Chapter 7 Mixtures of Acids and Bases Common Ion Effect Common Ion Effect Shift in position of an equilibrium caused by addition of an ion involved in the reaction Essentially the addition of a conjugate acid or base in the form of the appropriate salt Example pH Buffers Buffer Henderson Hasselbalch Equation Sample Exercise Basic Buffer An Environmental Buffer A Physiological Buffer Buffer Range and Capacity pH Change of Buffer vs Buffer Concentration Sample Exercise Buffering Action pH Indicators pH Titrations Titration Curves for NH3 and NaOH Titration Curves for Acetic and Hydrochloric Acids Detailed Sample Exercise on Strong Acid Weak Base Titration Suppose 50 00 mL of pH 10 0 water from a hot spring rich in carbonate and bicarbonate ions is titrated with 0 02075 M HCl A few drops of phenophthalein are added at the beginning of the titration and the solution turns pink It takes 11 21 mL of titrant to reach the pink to clear equivalence point Then a few drops of bromocresol green are added and it takes an additional 32 28 mL of titrant before the blue green color changes to yellow What are the initial concentrations of carbonate and bicarbonate in the sample We are asked to determine the concentrations of two analytes in one sample from the results of a single titration with a monoprotic strong acid HCl These determinations are based on the volumes of titrant needed to reach two equivalence points an initial one at which any CO32 in the sample has been converted to HCO3 1 H aq CO32 aq HCO3 aq and a second one at which HCO3 has been converted to H2CO3 2 H aq HCO3 aq H2CO3 aq The HCO3 titrated in reaction 2 includes any HCO3 in the original sample plus all the HCO3 produced in reaction 1 If there were no HCO3 present initially the volume of titrant needed to react with the HCO3 in reaction 2 would be exactly the same as the volume needed to react with CO32 11 21 mL in reaction 1 However the volume of the titrant required to reach the second equivalence point is much greater 32 28 mL The difference between these two volumes 32 28 11 21 21 07 mL is the volume of acid required to react with any HCO3 in the sample the value of HCO3 that we calculate for the original sample should be nearly twice its CO3 value Alkalinity Titration CO3 Ion 1 CO3 aq H aq HCO3 aq 2 HCO3 aq H aq H2CO3 aq Sample Exercise cont The stoichiometry of the reaction tells us that the titrant and carbonate react in a 1 1 mole ratio so at the first equivalence point mol HCl added mol CO32 consumed This means that the ratio nA nB for this reaction is 1 1 The same is true for the conversion of HCO3 into H2CO3 mol HCl added mol HCO3 consumed Using VAMA nA nB VBMB Note that nA nB is 1 1 in this example The titration results confirm that the bicarbonate concentration in the original sample was almost twice the carbonate concentration To check our two values from the calculations we can insert the results into the Henderson Hasselbalch equation and calculate what the initial pH of the sample should have been To do that we need the value for carbonic acid which is 10 33 This calculated pH value agrees with the pH given in the problem statement