wire.Buffered Fill SolutionpH Sensitive Glass
10 The Reference Electrode The purpose of the reference electrode is to provide stable and reproducible potential to which glass electrode potential may be referenced. It completes the circuit by contacting the sample solution through a liquid junction.The liquid junction allows diffusion of the electrolytes (ions) into and from the process, to maintain electrolytic contact.Most reference electrodes are termed ‘non-flowing’, because contact with the process is by ionic diffusion and not flow of the filling solution.Glass BodyKCl Fill SolutionAg/AgCl Internal WireLiquid JunctionThe internal portion of a reference electrode, called the reference cell, consists of a silver/silver chloride wire in a fill solution of concentrated potassium chloride (KCl). Some of the silver chloride dissolves off of the electrode and exists as free silver ion in the solution, and even more of the silver ion forms complexes with chloride ions. The concentration of silver ion in solution creates a potential on the silver/ sliver chloride wire about 200 mV at 25 C, that is used as the reference potential for the pH measurement. The reference potential increases linearly with temperature, and is balanced somewhat by the solver/silver chloride wire in the pH electrode itself.pH17
11 Liquid Junctions Liquid Junction KCl DiffusionRateLiquid JunctionA porous plug that allows liquid contact between the internal KCl solution and outside process solution but restricts the flow.The larger the porosity is, the lower the electrical resistance is and higher the diffusion rate is.The smaller the porosity is, the higher the electrical resistance is and the lower the diffusion rate.There is a tradeoff between high flow with good measurement accuracy, and low flow with longer reference life and less stable junction potentials.Potassium and Chloride ions diffuse out the reference at the essentially same rate.Positive and negative Ions in the process can diffuse into the reference at different rates, which leads to a build up of an electric charge across the liquid junction.This is called Liquid Junction Potential, which normally causes a small error in the pH measurement, which can be calibrated out by standardization5K ohms6-8 month lifeKCl DiffusionRate20KohmsThe liquid junction of a reference electrode maintains electrical contact with the process solution through the diffusions of potassium and chloride ions into the process solution. Liquid junctions with high porosity and higher diffusion rates tend to be more stable, and there may also be a small amount of actual flow of the reference fill solution.An added benefit of using potassium chloride for the reference fill solution is that the potassium and chloride ions are what is called “equitransferent”. This means that potassium and chloride ions diffuse at the same rate. So, when they are diffusing through the liquid junction, there is a positive ion (potassium) for every negative ion (chloride). Not all ions are equitransferent, and as a result, when ions with different diffusion rates, diffuse into the liquid junction, from the process solution, the net positive ions may diffuse faster than the net negative ions or vice versa. This sets up a charge across the liquid junction, called a liquid junction potential, which is another potential added into the pH measuring circuit.Liquid junction potential is typically less than + 15 mV and is usually calibrated out by standardization.1-2 year life
12 pH Sensor Construction This is a drawing of a pH sensor showing all the components used to measure pH. You might note that there is an outer liquid junction and fill solution in addition to the inner liquid junction, which leads to the fill solution containing the silver/silver chloride wire. As will be seen in the next section, this is done to provide some protection for the sliver/silver chloride wire and its fill solution.There is also a solution ground shown in the drawing, which will be discussed on the next slide.One thing that is missing is a temperature element for measuring temperature, which is used to temperature compensate the measurement. Temperature compensation will be covered shortly.
13 Basic circuits for a pH measurement Sensor Potential (E) using a pH sensor with a Solution Ground.E = (EpH - ESG) – (ERef – ESG)A solution ground makes it possible to measure reference electrode impedance and use reference impedance as a diagnostic tool.ESGTempEpHERefSensor Potential (E) using a pH sensor without a Solution Ground.pH measurement can use one of two circuits. The first uses a solution ground and the transmitter measures the difference of the pH and reference electrodes against he solution ground, and then their overall difference. Using a solution ground makes it possible to use impedance diagnostics for the reference electrode, as will be seen in the last section.The solution ground should be part of the manufactured pH sensor. In some cases, a grounding wire from a vessel to the solution ground input of the transmitter has been used as a solution ground with mixed results. In some cases, it appeared to perform adequately, but in others, it became the source of intermittent, random noise.pH can also be measured as the difference of the pH electrode and the reference electrode without the use of a solution ground.In both cases, a temperature element is show, which is not part of the pH measuring circuit, and does not actually have to be built into the pH sensor, but is important for temperature compensation.E = EpH – ERefTempEpHERef
14 PreamplificationpH sensor outputs (mV) are high impedance signals and can be susceptible to noise and interference.To lower the impedance and amplify the signal, a preamplifier is used:In the pH sensorIn a remote junction boxIn the transmitterCertain (rare) pH sensors use a 2nd glass electrode as a reference.Most pH transmitters can be configured to accommodate the high reference impedance of these sensors.PreampPreampPreampThe pH measuring circuit is a high impedance circuit because of the glass pH electrode in the circuit. In the past, this was often a major problem when long sensor cable runs were used, and interference could be picked up even from unlikely sources such as commercial radio transmitters in the general vicinity. Since that time, there have been improvements in the sensor cables used, so that in many cases the raw pH sensor millivolt signal can be run up to 50 feet. In these applications the preamplifier is in the pH transmitter.A more sure way to protect the measurement from noise and interference is to use a pH sensor with the preamplifier built in. The preamplified and lower impedance signal can be sent long distances with no problem. However, although a pH sensor with a preamplifier can be located a long way from its transmitter, it generally doesn’t make sense to do this because it makes maintenance and calibration more difficult.If a pH sensor without a preamplifier has to be located a long distance from its transmitter, a third option is to put a preamplifier in a junction box near the sensor and then the longer cable run will be preamplified.
15 Temperature Compensation and the Calculation of pH The effect of temperature on the pH sensor must be taken into account when determining pH to avoid large errors.How pH is Calculated with Temperature Compensation:Isopotential pHSensor potential (mV)Zero offset (mV)Slope (mV/pH K)Temperature (K)The plot shows the effect of temperature on the millivolt potential of the pH sensor consisting of a pH and reference electrode. As can be seen the slope of the line relating pH and millivolt potential increases with temperature. Also note that there is a point at 7.0 pH and 0.0 mV where the millivolt potential does not change with temperature. This is called the isopotential point, and for the vast majority of pH sensors it is 7.0 pH and 0.0 mV by design.The purpose of temperature compensation is to compensate the pH measurement for the effects of temperature, so that when a change in pH is observed, it is really due to a change in pH and not due to a change in temperature. The temperature compensation shown here only compensates the millivolt output of the pH sensor itself.The equation used for calculating temperature compensated pH is derived from the Nernst equation which governs the potential of pH and reference electrodes. By defining the isopotential point, the Nernst equation can be reduced to the simple equation shown. The slope and zero offset are calibration constants from a two point buffer calibration (more later), can indicate when a pH electrode has reached the end of its life (low slope), or reference electrode has been poisoned (contaminated) by ions or compounds in the process.
16 pH Changes with Temperature Change in Strong Acid and Base Solutions The pH of certain solutions can themselves change with temperature. The degree to which this happens is a function of the composition of the solution.In solutions containing only strong acids and bases, the only effect of temperature is to change the dissociation constant of water. The effects of this on solutions of 5 to 14 pH is shown. The green line is a neutral solution, which is 7.0 pH at 25 C, but increases at lower temperatures and decreases at higher temperatures.In acidic solutions below, which here are 5 and 6 pH, the effects of temperature are minimal or nonexistent. This is because the greater or lesser dissociation of water adds or removes a relatively small increment of hydrogen ion to an already large concentration of hydrogen ion.In basic solutions, 8 to 14 pH, the concentration of hydrogen ions is small, and so the change in hydrogen ions for the increased or decreased dissociation of water is significant. This can be seen by the drop in pH with increasing temperature. In general, the pH of all basic solutions will change with temperature, as will solutions of weak acids, due to changes in the dissociation constant of the weak acid. How much the pH of a solution changes with temperature ultimately depends upon its composition.The temperature compensation already seen which compensates millivolt potential of the pH sensor for temperature changes, does not compensate for pH changes in the solution itself. A second algorithm is provided for solution temperature compensation.In application where this is an issue, a second, solution temperature compensation is provided to correct the measured pH to 25 C.
17 Solution Temp. Coefficient SOLUTION pH CHANGE OF A DETERGENT Solution pH Change with TemperatureSolution Temp. CoefficientZ= pH/°C8.808.708.608.508.408.008.108.208.305045202540353055SOLUTION pH CHANGE OF A DETERGENTWITH TEMPERATURE8.909.00pHoThe plot shown is the pH change of a common detergent solution with temperature taken from a laboratory study. As can be seen the results are essentially linear, which allow a temperature coefficient to be calculated. This temperature coefficient can then be entered into the pH transmitter to be used by the solution temperature compensation algorithm to correct the pH to a 25 C value. So, in a case like this, the transmitter would be providing temperature compensation for the pH sensor millivolt potential, and also compensation for the solution temperature changes.Without solution temperature compensation, changes in solution pH with temperature can cause problems. This occurs because this effect does not seem to be widely known. Here are a couple of scenarios: The first is that the on line process measurement at an elevated temperature, will not match a grab sample of the process taken to the lab after it is cooled. The second is that someone not familiar with the process may be tempted to adjust the pH upwards, when the temperature is elevated, only to discover that the process is at a higher pH upon cooling, and out of spec.The most common involves taking a grab sample for the laboratory. Suppose the process runs at 35 C, and the pH when the sample is taken is 8.55 pH. By the time the sample has reached the lab and sat for a while, its temperature has dropped to room temperature, say 25 C. When the lab measures it, the pH will be around 8.70 pH, and so the lab tech tells the instrument tech that his pH transmitter is way out of calibration or has a problem.An argument can be made for using solution temperature compensation or not using it, but the important thing to know that basic solutions and solutions containing weak acids and bases will change pH with temperature. And if your process meets any of these criteria, its pH will change with temperature.
18 pH Transmitter Configuration All the Configuration Needed for pH Measurements Location of PreampImpedance of Reference Electrode (can choose High for special electrodes)Temp Comp On/OffTemp UnitManual Temp ValueSolution Temp TypeCoefficient for Linear Solution Temp CompIsopotential pH (can be changed for special electrodes)Configuration of a pH transmitter can readily be seen using a configuration window from an asset management system.On the left side of the window are parameters related to the pH sensor, including preamplifier location and reference impedance high or low setting. A high setting allows the transmitter to be used with pH sensor using a second glass or ceramic electrode as a reference. Transmitter output ranging is also located below.The right side is all about temperature and temperature compensation. Temperature compensation can be turned on or off, if it is turned off, a manually entered temperature is used by the transmitter. Solution temperature compensation can be selected and the temperature coefficient entered, and finally the sensor isopotential point can be changed to accommodate non-standard pH sensors.
19 Mounting pH SensorspH Sensors should be mounted at least 10 degrees above horizontal.Otherwise, the air bubble in the glass electrode can cover the inner surface of the glass bulb.The rule for mounting pH sensors is to mount them at least 10 degrees above horizontal to avoid a problem with bubbles inside the electrode.
21 Question 1 pH is proportional to: The concentration of hydrogen ion The concentration of hydroxyl ionThe logarithm of the concentration of hydrogen ionThe negative logarithm of the concentration of hydrogen ion
22 Question 2A neutral solution has an equal concentration of hydrogen and hydroxyl ions and its pH:is always 7.00 pHis only 7.00 pH at 25 deg Cdepends on the temperature of the solutionb and c
23 Question 3 The millivolt output of a pH sensor: changes with temperatureremains constant with temperature changescan be easily compensated for by measuring temperaturea and c
24 Question 4 The actual pH of a solution can: change with temperature change with temperature and does not require special temperature compensationcan be compensated by using special temperature compensationa and c
25 Question 5pH sensors must be mounted at least 10 degrees above horizontal.True or False?
26 Answers to Section 1 Test 1 – d: pH is proportional to the negative logarithm of hydrogen ion concentration2 – b and c: The pH of a neutral solution changes with temperature and is only 7.00 pH at 25C3 – a and c: The millivolt output of a pH sensor changes with temperature and can easily be compensated for by measuring temperature4 – a and c: The actual pH of a solution can change and can be compensated by using special temperature compensation5 – True: pH sensor need to be mounted 10 degrees above horizontal
27 Section 2When to Use pH and Special pH Applications
28 When to Use pH -- Acidic Solutions At pH values below 1.0 pH, a bad pH application becomes a good Conductivity application.The above curve of the relation between the pH and conductivity of a strong acid, shows that the acid concentration being measured by the typical pH measurement is a ppm concentration. This is possible because pH is a specific measurement, i.e., it responds only to hydrogen ions, so it can detect low (ppm and below) concentrations of hydrogen ions, in the presence of much larger concentrations of other substances.Conductivity, on the other hand, is non-specific, it responds to any and all electrolytes present in a solution. For conductivity to respond selectively to an acid, the acid has to be the only electrolyte present in the solution, or the acid has to be in a high enough concentration that its conductivity overwhelms the background conductivity of the solution. This makes conductivity the measurement of choice for more concentrated acid solutions (percent range), where pH measurement is difficult.The difficulties of measuring pH below 1.0 pH are large liquid junction potential caused by high concentration of hydrogen ions themselves. There is also an acid error, which is caused by the effect of high hydrogen ion concentrations on the surface of the pH glass.
29 When to use pH -- Basic Solutions At pH values above 13.0 pH, a bad pH application becomes a good Conductivity application.As was the case with strong acid solutions, the concentration of solutions of a strong base in the ppm range are best measured by pH, due to its ability to respond selectively to hydrogen ion, whose concentration is directly related to the hydroxyl ion (base) concentration.The reason for using conductivity to measure stronger, percent range concentrations of base is even more compelling, due to the fact that a pH electrode can be subject to sodium error at pH values in excess of 12 pH, as well as a large liquid junction potential due to hydroxyl ion, and outright destruction of the glass electrode by percent range caustic solutions.
31 Special pH Applications Most pH applications involve weak solutions or simply water at near room temperature.A general purpose pH sensor can be usedThe sensor should have a long, worry free lifeIn some pH applications, the temperature, pressure, and composition of the process can create issues with the pH measurementpH sensors with special features need to be chosen to best deal with these issuesWhen considering a pH application it is necessary to review the temperature, pressure, and composition of the process. If the solutions are concentrated acids or bases, simply go with conductivity. In some applications, certain components in the solution, or even the lack of components, can create problems for the pH measurement. These applications will be covered in the next few slides.
32 Pure Water (Conductivity < 5 mS/cm) Liquid Junction Potential (LJP)In normal applications, a potential at the liquid junction of the reference electrode is set up by the unequal diffusion of ions from the process into liquid junction.This is usually no more than 15 to 20 mV (~ pH) and can be calibrated out by standardization.In high purity water application this effect can become large and unstable resulting in major errors and drifting.This can be made worse by fluctuations in flow and static buildup on the pH sensor due to the low conductivity of the water.Pure water pH applications need a special sensor designed to take these effects into account.The unstable liquid junction potential when using a normal pH sensor in pure water applications, causes the measurement to drift around the actual pH. The only way to get a satisfactory pH measurement in high purity water is to use a pH sensor designed for pure water.
33 High Purity Water pH System Components 500 mlElectrolyte ReservoirVent TubeReservoir FilterReference TubingAir Bleed ScrewCombination ElectrodeThe solution to the problem of unstable liquid junction potential in high purity water measurements is the use of a flowing junction reference electrode, as shown in the high purity pH sensor above. A flowing junction maintains a constant concentration of potassium chloride within the liquid junction, which essentially eliminates liquid junction potential.Sample OUTDiffuser (inside flow cell)Sample INFlow Cell
34 High Temperature Applications Accelerates the ageing of pH sensor materials.Increases the impedance of the glass electrode.Results in a slower response time.Can quickly destroy a pH sensor not designed for it.A pH Sensor designed for High Temperature must be used.Traditional pH Sensor Lifetime versus TemperatureCurrent High Temperature pH Sensor RatingsSensor 1: Up to 155°C at 400 psigElevated temperature accelerates the aging of pH sensors. Elevated temperature not only ages the electrodes but also the materials of construction of the sensor, which can lead to high impedance shorts within the sensor. High impedance shorts reduce the apparent slope of the pH sensor, but the reduction in slope depends upon the temperature of the sensor. The lower the temperature, the more pronounced the reduction in slope.In the past, the life of a pH sensor could pretty much be summarized by the chart on the slide above. In recent years, new developments in pH glass and sensor design have provided sensors with much improved resistance to temperature and pressure, giving them a useful life at high temperature of months rather than weeks. They are the only choice if a pH measurement is to be made in a high temperature process.
35 Process Effects on Glass Electrodes Chemical Erosion and AttackHydrofluoric Acid (HF)Dissolves glassIf fluoride is present you need to know its concentration and the pH range.A special sensor may be needed or the application may not be possible with pH.Sodium and Potassium Hydroxide (NaOH and KOH)> 4 % (14 pH) will dissolve glass within 8 hours at elevated temperature – there’s no remedy -- go with conductivitySolutions containing AbrasivesTo prevent electrode damage or breakage, protect the electrode from the direct impact of the process flow.Glass electrodes crack or break in these cases.Hydrofluoric acid is a weak acid, and the undissociated acid (HF) attacks glass. If the pH range is high, a higher concentrations of fluoride can be tolerated because it is most of the HF is present as fluoride ions. pH electrodes have been developed that are more tolerant of HF and need to be specified if there is a significant concentration of HF in the process.As noted before, pH is not applicable to caustic solutions with a pH greater than 13 pH, and conductivity should be used.pH electrode need to be protected from solutions with abrasive particles or larger solids by using a shroud or baffle.
36 Sensor CoatingA sample velocity > 5 ft/sec will help minimize coatingCleaning Solutions for:Alkaline or Scale5 % HCl or vinegarAcidic CoatingsWeak caustic < 4 %Oil, grease, or organic compoundsDetergent / sensor friendly solventsUse a pH Sensor designed to resist coatingIn line Cleaning can be done using a jet spray cleaner.Coating increases sensor response time and can cause instability in pH control applications.Severe coating shuts down the pH measurement altogether.In process that have undissolved solids or liquids that can coat a pH sensor, the first line of defense is to mount the sensor so that it receives a sample velocity of 5 ft/second or greater. Coating problems occur when sensors are mounted so that they barely protrude through a pipe or vessel wall where the sample velocity is low.Coating increases the response time of the pH sensor and can cause instability in pH control application by increasing the lag time.pH sensors have been designed that resist the effects of coating, especially coating of the liquid junction, and should be used in applications where coating can be a problem.Sensor can be cleaned using a solution suitable for removing the material coating the sensor, if the solution does not damage the seals and o-rings of the sensor. Exposure to the cleaning solution should be minimized to reduce the amount of cleaning solution that migrates into the liquid junction of the sensor. In general, the time required for the cleaning solution components to diffuse out of the liquid junction is equal to the exposure time.As an alternative to removing and cleaning pH sensors, a jet spray cleaner can be mounted with the pH sensor. The jet spray cleaner directs a jet of water or cleaning solution on the face of the sensor to remove the coating from the glass electrode and liquid junction.
37 Reference Electrode Contamination PluggingPrecipitation of silver in the reference fill solution by ions in process.Typical villains are sulfide, bromide, and iodide ionsUse a triple junction electrode with a potassium nitrate outer fill.Plugging causes the pH measurement to drift.PoisoningDepletion of the silver in the reference solution by precipitation or complexation of free silver ion.Precipitation: sulfide, bromide, and iodideComplexation: ammonia; cyanide is deadly to referencesCheck the concentration of poisoning ions and use a triple junction electrode, or in extreme cases, a special electrode will be needed.Poisoning causes a large zero offset (> 60 mV).As discussed earlier, reference fill solutions contain free silver ion as well as silver and chloride ion complexes. Negative ions, called anions, that form compounds with silver that are less soluble than silver chloride can form a silver precipitate. This removes silver ion from the fill solution and can plug the liquid junction, preventing the diffusion of ions through it and creating an open in the pH measuring circuit. When this happens, the pH measurement will drift and is unusable.Poisoning means that the silver ion in the reference fill has been depleted to the extent that it no longer functions as a silver/ silver chloride reference. This can happen if the silver ion is precipitated by an anion, but it can also happen if the free silver ion forms a complex with the contaminant. This can happen with high concentrations of ammonia, and especially with cyanide ion. In fact, if a substantial concentration of cyanide is present in the process, a pH sensor with a flowing reference electrode must be use to prevent poisoning.The symptom of a poisoned reference electrode is a large zero offset. In addition to this, the effect of temperature on the reference will also change, which only adds to the pH measurement error.
38 What Happens During Reference Cell Poisoning Reference Poisoning can be a slow process. Nothing happens until the silver ion concentration in the reference is irreversibly depleted and the potential changes.TimeReference 3: OKReference Potential, mVThe above plot shows the rate of poisoning over time (180 days) of a reference element by potassium iodide (KI) as evidenced by the change in millivolt potential. Iodide ion forms a precipitate with silver ion. The results using three different reference electrodes are shown.As can be seen, not much happens for the first 90 days. After this point, there are relatively sudden changes in the reference potential of References 1 and 2, which indicates that they have been poisoned. This is characteristic of poisoning and the reason for it can be explained as follows: As the free silver ion in the reference fill solution is precipitated or complexed, silver chloride dissolves off of the silver/silver chloride wire and maintains equilibrium within the cell, so nothing happens. It is not until all of the silver chloride has dissolved off of the silver/ silver chloride wire that the equilibrium within the cell is disrupted, which changes its potential, creating the offset.Reference 1: PoisonedReference 2: Poisoned
39 Triple Junction Reference Electrode Reference technologySingle, Double, Triple Junction are used.Triple junctions slow the diffusion of harmful ions into the innermost portion of the reference.Reference electrode materialSilver-silver chloride wire in potassium chloride solutionStandard concentrations of silver and chloride ions maintain a standard potentialPoisoning ions disrupt these concentrationsJunction MaterialsCeramic, Teflon, quartz fibersElectrolyte Fill SolutionsGelled fill solution – Resists the transport of harmful ions by thermal convectionLiquid JunctionsPoisoning can be prevented by using a flowing reference electrode. But, flowing references require more maintenance due to the need to keep the reservoir of reference solution full, and will also require pressurization, if gravity alone cannot overcome the process pressure so fill solution flow can be maintained.An alternative to a flowing reference is a triple junction reference, whose goal is to retard the diffusion of poisoning ions into the reference cell. It does this by making the poisoning ions pass through 3 liquid junctions before reaching the silver/ silver chloride wire. Any lengthening of the time it takes to reach the innermost reference cell translates into longer reference life. Often the fill solutions are gelled. This does not slow the rate of ions diffusing, but prevents thermal convection currents from transporting ions, when there is a temperature change.
40 Applying pH You need Information: Process Pressure and Temperature Don’t forget to include transients—a short expose to high temperature or pressure can kill a sensor not designed for it.Process CompositionNot such a concern in common, well-known applications.In certain applications, knowing the process composition is essential.Choose the Sensor for ApplicationIn benign applications go with a sensor easy to mount and maintainSpecially Designed pH Sensors are needed for:High Purity WaterHigh Temperature ProcessesProcesses that CoatProcess with components that Poison
42 Question 1Conductivity is a better measurement than pH for concentrated acids and bases.True or False?
43 Question 2Measuring pH of a solution with a conductivity less than 5mS/cm can requires a special pH sensor.True or False?
44 Question 3When choosing a pH Sensor the following should be considered:Process temperature and pressureThe conductivity of the processThe composition of the processAll of the above
45 Question 4A process flow velocity greater than 5 ft/second will help minimize sensor coating.True or False?
46 Question 5pH measurements in processes with high temperature and pressure require a special pH sensor.True or False?
47 Answers to Section 2 Test 1 – True: Conductivity is a better measurement of concentrated acids and bases, due to problems with pH in these solutions.2 – True: Measuring the pH low conductivity solutions requires a special pH sensor.3 – d: The temperature, pressure, and composition of a process, including transients needs to be considered when applying pH.4 – True: a high sample velocity does help prevent sensor coating.5 – True: High sample temperatures and pressures require a special pH sensors; standard pH sensors have a short life in these applications.
49 pH Buffer SolutionsSolutions of known pH that can withstand moderate contamination or dilution without significant pH variation. The more concentrated a buffer solution is the more resistant it is to dilution and acid or base contamination.Buffers 4 and 7 or 10 are usually used – a difference of 3 pH between buffer values is recommended for (two point) buffer calibrationsRules of bufferingUse fresh bufferRinse between buffersAllow reading to stabilizeThe of a buffer solution can and does change with temperatureThis needs to be taken into account during a buffer calibrationSince glass and reference electrodes vary in the potential that they produces in a given solution, standard solutions, known as buffers are used calibrate the glass and reference electrodes. Buffer solution are prepared from a weak acid and its salt, and are designed to maintain a constant pH at a given temperature, and resist changes in pH due to contamination and dilution. The higher the concentrations of weak acid and salt used to prepare the buffer, the greater its resistance to changes in pH. This resistance is referred to as the buffering capacity of the buffer.The pH of a buffer solution is temperature dependent, and this temperature dependence generally increases with the pH of the buffer. So, the pH of a 7 pH buffer is more temperature dependent than a 4 pH buffer. Commercially available buffers typically have the pH values of the buffer at different temperatures printed on the label, which should be used when calibrating.
50 pH Buffer Calibration Two Point Calibration Verifies Sensor Response to pH ChangeDetermines Slope and Zero OffsetTransmitter Automatic Buffer Calibration FeaturesIdentifies the Buffer ValueCompensates for Changes in Buffer pH with TemperatureAccepts Calibration only upon Stabilization of the Millivolt SignalBuffer 2ZeroBuffer 1Buffer calibrating of pH sensor verifies its response to pH changes and calculates the slope, which is a calibration constant and quantifies the response of the pH electrode in millivolts per pH. A low slope indicates that a pH electrode has reached the end of its life.A second calibration constant calculated is the zero offset, which is a measure, in millivolts, of how close the reference electrode is to theoretical zero point. A large positive or negative zero offset is an indication of poisoning or other problems with the reference electrode.Two of the most common errors associated with buffer calibration is the failure to correct the buffer value for temperatures other than 25 deg C, and accepting a calibration value before the sensor has had time to fully respond. This also occurs as a result of a warm pH sensor being placed in a cool buffer solution; full stabilization and a correct pH reading will not occur until, all components of the pH sensor, and the buffer have reached the same temperature.Automatic buffer calibration features help eliminate these errors by identifying and temperature correcting the calibration buffer, and not allowing the calibration procedure to be completed until a stabile reading has been reached.
51 pH Standardization Performed on-line by grab sample evaluation Use a calibrated portable analyzerTake sample at the or near the sensor installation pointAnalyze grab sample immediately for best resultsCalibrates the sensor in the process environmentCompensates for minor coatingsCompensates for small offsets due to liquid junction potentialBut…Even a broken pH electrode can be standardized.pH standardization is a single point calibration that adjusts the zero offset of the pH measurement. It is used to correct a pH measurement to a standard or the reading of a a referee pH analyzer.It does not demonstrate that the pH electrode is responding to pH changes. A broken pH electrode produces a false pH reading around 7 pH, but even this false pH reading can be standardized.
52 Configuring pH Calibration The current live measurements and their statusBegin Buffer or Temperature Cal or pH StandardizationCal Constants from the last calibrationSet the maximum zero offset limitThe calibration window from an asset management system is shown above.The top of the window shows the current pH, temperature, and pH sensor millivolt measurements and their status, which shows that the transmitter is operating normally.The middle section of the window has buttons for launching buffer calibration and standardization routines. Also shown are the slope and zero offset from the last calibration. The slope and zero offset values can be written to, which makes it possible to calibrate a pH sensor in a laboratory or shop, write down the resultant slope and zero offset, and simple enter these values into the transmitter.The lower section of the window has controls for selecting automatic or manual buffer calibration, and choosing the buffer type used for automatic calibration. Stabilization time and span can be chosen, which determine how stabile the pH measurement has to be, before calibration can proceed. The final control sets the limit to the magnitude of the zero offset which will not trigger a zero offset alert.Select: Manual or Auto Buffer CalSelect the buffer type you want to use for Auto CalSelect Stabilization Span and Time
54 pH Diagnostic Types Sensor Diagnostics Calibration Diagnostics Transmitter DiagnosticsEventsModern pH transmitters are supplied with a number of diagnostic alerts. These include problems with the pH sensor, issues arising from calibrations, and issues with the transmitter electronics. A fourth category termed “Events” simply indicate that an event has occurred, such as a calibration. This information provides useful information to the plant or batch historian.Diagnostic messages are displayed locally on the pH transmitter’s display, and when smart transmitters are used they are transmitted to the control system or asset management system. When displayed on a remote system, a description of the error and recommended action or a trouble shooting procedure can accompany the diagnostic alert.
55 Sensor Diagnostics Electrode Impedance Diagnostics With a Solution GroundGlass Electrode ImpedanceRange: 10’s to 100’s of MohmGlass impedance is highly temperature dependent and uses impedance temperature compensation.Best use is detecting cracked or broken electrode (R < 1Mohm)The pH of a broken or cracked electrode is a constant pH near 7.00 pH.It can easily go undetected without diagnosticsReference ImpedanceNeeds a pH sensor with a solution ground for measurementRange: 1 to 100’s of kohmDetects plugged or coated reference electrodesWithout a Solution GroundImpedance diagnostics provide a real time indication of problems with the pH sensor, such as coated liquid junction or a broken pH electrode.
56 More Sensor Diagnostics Temperature DiagnosticsHigh / Low TemperatureTemperature is outside the range of the pH sensor, which can be damagedTemperature Open / ShortedThe measured temperature will appear either extremely high or lowThe measured pH will be about 7 pHThis error can go undetected without diagnosticsSolution Ground OpenThe solution ground input is an open circuitThe millivolt input is out of rangeThe solution ground lead must be connectedThe solution ground on the sensor must be in contact with the process solutionIf there is no solution ground the solution ground input must be jumpered to the reference electrode input.Sensor potential (mV)TemperatureA good Temperature measurement is as important as a good Millivolt input.As the equation on the right side of the slide indicates, temperature is a key measurement used in calculating pH. Problems with the temperature measurement cause errors in the pH measurement.
57 Calibration Diagnostics pH Slope LowUsual low limit is 40 mV/pH (Ideal slope is mV/pH)Indicates that a pH electrode is worn outThe sensor is worn out (usually has a high impedance short)The sensor is coatedAn error was made during calibrationpH Slope HighUsual high limit is 62 mV/pHThere could be a sensor problem – check for instabilityAn error was made during calibration (most likely)Zero Offset to highDefault setpoint is + 60 mVCan indicate a poisoned reference electrodeThe slope of the pH electrode is the key indication of its responsiveness to pH changes. A low slope indicates that the pH electrode is at the end of its useful life, or there is a short within the pH sensor; in either case, the sensor must be replaced. High pH slope is most often due to poor technique or errors made during pH calibration, which means that the buffer calibration should be repeated.A high zero offset typically indicates problems with the reference electrode, such as poisoning, or an error made during calibration.
58 Transmitter Diagnostics Electronic ErrorsCan be an input out of range (A to D Converter Overrange)Likely a sensor problemCan indicate a fatal error (Ground > 10% Off)Transmitter must be replacedMemory ErrorsUsually fatal errors requiring transmitter replacementThere are a number of diagnostic messages which relate to the health of the transmitter itself.
59 EventsAlerts user and control system that certain events are or have taken place, such as:Buffer calibrationStandardizationTemperature standardizationTransmitter Out of ServiceWhile these events appear on the transmitter’s display, their most important role is in control systems using smart transmitters, where they provide important information in batch and control system records.Events provide a history of maintenance procedures performed on the transmitter and sensor. In cases where a calibration or standardization is required at the start of a batch, an event alert can provide the proof that procedure was followed and a calibration was performed.
60 An Example of pH Diagnostics Sensor DiagnosticsCalibration DiagnosticsThis slide shows the diagnostic alerts available in a smart pH transmitter.EventsTransmitter Diagnostics
61 An Example of a Diagnostic Message in an Asset Management System The above slide shows a diagnostic message in an asset management system. In this case the alert is an open or short in the temperature measurement circuit. The window shows the current temperature measurement which is out of range, its status, and the resistance of the RTD. In addition, a brief description of the alert is shown along with the recommended action.
63 What Smart pH Sensors Do Smart pH Sensors have a preamplifier with a memory chipUpon connection to a Smart enabled pH Transmitter they upload their latest calibration data, which is used by the transmitter to measure pHThis makes it possible to buffer calibrate a sensor in the shop or laboratory and simply install it in the process.Historical data acquired during calibration is also uploaded to the transmitter for display by it or an asset management systemNew calibration data and maximum and minimum temperature measured are downloaded from the transmitter to the Smart sensor
64 Smart pH Sensor Information Serial numberManufacturer date codeCalibration DataSlopeZero offsetTemp cal offsetGlass impedanceReference impedanceSensor run time at each calibrationsHistorical InformationLast 5 calibration data sets for troubleshootingPlug and PlayThe slide shows the information stored in the smart sensor’s memory chip.
65 Smart Sensor : Basic Sensor Information in an Asset Management System The basic sensor information is the model code of the sensor, its manufacture date, serial and software version. The maximum and minimum exposure temperatures are also shown.
66 Smart Sensor : Calibration History in an Asset Management System Calibration history is stored in the sensor includes the original factory calibration, and the last five calibrations and standardizations. The data captured for each calibration are the calibration method, run time, slope, zero offset, temperature at calibration, and the glass and reference electrode impedances at each calibration.
67 Troubleshooting pH Applications What do you do when sensors fail prematurely?Get the application information:Process temperature and pressure (and transients)Process composition (and transients)How is it failing? Broken glass...poisoned reference…coating…How often is it failing?What do the diagnostics say?If calibration information is available, get itHave sensors always failed in this application or is it a new phenomenon?
68 Troubleshooting pH Applications Tools for diagnosing problemsUse the transmitter’s diagnosticsSmart pH sensors capture calibration data, and min/max temperature; this is very useful for troubleshooting.If asset management software is used, there is a lot of information captured an audit trail.Data LoggingLook at measurement data, including temperature, millivolts, and impedances if they are availableCheck events, such as diagnostics alarms, if they are also recorded.
70 Question 1A two point buffer calibration is the only way to tell if a glass electrode is adequately responding to changes in pH.True or False?
71 Question 2 If a glass pH electrode is broken, can it be standardized? Yes or No?
72 Question 3When choosing a pH Sensor the following should be considered:Process temperature and pressureThe conductivity of the processThe composition of the processAll of the above
73 Question 2 The pH of a solution can change with temperature. True or False?
74 Question 5When pH sensors consistently fail prematurely, the following should be considered and examined:Process temperature and pressureSensor or calibration diagnostic historyThe composition of the processAll of the above