India)
'i'hc work cliscussctl in this scrics of papers, has hccn tlividcd into tlu-ce sections unclcr the licaclings indicated in an carlicr publication 1. Some separations of alkaline earth metals coming under the first licading have been clcalt with in two papers already publisl~ctl *02. In this paper the remaining separations of alkaline cart11 metals from clemcnts tliat form thiosalts arc dealt with.
Standard solutions of sodium arscnatc, potassium antimony tartratc, potassium tclluritc, sclenious acid, ammonium molybdatc and strontium chloriclc wcrc used. Mcasurcd volumes of the two solutions were mixccl together. 'I'hc mixture was trcatccl with ZN sodium sulphidc rcagcnt in cxccss. Tlic solution containing the thiosalt was acidified with a mcasurccl cluantity of hydrochloric acid. 'l'hc mixture was hcatcd to boiling and allowccl to cool to room temperature. 'I'l~t: prccipitatc was filtcrcd through a weighccl sintercd glass crucible (porosity 4), and the metals wcrc estimated as As2Sa3, Sb2Ss4, 'I'cS,~, SCS,~ or MoS,*zH205 rcspcctivcly.
'l'hc following points must bc borne in mint! when precipitating the cliffcrcnt sulphides.
(I) If a white precipitate appears 011 mixing the two solutions, it sl~oulcl bc dissolved in the minimum quantity of dilute hydrochloric acid.
(2) In the cast of molybdenum sufficient acicl should be present in the solution to which sodium &lphiclc is added and the reagent sl~ould bc adclcd dropwisc with constant shaking, othcrwisc molybdenum blue may form and pass into the filtrate. (3) l'hc solution containing tllc thiosalt of arsenic slloulcl be acidified with a mcasurccl volume of concentratccl hydrochloric acid, to produce an acidity of approximatcly CV in the resulting mixture. In the cast of otlict-clcmcnts tlic solution containing the thiosalt is ncidificcl with 2N hydrochloric acid to proclucc an acidity of approximately IN in tlic resulting niixturc. (4) In the cast of sclcniuin a more easily filterable prccipitatc is obtained, if tlic concentration of the acid in the resulting mixture is about 4N. After acidification the mixture .should not bc boiled. It should bc kept at a lower tcmpcraturc (700-800), until the yellow precipitate becomes orange in colour.