2 Measuring pH Three ways to measure pH pH paper pH meter turns a specific color for each pH valuepH metermeasures the voltage difference between a reference solution in the probe andthe solution it’s immersed in andtranslates it into a pH reading.
3 Measuring pH Indicators HIn + H2O H3O+ + In- yellow red weak acids/bases that dissociates in a certain pH range, changing colors as it does so.HIn + H2O H3O+ + In-yellow redHIn = indicator in the acidic form (H = acidic, In = indicator)In- = indicator in the basic form
4 Indicators HIn + H2O H3O+ + In- yellow red What color will it be when you add acid?yellowWhat color will it be when you add a base?red
5 Shows various indicator color ranges – pg. 580 in your text Which indicator would you use to show that a reaction solution has changed from pH 10-11?Alizarin Yellow R
6 IndicatorsThe change in the two forms occurs in a range of about 2 pH units for most indicators.In this range it is a mix of the two colors.Below this range it has the acidic color.Above this range it has a basic color.
7 Limitations of Indicators Solution should be colorlessNeed the ability to detect slight color changesCan only give approximate information on pHIs dependent on temperatureTo test the pH of a liquid over a wide range a combination of indicators are needed. (Universal Indicator)
8 TitrationsTitration – a method in which a standard solution is used to determine the concentration of another solution.Standard solution – one for which the concentration is known.
9 TitrationYou would know the concentration of one of the solutions – either the solution in the buret or the solution in the flask.
10 Titration and Neutralization When a strong acid and a strong base are mixed, the H+ and OH- react to form H2OH+ (aq) + OH- (aq) H2O (aq)This is called a neutralization reactionEventually, a neutral solution will result!To analyze the acid or base content of a solution, chemists use titrations
11 TitrationTitration uses a buret to slowly add one solution (called the titrant) to the other (called the analyte)One of the solutions concentrations is knownCompare the number of moles added to reach the endpointThe endpoint is known as the equivalence pointThe endpoint would be when the solution has a pH of 7
13 Titration and Neutralization What has to be true about the concentration of the ions present when the neutralization is complete?The moles of OH- added must be exactly the same as the moles of H+.How could we see the endpoint with an indicator?Use an indicator that changes colors when it changes from an acid to a base![OH-] add = [H+] cuz the solution ends-up being neutral!!
14 Titration Endpoint or Equivalence point: When enough titrant has been added to react exactly with the substance in solution that is being titrated.The portion of the curve with the steepest slope.Occurs when solution stays colored for 15 sec with swirling
15 TitrationWhat happens to the graph as the titration nears its equivalence point?Steep slope shows a rapid change in pHWhat happens to the pH if you continue to add base?The pH will increases
16 Titration Calculations How many mL of M KOH will neutralize 15.0 mL of 0.400M HCl?Write a balanced chemical equation.HCl + KOH KCl + HOHDevelop a conversion strategyL HCl mol HCl mol KOH L KOHSince M = mol/L we can use it as a conversion factor to change from liters to moles!!!
17 TitrationWhat would happen if we left everything the same, but used H2SO4 instead?It would take twice as much KOH because we have two H+ in every acid that need to be neutralized.The amount of H+ and OH- must be equal if complete neutralization is to occur.
18 Titration Write a balanced chemical equation. H2SO4 + 2KOH K2SO4 + 2HOHHow do we use the mole ratio when we are looking for amount of base?mol H2SO4 x mol ratio = mol KOHMa. Va . b/a = Mb . Vbb/a is the mole ratio of base to acid (put on the acid side)Can use (Ma)(Va)(b/a) = (Mb)(Vb) OR stoich way…it doesn’t matter!! You get the same answer EVERY time!!