Stability of dilute standard solutions of antimony, arsenic, iron and rhenium used in colorimetry

A study has been made of the stability of dilute standard solutions of antimony (4 µg ml–1), arsenic (20 µg ml–1), iron (50 µg ml–1) and rhenium (5 µg ml–1) used in colorimetry. The standard elements in these solutions were determined over a period of 2 months by using colorimetric procedures developed in this laboratory and reported previously. Tests were carried out on standard solutions stored in soda-glass, in borosilicate glass and in rigid polyethylene containers.The dilute standard antimony solutions, prepared either by dissolving antimony potassium tartrate in water, or by dissolving elemental antimony in sulphuric acid and diluting the solution with water, were found to be stable (i.e., to deteriorate by less than 2 per cent.) over a period of 50 days. Similar dilute standard antimony solutions containing hydrochloric acid deteriorated rapidly, however.The dilute standard arsenic solutions prepared either by dissolving arsenic(III) oxide in sodium hydroxide solution and then neutralising the solution with hydrochloric acid, or by dissolving disodium hydrogen arsenate heptahydrate in water, were found to be stable. Arsenic(III) in the former standard solution was oxidised slowly by dissolved oxygen, but the total arsenic present in the solution remained unchanged and could be determined by the molybdenum-blue method.An iron(III) standard solution, 0·06 M in hydrochloric acid and prepared from ammonium iron(III) sulphate, was stable for at least 2 months, as was a standard potassium perrhenate solution in a buffer solution of pH 6.Light in the laboratory and the material of the containers did not adversely affect the solutions reported to be stable. Light accelerated the deterioration of the antimony solutions that contained hydrochloric acid, and the material of the containers had a slight effect on the rate of deterioration.