Ferric alums chromate

Chlorides, bromides, and iodides can be quantitatively determined by treatment with silver nitrate, and, with suitable precautions, the precipitated halide is washed, dried, and weighed. Chlorides in neutral soln. can be determined by F. Mohr s volumetric process 27 by titration with a standard soln. of silver nitrate with a little potassium chromate or sodium phosphate as indicator. When all the chloride has reacted with the silver nitrate, any further addition of this salt gives a yellow coloration with the phosphate, and a red coloration with the chromate. In J. Volhard s volumetric process, the chloride is treated with an excess of an acidified soln. of silver nitrate of known concentration. The excess of silver nitrate is filtered from the precipitated chloride, and titrated with a standard soln. of ammonium thiocyanate, NH4CN8—a little ferric alum is used as indicator. When the silver nitrate is all converted into thiocyanate AgN03-fNH4CNS=AgCNS +NH4NOS, the blood-red coloration of ferric thiocyanate appears. 

Volumetric estimation4 in neutral solution can be effected by titration with standard sodium chloride, potassium chromate being employed as indicator and in nitric-acid solution with thiocyanate, using ferric alum as indicator, or with sodium chloride without any external indicator. 

In a back-titration of silver by chloride ions, potassium chromate can be used to indicate the endpoint (Mohr s method) but with potassium thiocyanate as tifrant, anunonium iron(III) sulfate ( ferric alum ) is preferred. In the direct titration (Gay-Lussac s method) the location of the turbidimetric endpoint has been improved in detail. ... 

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