THE VOLUMETRIC DETERMINATION OF BARIUM 

 AND SULPHATE. 



By J. L. B. Smith. M.Sc. 



Read July 9, 191 9. 



(i) Barium. 



Principle. — Excess of a standard tenth-normal KgCrO^ solution 

 is added to the neutral solution of a barium salt, when BaCrO. is 

 precipitated : — 



BaClg + K^CrO^ == BaCrO^ + 2KC1. 



The excess of KgCrO^ is filtered off, KI and HCl added in 

 excess, and the liberated iodine is titrated with tenth-normal 

 Na^SgOg. 



2K2CrO^ -f 16HC1 + 6KI = lOKCl + 2CrCl3 + SI^ 

 3I2 + SNa^S^Og = 6NaI -f SNa^S^Og. 



Procedure. — The solution of barium salt should not contain less 

 than 03 per cent, or more than 1 per cent, of BaO. 50 c.c. of this 

 solution is taken, and — 



(A) If acid, finely divided pure precipitated calcium carbonate 

 is added till there remains about ^ gm. in the flask. 

 Then raise to boil and allow to boil for about three 

 minutes. 



(B) If alkaline, add to^CI until it is slightly acid, and then 

 proceed as from (A). 



In any case, even if the barium solution is neutral, add 

 about \ gm. finely divided pure precipitated CaCOg and 

 raise to boil. 



Then from a burette add slowly, with continual shaking, to 

 the hot solution '^ KgCrO^ till in excess. (About 5 c.c. in excess is 

 best.) 



Raise to boil slowly, with constant shaking, and allow to boil 

 very gently for about two minutes. 



Allow to stand for a few seconds and then filter rapidly into a 

 clean flask. In the precipitation flask raise to boil two lots of 

 distilled water, first 50 c c. and secondly 25 c.c, and wash the 

 precipitate with these, adding the washings to the filtrate. 



Cool completely . Add excess of KI solution, then acidify with 

 pure HCl (about 3 c c. concentrated acid for every 100 c.c. of the 

 solution), and titrate the liberated iodine with ^^ Na^SaOg, using 

 starch as indicator. 



