As_{4}O_{6} + 4Cl_{2} + 4H_{2}O —­> 2As_{2}O_{5} + 8HCl

Note that only one twentieth of the original weight of bleaching powder enters into the reaction.

[Note 1:  The powder must be triturated until it is fine, otherwise the lumps will inclose calcium hypochlorite, which will fail to react with the arsenious acid.  The clear supernatant liquid gives percentages which are below, and the sediment percentages which are above, the average.  The liquid measured off should, therefore, carry with it its proper proportion of the sediment, so far as that can be brought about by shaking the solution just before removal of the aliquot part for titration.]

[Note 2:  Bleaching powder is easily acted upon by the carbonic acid in the air, which liberates the weak hypochlorous acid.  This, of course, results in a loss of available chlorine.  The original material for analysis should be kept in a closed container and protected form the air as far as possible.  It is difficult to obtain analytical samples which are accurately representative of a large quantity of the bleaching powder.  The procedure, as outlined, will yield results which are sufficiently exact for technical purposes.]

III.  PRECIPITATION METHODS

DETERMINATION OF SILVER BY THE THIOCYANATE PROCESS

The addition of a solution of potassium or ammonium thiocyanate to one of silver in nitric acid causes a deposition of silver thiocyanate as a white, curdy precipitate.  If ferric nitrate is also present, the slightest excess of the thiocyanate over that required to combine with the silver is indicated by the deep red which is characteristic of the thiocyanate test for iron.

The reactions involved are: 

AgNO_{3} + KSCN —­> AgSCN + KNO_{3},
3KSCN + Fe(no_{3})_{3} —­> Fe(SCN)_{3} + 3KNO_{3}.

The ferric thiocyanate differs from the great majority of salts in that it is but very little dissociated in aqueous solutions, and the characteristic color appears to be occasioned by the formation of the un-ionized ferric salt.

The normal solution of potassium thiocyanate should contain an amount of the salt per liter of solution which would yield sufficient (CNS)^{-} to combine with one gram of hydrogen to form HCNS, i.e., a gram-molecular weight of the salt or 97.17 grams.  If the ammonium thiocyanate is used, the amount is 76.08 grams.  To prepare the solution for this determination, which should be approximately 0.05 N, dissolve about 5 grams of potassium thiocyanate, or 4 grams of ammonium thiocyanate, in a small amount of water; dilute this solution to 1000 cc. in a liter bottle and mix as usual.

Prepare 20 cc. of a saturated solution of ferric alum and add 5 cc. of dilute nitric acid (sp. gr. 1.20).  About 5 cc. of this solution should be used as an indicator.

STANDARDIZATION