22 LEAD ARSENATE. 



great advantage of this test is that it is not necessary to filter the 

 solution or wait for it to settle. 



If the paper is not at hand, the test may be made by adding a few 

 drops of a solution of potassium iodid, when, if lead is in excess, the 

 instant the drops touch the solution a bright yellow compound, lead 

 iodid, will be formed. 



It is very essential that the lead salt be added in slight excess, but a 

 large excess should be avoided. 



If the material has been carefully prepared with a good grade of 

 chemicals it will not be necessary to filter and wash the lead arsenate 

 formed, though it would be a safe precaution to allow the lead arsenate 

 to settle, then decant the clear solution and discard it. Approximately 

 1 pound of actual lead arsenate will be obtained by using the amounts 

 of chemicals specified, which is equivalent to practically 2 pounds of 

 commercial lead arsenate in the paste form. It may be made up to 

 50 gallons with water if a formula is being used which calls for 2 

 pounds of commercial lead arsenate to 50 gallons, or if a stronger 

 application is desired add less water. 



As these chemicals are all extremely poisonous, vessels in which 

 they have been dissolved or mixed should be plainly marked and 

 not used for any other purpose. 



COMPARATIVE MERITS OF LEAD ACETATE AND LEAD NITRATE. 



As far as expense is concerned it makes little difference which of 

 these lead salts is used, as their price per pound is practically the same. 

 The nitrate may be slightly cheaper, as it contains a higher per cent 

 of lead, though some of the commercial samples of lead acetate which 

 are nearly free from moisture contain almost as much. A little less 

 lead nitrate is required to make the same quantity of lead arsenate, 

 since when made from this salt more of the lead hydrogen arse-nate is 

 formed, which contains a larger per cent of arsenic on an average 

 about 4 per cent more. This compound has also m'ore desirable phys- 

 ical properties, as it remains in suspension better. Kirkland a has 

 shown that the lead hydrogen arsenate is slightly more poisonous 

 than the tri-plumbic arsenate. This may be due to the fact that the 

 former has a larger per cent of arsenic and therefore a smaller quan- 

 tity of it would give the same effect. It is probable, however, that 

 the lead would possess some poisonous properties in this compound, 

 and therefore the larger amount of lead in the one may somewhat 

 offset the excess of arsenic in the other. Some have claimed that 

 the lead hydrogen arsenate was more injurious to foliage than the tri- 

 plumbic arsenate, but this was not found to be the case during the 

 three years of the experiments here reported. Taking all of these 



Agriculture of Massachusetts, 1897, p. 386. 



