Monograph - Metrohm

Titration of acidsResultsA) Benzoic acid (pK A = 4.20)KOH normally produces larger potential jumps than TBAOH, but TBAOH yieldsmore symmetrical and steeper titration curves. In 11 (63%) out of 16 determinationslarger jumps were obtained with Electrode 2 (underlined values).Summary of the delta pH values (mean value of 3 determinations, delta «pH init»to «pH end»):Solvent KOH/El.1 TBAOH/El.1 KOH/El.2 TBAOH/El.2Ethanol 7.6 7.3 8.1 7.5IPA 8.5 7.1 8.7 8.5tert. Butanol/IPA 8.9 9.5 8.2 8.9Acetone 9.0 6.4 8.1 7.6MIBK 7.7 7.6 7.1 7.0DMF 7.6 5.8 8.7 6.5Pyridine 6.1 6.1 8.0 7.0Acetonitrile 7.3 6.7 10.2 7.7The highest pH values at the end of the titration were:with KOH; 18.2 in DMF/El.2, 16.8 in DMF/El.1 and in pyridine/El.2with TBAOH; 16.0 in pyridine/El.2, 15.9 in tert. butanol/El.2The lowest pH values at the end of the titration were:with KOH; 11.6 in ethanol/El.1, 12.4 in MIBK/El.1with TBAOH; 11.1 in ethanol/El.1, 11.7 in MIBK/El.1Li ions therefore appear to have a certain influence on the pH value (alkali error).The largest jump was obtained with KOH/El.2 in acetonitrile (delta pH = 10.2).However, acetonitrile is a problematic solvent. During the titration sticky precipitatesformed with both titrants, which lead to blockage of the diaphragm. Thismeans that with this solvent the electrode should be cleaned after each titration(H 2 O and ethanol).Very good solvents for medium-strength acids are tert. butanol/IPA, acetone,DMF and isopropanol (IPA).B) Phenol (pK A = 9.95)With TBAOH as titrant the endpoint is normally reached sooner. Electrode 2 usuallygives titration curves that are easier to evaluate.10 Peter Bruttel