86 POISONS : THEIR EFFECTS AND DETECTION. [§ 56 . 
§ 56. Detection. —Both ammonium and hydric sulphides blacken 
silver and filter-paper moistened with acetate of lead solution. To test 
for hydric sulphide in air, a known quantity may be aspirated through a 
little solution of lead acetate. To estimate the quantity a decinormal 
solution of iodine in potassium iodide 1 solution is used, and its exact 
strength determined by d.n. sodic hyposulphite solution ; 2 the hypo¬ 
sulphite is run in from a burette into a known volume, e.g. 50 c.c., of 
the d.n. iodine solution, until the yellow colour is almost gone ; then a 
drop or two of fresh starch solution is added and the hyposulphite run 
in carefully, drop by drop, until the blue colour of the starch dis¬ 
appears. If now a known volume of air is drawn through 50 c.c. of the 
d.n. iodine solution, the reaction I 2 +SH 2 =2HI-f-S will take place, and 
for every 127 parts of iodine which have been converted into hydriodic 
acid 17 parts by weight of SH 2 will be necessary ; hence on titrating 
the 50 c.c. of d.n. iodine solution, through which air containing SH 2 has 
been passed, less hyposulphite will be used than on the previous occa¬ 
sion, each c.c. of the hyposulphite solution being equal to 1*11 c.c. or 
to 1*7 mgrm. of SH 2 . 
1 12-7 grms. of iodine, 16-6 grms. of potassium iodide, dissolved in a litre of water. 
2 24-8 grms. of sodic hyposulphite, dissolved in a litre of water. 
