148 



CHEMICAL CONTROL OF DIPPING TANKS. 

 OLD TANK. 



These figures show that oxidation of the arsenic proceeded 

 continuously at a serious rate in both tanks, although they were 

 in continuous use. The same trouble is liable to occur in other 

 dipping tanks, and for this reason, among several others, farmers 

 should be encouraged to forward samples periodically to the labora- 

 tory of the nearest agricultural college in order to have a proper 

 chemical analysis made, as a check on the tests carried out by 

 themselves. 



Effect of Organic Matter and other Impurities on the 

 Results of the Estimation of Arsenic by the Iodine Method. 



In the laboratory at Cedara, Mohr's method of titration with 

 standard iodine solution is utilised for the estimation of the arsenic, 

 in both the arsenite and arsenate states. 



The dip fluid is first clarified by the aid of a few cubic centi- 

 metres of either strong hydrochloric or sulphuric acid. For the 

 '■estimation of the arsenious oxide the filtrate, after neutralising 

 with sodium carbonate, is titrated with N/10 iodine, after adding 

 an excess of sodium bicarbonate as usual. 



In estimating the total arsenic, another portion of the filtrate 

 is treated with a large excess of either concentrated hydrochloric 

 or sulphuric acid and a gram or two of potassium iodide, in order 

 to reduce the arsenate present to the arsenite condition. The 

 reduction is complete after warming the flask for a few minutes, 

 the free iodine is got rid of by meanr. of sodium thiosulphate, and 

 the arsenic is then estimated as before 



In the dip fluid as taken out of the tank there is a fairly 

 large proportion of extraneous matter present, besides arsenious 

 oxide, that causes the absorption of iodine, but by the addition 

 of an acid to clarify the fluid the greater proportion of these sub- 

 stances is got rid of. In order to ascertain the amount of iodine 

 absorption actually taking place in this method of estimation, a 

 liquid was made up of spring water containing about 10 per cent, 

 •of drainage fluid from the cow byre, thus approximating the con- 



