Abstract
When the following potentiometric titrations using a glass electrode of pH-meter as an end point indicator were carried out in the presence of polar organic solvent in the aqueous solution, steep changes of apparent pH on the scale reading of ordinary pH-meter ( : glass electrode potential) of the solution were found near the equivalence points of the titration.
(1) Argentometry of thiocyanate, ferricyanide and cyanide.
(2) Precipitimetry of ferrocyanide, phosphate, secondary phosphate, sulfate, chromate and iodate using a titrant of lead nitrate solution.
(3) Precipitimetry of various heavy metal ions, for example, silver ion, cadmium ion, nickel ion and zinc ion using a titrant of sodium sulfide solution.
(4) Direct titration of a few weak Bronsted acids (pKa) more than 9 such as ammonium ion, methylammonium ion using a titrant of sodium hydroxide. Therefore, the end point can be detected graphically and a pH indicator electrode is available to indicate the end point in such titrimetry.
The pH of the titration medium is of importance. End points precisely reproduce at the equivalence point over the limited range of pH. For example, the argentometry of ferricyanide could be achieved by the addition of about 10-3M sodium hydroxide solution into the solution to be tested to elevate its pH to about 8.59.0.
Many titration curves were drawn under various conditions to find suitable procedure for this purpose.
The recommended conditions, for example, in the precipitimetry using a titrant of lead nitrate were as follows : the optimum amount of acetonitrile in 30 ml total volume should be 3070% for K4Fe (CN)6, 3070 for Na3PO4, 4080 for Na2SO4, 4060 K2CrO4, 3040 for Na2HPO4, 5080 for KIO3, the minimum detectable concentration by this method was about 2 × 10-3M for all cases.
The most suitable organic solvent for obtaining a sharp end point is acetone in (1) (3) (4) titrations and acetonitrile in (2). Other polar organic solvents belong to protophilic or water-like amphiprotic solvents such as dioxane or ethanol could be used for this purpose.
In this potentiometric precipitation titrations, the more insoluble a precipitate, the more complete is the reaction at the equivalence point and the larger in the change in concentration of the reacting ions. It is to be desired that the Ksp of the precipitate M+A-is 10-10 or less.
In spite of the large number of ammonium determinations being performed at the present time, there is still no simple direct titration procedure available to analytical chemist. But the above stated end point determination is applied to not only various precipitimetry but also direct titration of ammonium compounds.
The quantitative graphimetric end point determination by using a commercial pH-meter in mixed solvents is simple, accurate and practical. Therefore, the method will be especially appreciated in routine analytical laboratories where many daily samples must be quickly analyzed.
