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CHEMICAL CONTROL OF DIPPING TANKS. 



2. On. Arsenate of Soda: — 



In order to find out whether the above compounds precipitate: 

 any appreciable amount of the oxidised arsenic in the dipping 

 tank, a solution of arsenate of soda was made up and tested as in 

 the case of the arsenite of soda. 



Proportion of Arsenate (calculated as As 2 3 ) in the solution 

 originally =0114%. 



The above results show that spring water containing an 

 appreciable amount of saline matter (especially salts of iron and 

 of the alkaline earths), when used in making up a dip fluid, may 

 cause a serious precipitation of both the unoxidised and oxidised 

 arsenic in a dipping tank, and the weakening of dip fluids in tanks 

 might sometimes have possibly been due to this cause. Of course, 

 salts of sodium and potassium in the water would have no such 

 effect. 



Summary. 



1. When estimating the arsenic in ordinary dip fluids by 

 titration with standard iodine solution, the error due to absorp- 

 tion of the iodine by the organic matter in the dip fluid is very 

 small, and even negligible, if the fluid is first clarified by the aid 

 of a few cubic centimetres of either strong hydrochloric or sulphuric 

 acid. 



2. It has been stated by more than one authority that the 

 frequent addition of fresh excretory matter to the dipping tank 

 will not only retard the oxidising action that usually goes on in 

 arsenical dip fluids, but will even cause the speedy reduction of 

 the oxidised arsenic. According to this contention, a tank in 

 constant use will not show any appreciable oxidation, but experi- 

 ence has shown that some dipping tanks, although fulfilling these 

 conditions, still show serious weakening of strength by oxidation. 

 It is, therefore, very necessary that farmers should forward 

 samples periodically to a laboratory in order to have a proper 

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

 themselves. 



3. The standard solutions of iodine supplied by certain dealers 

 to farmers for testing their dip fluids seem to keep up their strength 

 fairly well for about three Or four months, but after that period 

 has elapsed they usually deteriorate fairly rapidly. 



