48 PUBLIC HEALTH CHEMISTRY 



grm. silver nitrite are dissolved in boiling distilled water, 

 and precipitated by slight excess of KC1, i grm. of nitrous 

 acid as N 2 O 3 is left in solution in combination with 

 potassium. The bulk is made up to i litre, the precipitate 

 allowed to settle, and 10 c.c. are taken and diluted to i litre, 

 then i c.c. = o-oi mgr. N* 2 O 3 (equal to i per 100,000). 

 Standard nitrite solution is also made so that i c.c. 

 = o-oi mgr. N, and also of millinormal strength, and 

 then i c.c. N /iooo = 0-046 mgr. NO 2 . 



PROCESS. To 100 c.c. of sample in a Nessler glass, i c.c. 

 of the dilute sulphuric acid and i c.c. of the metaphenylene- 

 diamine solution are added as a preliminary test. If an 

 orange colour is immediately produced, the tint will prove 

 too deep for comparison. In such a case 50 c.c. should 

 be tried, and if found suitable such an amount diluted to 

 100 c.c. with aq. dest. is to be used in the real test, which 

 is done thus : Having decided the amount of the sample 

 to be used, it is taken in a 100 c.c. Nessler glass .and made 

 up to 100 c.c. if required. Three other Nessler glasses are 

 taken, and i c.c., 2. c.c., and 3 c.c. of standard nitrite 

 solution added to each respectively, and the bulk is made 

 up to 100 c.c. in each case with aq. dest. To all the glasses 

 is added i c.c. of each of the reagents, namely M-P-D 

 and H 2 SO 4 , and this is done as quickly as possible, so 

 that the colours in the glasses may develop from as nearly 

 as possible the same time. The glasses are set aside for 

 fifteen to twenty minutes and are then compared in the 

 manner known as " Nesslerizing." If the sample matches 

 one of the standards, then the amount in it is known. 

 If not, fresh trial glasses are put up, the amount of standard 

 being gauged from the preceding experiment. 



The colour produced is Bismarck brown or triamido-azo- 

 benzol. Griess's test is a very accurate one but requires that 

 the water and the reagent should be colourless or be decolor- 

 ized. The reagent may be bleached by pure animal charcoal. 



Ilosvay's Naphthylamme Test. 



a. Solution of sulphanilic acid 0*5 grm. in 150 c.c. of 

 diluted acetic acid (specific gravity 1-04). 



b. Solution of naphthylamine made by dissolving o-i 

 grm. in 20 c.c. of aq. dest., filtering, and adding 180 c.c. 

 diluted acetic. 



