136 PUBLIC HEALTH CHEMISTRY 



250 c.c. of water, along with 25 grm. of sodic carbonate. 

 Boil for some time until all the oxide is dissolved, cool, and 

 make up to 1 litre. This is decinormal strength and can 

 be standardized against N/10 I. ; (2) N/10 iodine solution ; 

 (3) Starch and KI paper as indicator. Take 2-5 grm. 

 of sample and rub up with successive quantities of 

 water until all the powder is transferred to a 250 c.c. 

 flask to which the washings are added. Make up to the 

 mark, shake well, and taking 20 c.c. of the milky fluid, 

 titrate with the standard arsenious solution until starch and 

 KI paper is no longer blued on being touched with a drop 

 of the mixture removed on a glass rod. As excess of the 

 arsenious solution is easily added, it is usual to now add 

 starch to the titrated liquid and titrate with N/10 iodine 

 until a permanent blue is got. The amount of the N/10 I 

 used is deducted xrom the amount of arsenious solution 

 required, and as 1 c.c. As 2 3 solution = 0-00355 g rm - 

 CI, or 3 -55 mgr. CI, the amount of CI is easily calculated 

 and is returned as a percentage. 



2 Ca(OCl)Cl+ 2 H 2 0+ As 2 3 = As 2 5 +2 CaCl 2 + 2 H 2 0. 



Formalin is a 40 per cent solution in water of formalde- 

 hyde (CH 2 0), the qualitative tests for which have been 

 given on page 80. Quantitatively it is tested as follows : 

 Weigh out 2-075 grm. of formaldehyde sample, and dilute 

 with water to 500 c.c. Take 10 c.c. of this dilution and 

 mix with 25 c.c. of N/10 iodine solution in a flask. Add 

 NaOH solution (15 per cent) drop by drop until the liquid 

 becomes clear yellow, and allow to stand for ten minutes. 

 Then add sufficient dilute HC1 to liberate the iodine not 

 acted on by the formalin, and titrate the amount of I with 

 N/10 thiosulphate ; the number of c.c. of thiosulphate 

 required subtracted from 25, gives the number of c.c. of 

 N/10 I absorbed. 1 c.c. N/10 I = 0-0015 grm. CH 2 0. 

 HCHO + H 2 O + 1 2 = H-COOH -f 2HI. 



Permanganate of Potash. The solution is titrated 

 with standard ferrous sulphate solution in presence of 

 sulphuric acid. Take 5 c.c.or 10 c.c. of sample (the smaller 

 the amount the stronger the sample), add 10 c.c. of 25 

 per cent H 2 S0 4 , and from a burette add standard FeS0 4 

 solution until colourless. Calculate to a percentage. 



