A Semimicro Scheme of Qualitative Analysis for the Cations Without the use of Hydrogen Sulfide -
J. T.DOBBINS and E. S . GILREATH University of Nor& Carolina, Chapel Hill, North Carolina
T .
HE first edition of the Fresenius "Qualitative Analysis" was published in 1840. This scheme, although modified and improved by subsequent investigators, has remained the standard procedure for the analysis of the cations until the present time. The greatest objection to the use of hydrogen sulfide in qualitative analysis is the poisonous nature of the gas when improperly used. The production and use of hydrogen sulfide, by any method, also limits the rapidity of the analysis. The slowness of the hydrogen sulfide scheme not only greatly reduces the number of the student's laboratory operations, but necessarily cuts down the scope of the accompanying lectures. As early as 1842 attempts were made to replace hydrogen sulfide with less objectionable precipitating reagents. From that date until the present, a t least 50 nonhydrogensulfide schemes have been developed. In spite of the many undesirable features of the Fresenius separation of the cations, the proponents of nonhydrogen-sulfide schemes have not produced a satisfactory substitute during the period of over a hundred
years. An examination of solubility tables for possible precipitating anions other than the sulfide ion, leads to the conclusion that the difference in the solubility products of any other anion is not sufficient to give complete and clear-cut separations. With the experience of past investigators as a guide, the development of a nonhydrogen-sulfide scheme must involve an entirely new approach. Such an approach has been achieved by not only controlling precipitation by differences in solubility products, but by combining this principle with control of concentration of cations through instability constants of complex compounds. Using the phosphate ion as a precipitating agent, and lactic acid as a complexing agent, two phosphate groups have been separated completely; one group of cations being precipitated in acid solution and the other precipitated in alkaline solution. A thud group is precipitated as the insoluble pyridinethiocyanate complexes. Briefly the scheme divides the common cations into five groups: Group I is composed of those cations whose chlorides
'
are insoluble in dilute acids. These are silver, mercurous mercury, and lead. Group I1 is composed of those cations whose phosphates are precipitated in a solution containing monobasic ammonium phosphate and lactic acid. These cations are lead, bismuth, iron, and chromium. Tin precipitates in this group as the insoluble beta stannic acid. Group I11 is composed of those cations whose phosphates are insoluble in a solution made alkaline with ammonium hydroxide and containing the lactate ion. This group includes aluminum, manganese, barium, strontium, calcium, and magnesium. Group IV is composed of those cations which form insoluble pyridine-thiocyauate complexes in very dilute acetic acid. These ions are copper, zinc, nickel, cobalt, and cadmium. Group V is composed of those cations which do not precipitate under any of the conditions given for the precipitation of the first four groups., These ions are mercury, antimony, arsenic, sodium, potassium, and ammonium. The precipitation of the cations of Group I1 depends uDon the verv small solubilitv D ~ O ~ Uconstants C ~ of tieir phosphites; this fact &a
