20 LEAD ARSENATE. 



The amount of water recommended to be added to these quantities 

 varies from 16 to 200 gallons. In some cases one is directed to mix 

 the chemicals, then add the water; in other cases to dissolve the chem- 

 icals in separate portions of w r ater and then mix the solutions. But in 

 only a few cases is attention called to the necessity of having the lead 

 salt in excess or a method given for determining when it is in excess. 

 The grade of arsenate of soda to be used is sometimes given, but 

 usually no reference is made to it. The 4 to 11 formula is based on 

 arsenate of soda of 50 per cent strength and the 3 to 7 formula on 

 arsenate of soda of 65 per cent strength; that is, 50 per cent and 65 

 per cent of arsenic oxid (As 2 O 5 ) . 



Some confusion seems to have arisen in regard to arsenate of soda 

 and arsenite of soda, as some of the formulas call for the latter, 

 though the other salt is no doubt intended. Arsenite of soda is not 

 suitable for the purpose. In a few instances the objection to the 

 presence of chlorids in the sodium arsenate is referred to, but usually 

 this is not mentioned. 



It was the practice originally to add glucose or thick molasses at 

 the rate of 2 quarts to 100 gallons for the purpose of increasing the 

 adhesive qualities of the mixture. This practice has since been dis- 

 continued, as it was found that these substances did not increase 

 adhesion, nor was the material eaten any more readily when they 

 were present. 



According to some of the published formulas there would be present 

 in the prepared mixture less than one-half pound of actual arsenate 

 of lead to 150 gallons of water. It is very doubtful whether the appli- 

 cation of such a small amount would be of sufficient benefit to pay 

 for the trouble of applying it. 



In all of the formulas the lead salt is present in large enough pro- 

 portions, under ordinary conditions, to combine with all of the arsenic 

 and still be in excess. In extreme cases, however, when sodium 

 arsenate was used which contained an unusually large per cent of 

 arsenic or of sodium chlorid this would not be true. It is necessary 

 that a different formula should be used for different grades of chem- 

 icals, and unless the person making the lead arsenate knows the grade 

 of material he is working with he will be in the dark as to which for- 

 mula to employ. This shows how necessary it is to apply some test 

 to determine when sufficient lead has been added, instead of using 

 definite amounts of the two salts. The following tests for this purpose 

 have been given : 



After mixing the salts, filter a portion and to the clear filtrate add a 

 few drops of dilute sulphuric acid, when, if lead is in excess, a white 

 precipitate of lead sulphate will be formed. Instead of sulphuric 

 acid, there may be added to the clear filtered liquid a few drops of 

 chrornate or dichromate of potash, when, if lead is in excess, a yellow 



