42 Preparation of Substances 



water. Mix the two solutions. Test the 

 liquid with pieces of red and blue litmus 

 paper. 



As the precipitate of white lead arsenate 

 settles decant the clear supernatant liquid 

 — best over the edge of the beaker, not using 

 the lip — fill the beaker with fresh water, stir 

 the mixture and again allow the precipitate to 

 settle. Repeat this washing until the soluble 

 salts are removed, and the precipitate, be- 

 coming colloidal in character and refusing to 

 settle completely, is partially dispersed thru 

 the liquid. 



This condition being reacht allow the mix- 

 ture to stand over night, so that as much as 

 possible of the lead arsenate will settle out, 

 then decant most of the liquid, neglecting the 

 loss of the comparatively small amount of 

 precipitate in colloidal condition, and bring 

 all the remaining precipitate gradually onto 

 one 15-cm. filter folded in the ordinary way. 

 Allow the precipitate to drain in the funnel 

 for several days; even a week, as a rule, is 

 not too long. 



"When the amount of moisture is reduced 

 to about 50 per cent the mass will separate 

 easily from the paper and should be handed 

 in. 



