Abstract
Mercurometry (ISO 2481), potentiometry (ISO 6227), and argentometry (JTS) for determination of chloride in common salt were studied as mutual comparison regarding the operation, error, time required and cost. Mercurometry was effected by pH and quantity of diphenylcarbazone. Excessive addition of nitric acid caused a decrease in repeatability because of an increase in blank value and unstable color at the end-point. Too little addition of nitric acid gave too little determination value. So addition of 0.5 to 1.0ml of 2 N-nitric acid and preparation about pH 2 were recommended. The color of the end-point was changed by the quantity of diphenylcarbazon, but determination value was not affected. A serious disadvantage in mercurometry was a disagreement of color with the color-matching solution, and the instability of end-point color gave frequently some anxiety to, the operator.
Repeatability in potentiometry in the ruled six combinations of electrodes was not different from others, but a combination of silver-ion selective electrode and calomel electrode gave relatively good accurate results. Repeatability of titration was 1: potentiometry, 2: argentometry, 3: mercurometry in the other of excellence. But each standard deviation in the total operation of determination was nearly equal because the dilution error was greater than the titration error. The comparison of accuracy of each method was obtained from the deviation from the standard value of the International Standard Seawater. The argentometry was the most accurate, the mercurometry gave lower value, and the potentiometry gave higher value.
Time required for making a sample was about 25 min in mercurometry, about 30min in potentiometry, and about 20min in argentometry after pipetting the sample solution. The reagent cost, apparatus cost and personnel expenses were estimated, and the total cost of argentometry was more reasonable than mercurometry in spite of expensive reagent cost because the weight of personnel expenses was greater than reagents and apparatus cost.
