Determination of a mixture of chloride, hypochlorite, chlorite, chlorate and perchlorate ions by short-circuit amperometric titration

Abstract

Quantitative determination of chloride, hypochlorite, chlorite, chlorate and perchlorate ion mixtures was studied.
Hypochlorite, chlorite, chlorate and perchlorate ions were reduced to chloride ions one after another with proper reducing agents, according to the oxidationreduction potential.
After the reduction, these chlorides could be titrated with a silver nitrate solution by means of shortcircuit amperometry, which uses a rotating platinum wire electrode (1000 rpm) as the indicative electrode and a saturated calomel electrode(SCE) as the reference electrode.
In the reducing operations, hypochlorite (ClO^-) could be reduced within a few minutes by sodium arsenite at pH 810, hypochlorite and chlorite (ClO₂^-) could be reduced within ca. 10 minutes at room temperature by potassium borohydride at pH 27, and hypochlorite, chlorite and chlorate (ClO₃^-) by sulfurous acid in acidic region.
Hypochlorite, chlorite, chlorate and perchlorate(ClO4^-) could be reduced by fusing with sodium nitrite for ca. 30 minutes in a nickel cruciple. In the amperometric titration of these chloride solutions, the procedure employed is as follow. pH of the sample solution was adjusted 88.5 with 0.1 N acetic acid solution and 0.1 N sodium acetate solution, and then 10 ml of 1 M potassium nitrate solution (as a supporting electrolyte) and 10 m l of 0.2% gelatin solution was added. Finally the total volume of the solution was diluted to 100 ml with water.
When the concentration of chloride was ca. 10^-4M, 5 ml of 0.1 M barium nitrate and 25 vol% methanol should be added to remove carbonates dissolved in the sample and to decrease the solubility of silver chloride.
In the case of a sample chloride solution containing ClO^- (1st case), or ClO^- and ClO₂^-(2nd case), the chloride could be determined by titration under the following conditions.
Excess chlorate(ca. 2.5 M) solution had to be added, and the temperature had to be kept below 2°C, as soon as the sample was take in.
Moreover, for the 2nd case, the concentration of either ClO^- or ClO₂^- had to be held below ca. 10^-3 M.
Because, otherwise, the disproportionation and interactions between these ions occur during the titrating operation and thus the determination becomes difficult.
In general, the amount of the designated ion can be determined from the difference in the volumes of the silver nitrate standard solution consumed until the end points in two proper titrations.
By these experiments, the lower limiting concentration range was found to be ca. 10^-4 M for each ion and, in these cases, the relative percentage errors were ca. ±3%.
However, if the ratios of any ion to the other ions are higher than 50 and chloride is titrated by using a certain silver nitrate standard solution, these errors become over ± 10%, as shown in Table I.