GF Piping Systems 4 What is pH? Expressing Concentration in Terms of pH pH is a Unit of Measurement Defined as the Negative Logarithm of Hydrogen Ion Activity (concentration and activity are closely related) pH = -log [H + ] – Example: if the hydrogen concentration is 1.0 x 10 -4 moles/liter, the pH is 4.0 O 2- H+H+ H+H+ H+H+ H+H+ +
GF Piping Systems 5 What is pH? pH measures the relative amount of hydrogen (H + ) ions compared to hydroxide ions in an aqueous solution: H 2 0 = H + + OH - In an aqueous (water based) solution, the concentration of hydrogen ions multiplied by the concentration of hydroxide ions is a constant number: K w = [H + ][OH - ], K w = dissociation constant for water At 25ºC, K w = 1 X 10 -14 ; at 35ºC, K w = 1 X 10 -14 O 2- H+H+ H+H+ H+H+ H+H+ +
GF Piping Systems 6 What is pH? Acids and Bases Acid dissolves in water to furnish H + ions HCl H + + Cl - Hydrochloric Acid HNO 3 H + + NO 3 - Nitric Acid HF H + + F - Hydrofluoric Acid Base dissolves in water to furnish OH - ions NaOH Na + + OH - Sodium Hydroxide KOH K + + OH - Potassium Hydroxide NH 4 OH NH 4 + + OH - Ammonium Hydroxide pH
GF Piping Systems 7 What is pH? pH is a measure of hydrogen ions in a water based solution It is expressed in hydrogen ion concentration: pH = -log [H + ] Acids and bases added to water changes the amount of H + ions.
GF Piping Systems 8 How is pH expressed? pH measurement is expressed on a scale of 0.0 to 14.0 pH electrode will produce 59.16 mv per pH unit at 25 °C pH of 7.0 is neutral (H + ions = [10 -7 ] and OH - ions = [10 -7 ]) More hydrogen ions (H + ) is considered an acid More hydroxyl ions (OH - ) is considered alkaline (base) pH mV Acids Bases 01234567891011121314 +414+355+296+237+177+118+5900-59-118-177-237-296-355-414
GF Piping Systems 9 pH Values of Common Items NaOH = Sodium Hydroxide H2SO4 = Sulfuric Acid
GF Piping Systems 11 Traditional pH Sensors A combination pH sensor consists of: Reference electrode pH electrode Temperature element Reference electrodepH electrode Temperature element
GF Piping Systems 12 pH Sensing Electrode Hydrogen sensitive glass Inert glass stem A silver/silver chloride wire Buffered electrolyte solution Hydrogen sensitive glass bulb Electrolyte Solution: Silver/Silver Chloride Wire Sol-gel (silica oxide layer) on both sides of glass Glass membrane thickness 0.2-0.5 mm Glass stem
GF Piping Systems 13 Silver/Silver chloride wire Front Porous Junction 2nd reference chamber: KCl Electrolyte 1st reference chamber: 3.5 M KCL 2nd Junction Silver wire Sealed barrier Plastic tubing Silver Chloride 3.5 M KCL Sealed barrier Traditional Reference Electrode
GF Piping Systems 14 To make a pH measurement, a pH and a Reference electrode are required. A combination pH sensor consists of a pH electrode and a reference electrode built into one body. – A pH sensor has a glass membrane which is sensitive to H + ions – A reference sensor has a porous membrane with electrolyte on the inside Electrode Construction pH SensorReference Sensor= Combination Sensor and
GF Piping Systems 15 Temperature Measuring Elements Pt 100, Pt 1000, 3K Balco, etc… Glass error of 0.03 pH /per pH unit/per10 degree C °C23456789101112 22.214.171.124.12.060.12.18.24.30 15.15.12.09.06.030.06.09.12.15 2500000000000 35.15.12.09.06.030.06.09.12.15 126.96.36.199.12.060.12.18.24.30 188.8.131.52.18.090.18.27.36.45 184.108.40.206.220.127.116.11.48.60 18.104.22.168.22.214.171.124.60.75 126.96.36.199.188.8.131.52.72.90 0 pH error range 0.1 pH error range
GF Piping Systems 16 pH and Sensor Calibration pH Theoretical slope; 59.16 mV/pH unit at 25C. Calibrate in pH buffer solutions to determined actual slope. Slope indicates the true performance of the pH glass. A slope of 49 to 59 is typically acceptable for a working electrode. Below 49, the pH glass is bad. 177 mV 0 mV 3 pH units
GF Piping Systems 17 Installation Recommendations
GF Piping Systems 18 Preamplification Preamplifiers are required for pH sensor Preamplifiers boost high impedance signals Preamplification allows separation for hundreds of feet
GF Piping Systems 19 Installation Tips Hints for Installing sensors with preamplifiers Mount electrodes in a location with ample clearance for removal for periodic cleaning and recalibration. Choose a location that keeps the electrode glass completely submerged at all times. Place the electrode tip in pH 4 buffer during system maintenance or storage to avoid dehydration and reduce chemical activity. Be aware of mounting positions-some sensors must be mounted +/- 45 degrees of the vertical.
GF Piping Systems 20 Installation – In-Line In larger diameter tee’s watch for stack height of tee fitting, and flush style reducer bushings making sure sensor tip is in process < 1.5”
GF Piping Systems 21 Installation : In-Line 2 ½ in Tee 2 ½ X 1½ in Flush style reducer bushing 1½ X ¾ in Flush style reducer bushing 12
GF Piping Systems 22 Installation : Submersion and seal tape threads Use “o”-ring lubricant
GF Piping Systems 23 pH Sensor In-Line Installation Caution: Air Bubbles and Air Pockets Caution: Sediment and debris Build-up Caution: Sensor needs to stay wet
GF Piping Systems 25 pH Sensor Calibration Two point calibration: Standardize / Slope Single point calibration: Grab sample pH 4pH 7pH 10
GF Piping Systems 26 pH / ORP Sensor Calibration Two Point Buffer Calibration Calibrate when: The sensor is initially installed to insure proper functioning pH sensor is replaced Calibration establishes sensor slope; ensures linear output Slope decreases with: –age, coating, elevated temp. –pH glass erosion by abrasion, strong NaOH, KOH or HF –coating from grease and oils Acceptable mV/pH slope is typically 49 mV/pH to 59 mV/pH –Theoretical pH/mV slope @ 25 º C is 59.16 mV/pH
GF Piping Systems 27 pH Sensor Calibration Single Point pH Calibration (Standardize) To Perform a grab sample evaluation Take sample at or near pH sensor installation point Analyze grab sample ASAP using a portable or laboratory analyzer (avoid temperature change) Change the reading in the instrument to match the portable or laboratory analyzer Single point standardization: Compensates for minor coating of the pH electrode. Compensates for small offsets in the liquid junction potential
GF Piping Systems 28 pH System Start-up Recommendations for simple start-up : Place the sensor in 7.0 pH Buffer solution Wait 10 minutes for temperature to equilibrate and cal temp Using 4 and 7 pH buffers cal meter Use fresh off-the-shelf buffer Triple rinse and place in process Allow minimum time to equilibrate Repeat Buffer calibration in 24 hours. Determine appropriate cleaning and calibrating schedule for your application. Try weekly and based upon observed results increase or decrease frequency for desired results.
GF Piping Systems 29 Sensor Storage & Maintenance
GF Piping Systems 30 pH Storage & Care When storing boxed sensors, lay the sensor flat to maximize hydration of the reference surface. Keep the reference surface wet at all times. Store sensors at a stable room temperature. Remove storage boot that sensor is shipped in; keep boot and re- boot with pH 4 buffer to keep sensors wetted during storage and transportation. If the sensor dehydrates: Clean sensor, and soak the sensor tip in pH 4 buffer for 24 to 48 hours, then visually inspect the electrode for surface cracks, swelling, or discoloration. Severely dehydrated electrodes cannot be restored to reliable operation. Never expose electrode to temperatures below 0° C or allow it to dehydrate. These conditions may damage the electrode.
GF Piping Systems 31 pH Storage & Care Store between 10 and 30°C Use protective caps with potassium chloride KCl solution or pH 4 buffer Do not allow to dry out, else problems Slow response High glass resistance To fix, re-hydrate for 24-48 hours in 4 buffer or KCl solution Factors that decrease sensor lifetime Heat Extreme cold Vibrations
GF Piping Systems 32 Sensor Cleaning Rinse with DI or warm tap water Soak for a few minutes in one of the solutions below Rinse with DI or tap water Re-hydrate in pH 4 buffer or KCl solution, 1-24 hours Gentle use of toothbrush is ok Cleaning Solvent/SolutionApplication Warm water & detergentFor normal applications Less than 5% HCl acidFor coatings of lime and hydroxides Acetone or isopropyl alcoholFor coatings containing oils & fats Dilute bleach, 1-5%For coatings with algae, bacteria or molds
GF Piping Systems 33 Cleaning Frequency Keep sensor reasonably clean to maintain accuracy Frequency depends on… Process Accuracy required Experience with application
GF Piping Systems 34 pH/ORP System Troubleshooting
GF Piping Systems 35 Possible Problems in Existing Installations Electrode cleaning and calibration. Electrodes should be removed and cleaned on a regular maintenance schedule depending on the application. Faulty electrode. Sensor life is application specific. High temperature and chemicals shorten lifetimes. Faulty amplifier assembly Electrical wire problems. Some environments can cause the wiring to corrode and cause faulty connections.
GF Piping Systems 36 Potential Measuring Problems-Troubleshooting Use simulator to check electronics Use raw mV values to investigate sensors Use buffers to gauge: response time accuracy stability Best Practice: Use logs to record maintenance intervals and events Check critical connections make sure they are dry Check seal areas Check sensor surfaces for breakage or fouling Clean sensor and calibrate; if the sensor cannot be calibrated, it is probably time to replace it
GF Piping Systems 37 Potential Measuring Problems Reference Cell pH Sensitive Glass Heavy metals like Hg ++, Cu +, Pb ++, ClO 4 - Penetrate the Liquid Junction and react with KCl Br-, I-, CN-, S- compounds penetrate liquid junction and react with Ag+ pH Measuring Electrode: Internal solution and silver chloride (AgCl) wire is protected from outside elements. Extreme pH or temperature values, harsh chemicals, and hairline cracks will affect the reading Silver Chloride Billet pH (glass) Silver Chloride Billet Porous Reference Junction KCl solution Oils, grease, dirt, from the outside and silver compounds formed on the inside will clog the reference junction Reference Electrode: There are many elements that can affect the reference electrode. What can Affect a pH Measuring or Reference Electrode Output? Stray electrical currents
GF Piping Systems 39 pH Troubleshooting Measuring Offset : Thoroughly decontaminate electrode with clean water Clean electrode tip in beaker of 5% HCl solution, < 30 seconds Thoroughly rinse off HCl residue with clean water Place electrode in beaker of fresh 7.0 pH Buffer solution Using electrode gently stir buffer for about 10 seconds. Wait for mV on controller to stabilize ( should only need a few seconds ) mV value on controller display should be 0 mV, +/- 50 mV Make note of mV value and retain for determining Span. Example: mV value measured in 7.0 pH Buffer = - 10 mV Caution: Always wear appropriate safety gear and exercise proper safety practices when working with or near liquids & chemicals
GF Piping Systems 40 pH Troubleshooting / Maintenance Measuring Span : Remove electrode from 7.0 pH Buffer solution Thoroughly rinse electrode with clean water Place electrode in beaker of fresh 4.0 pH Buffer solution Using electrode gently stir buffer for about 10 seconds. Wait for mV on controller to stabilize ( should only need a few seconds ) mV value on controller display should be +177 mV away from the value measured in 7.0 pH Buffer, +/- 50 mV. Example: Offset value was -10 mV, therefore Span value should be +167 mV, +/- 50 mV (+117 to +217 mV) Caution: Always wear appropriate safety gear and exercise proper safety practices when working with or near liquids & chemicals
GF Piping Systems 41 pH Troubleshooting Occurrence :Effect: Natural agingmV slowly drift more positive Clogged ReferenceSlow response in same direction Coated pH Glass ORP TipErratic drifting multiple direction Damaged pH GlassShort Span Sodium Ion ErrorReads low above 12.5 pH Attacked ReferenceLarge Offset (prematurely) Depleted ReferenceLarge Offset (over time) Cracked GlassmV value always around +55, or 0mV Shorted WiringmV value always around 0 mV Moisture intrusionVery wrong mV values, erratic, stuck Glass pH Etched>2% HF, or >12.5 pH Note: Offset problems are typically associated with Reference half Span problems are typically associated with Measurement half
GF Piping Systems 42 Potential Measuring Problems Ground Loop Problems Sensor picks up current from improperly grounded circuit, typically from pumps, motors, or other closely located electrically driven equipment NOTE: Plastic piping can be a source of this problem because: 1. It picks up current or static charges from liquid flow 2. And/or it picks up electrical current from pumps and other equipment but may not have grounding Reference Element can be stripped, shortening sensor life Differential Sensors eliminate the issue of stripped reference element Symptoms include sensor reading drifting, noisy, or erratic Eliminating Ground Loops: 1. Find and Fix Grounding Problem 2. Use a Solution Ground 3. Use a Differential Sensor with Solution Ground
GF Piping Systems 43 pH: Assessing pH Application (step 1) pH / ORP application: Application (scrubber adjust, waste treat adj, process adj, effluent monitoring, etc) Process range e.g., 3 – 5 pH Desired process value range e.g., 7.5 – 9.5 pH Process conductivity Process temperature range Solution chemistry (what, percentage) Solution quality (DI water, oils / grease, sludge, crystals, solids) Chemical compatibility of all wetted components Distance from sensor to indicator Inline (pipe size, material, flow rate, pressure, recirculation or single pass) In tank (tank size, flow rate, type of mixing, retention time, flow through or batch) Reagent type and concentration Chemical dosing (proportional, on/ off)
GF Piping Systems 44 pH Things To Watch For Dry Glass / Junction ( keep submerged in liquid ) Clogging of reference ( avoid build-up of solids, oils/grease, salts, etc ) Coating of Glass ( avoid build-up of oils/grease, solids, salts, etc ) Etching of Glass ( avoid >2% hydrofluoric, caustics above 12.5 pH, etc ) Stray electrical currents ( plating baths, poor grounding ) Chemical attack of Reference Fill ( minimize exposure to mercury, copper, lead, perchlorate, etc ) Chemical attack of Reference Element ( minimize exposure to bromine, ammonia, iodine, sulphur, cyanide, etc ) Moisture intrusion at back of pre-amp ( properly seal seal chemistry, condensate ) Moisture intrusion between electrode & pre-amp ( select proper “O”-Ring and lubricant to properly seal out chemistry ) Cracked Glass ( mount in an easily accessible area for maintenance, away from mixer blades, do not drop )
GF Piping Systems 45 pH Things To Watch For - continued Controlling pH ( should only be performed in tank or recirculation loop ) Retention time ( in flow through tank, size the tank to allow at least 10 minutes for every 3 pH units of neutralization ) Improve pH adjustment ( use proportionally controlled metering pumps, and provide proper mixing ) Elevated Temperature ( increases ionic activity, decreasing electrode life ) Temperature fluctuations ( cause pumping of fill solution, decreasing electrode life ) Low conductivity solutions ( deplete electrode fill solution, decreasing electrode life) Mounting angles ( use plunger type electrodes for mounting greater than 70° from vertical, or less than 30° from horizontal )