For the calorimetric determination of low concentrations of chloride, methods based on silver chromate and diphenylcarbazide, or silver phosphate and lpolybdate have been reported 1, but their sensitivity is as low as that of turbidimetryz. A small amount of chloride can also be estimated by its suppression of reaction of mercury(I1) ions with diphenylcarbazide or diphenylcarbazone 1~3. This method is sensitive but is effective only in a narrow concentration range under strictly controlled pH conditions. UTSUMI et aZ.486 and TOMONARI~ developed a method based on the calorimetric determination of a specific amount of thiocyanate ions liberated from slightly dissociated mercury(I1) thiocyanate by substitution with chloride ions, using iron(III) alum as a color developer. In these conventional calorimetric methods, care is needed to avoid errors caused by the fading of the color intensity of iron(II1) thiocyanate and mercury( II)-diphenylcarbazide or -diphenylcarbazone.
Recently, a new and consistent method for thiocyanate ions was suggested'; thiocyanate, as low as 2. IO-" M, was extracted from aqueous solution into nitrobenzene as tris(I,xo-phenanthroline)iron(II) thiocyanate and determined colorimetritally. In this method, the absorbance was very stable, and chloride, even if present in a large amount, was not extracted and did not interfere with the determination of thiocyanate. Accordingly, if this solvent extraction with tris(I,Io-phenanthroline)iron(I1) is employed for the determination of chloride ions instead of using iron(II1) as the color developer for thiocyanate released from mercury(I1) thiocyanate, higher sensitivity together with higher color stability should be achieved. This consideration led to the present study, in which an indirect calorimetric method for the @termination of a small amount of chloride has been established and successfully applied to analyses for chloride in fresh water.