MODIFIED PROCEDURE for the ARSENIC GROUP A. T. LINCOLN
AND
EARL OLSON
Carleton College, Northfield, Minnesota HE difficulties experienced by students in qualita- Asss by dissolving arsenic pentasulfide in aqueous tive analysis in the separation and detection of ammonia. This may be represented thus: AszSsf the members of the Arsenic Group or Group IIB, 6NHaOH -+ (NH4)aAsOjS f (NHa)jAsS4 f 3Hz0. as i t is familiarly called, are not confined to a few No reference can be found to the analogous reaction laboratories nor to any one procedure. Some institu- for antimony, but it is well known that antimony tions even evade these difficulties by omitting this sub- pentasulfide does dissolve in cold dilute ammonia and group from their procedure and do not-give arsenic is more soluble when warmed. We find, however, that and antimony in the samples issued to students for in the presence of ammonium oxalate the solubility of analysis. There are quite a number of different pro- antimony is practically nil. cedures for these specific separations and detections. Various confirmatory tests may be utilized for the At Carleton College we have made use of most of them, identification of arsenic and antimony but we have and the various classes in qualitative analysis, year found the molybdate test-for arsenic and the sulfide after year, have met with indifferent success, in com- test for antimony to"yie1d satisfactory results in the mon with students elsewhere. bands of the average freshman. We shall not review these various procedures but The relative solubilities of the sulfides of arsenic and merely utilize the first of the two methods (Method A) antimony have been checked in the ammoniacal soluthat Dr. J. H. Reedy gives in his "Elementary Quali- tions of the common ammonium salts including NHICI, tative Analysis" as the basis for our modified procedure. (NH4)zSO4, NHaNOa, (NHa)zCOa, NHiCzHaOs, and Bridy, this consists in treatin the yellow ammonium (NH4)2C204.From these numerous tests it became 5 sulfide solution of the sulfides of Group IIB with ox- apparent that only ammoniacal (NH4)GOa dissolves alic acid and 3% hydrogen peroxide which dissolves the sulfides of arsenic completely, whereas its solvent the tin compound and leaves the insoluble sulfides of action on the sulfides of antimony is practically nil; arsenic and antimony and possibly some sulfur. The hence, the use of (NH4)2C20a as the solvent for the oxalic acid converts the tin sulfides into stannic oxalate, separation of these two substances. As an example Sn(C204)z,and the hydrogen peroxide greatly increases of our numerous tests, we digested an excess of AS& its solubility, while it also tends to retain the arsenic and Sb& for five minutes with 20 ml. N/2(NH4)2C104 and antimony in their highest state of oxidation, which and 4 ml. 5N NH4OH. No trace of antimony was deis essential. tected in the filtrate but 0.665 gram of AS& was found According to our modified procedure, the precipi- dissolved. tated residue is then treated with (NH4)E?O&and Tests were made to ascertain the sensitiveness of NH40H. The arsenic sulfide is dissolved while the this procedure by employing an ammoniacal solution antimony sulfide is practically insoluble, thus com- of As& containing 0.0031 gram of As per ml. Differpletely and readily separating them. J. W. Mellor,' ent quantities of this solution were treated according in referring to the work of L. W. McCoy, states that he to the detailed procedure with 24ml. portions of amobtained a solution of amrnoniumsulfarsenate, (NH& moniacal fNH4hC*Oa. , .,... The solutions were then acidified and boiled, causing the precipitate to coagulate in large ~MELLOR; J. W., "A comprehensive treatise on inorganic of this treatmentas set and theoreticalchemis~."Volume IX, Longmans, Green & Co.. curdy particles. The forth in the last column of the following table show New York City. 1929, page 327.
T
that arsenic can be detected readily if the concentration is over 0.0009 gram in less than 3 ml. by diluting with 24 ml. of the reagent. Amou* in ml. of Slandnrd Ar Solufia
Amount (gram) of Ar
1 2 3
0.00031 0.00062 0.00093 0.00124 0.00155
4
5
Rcsulling Tcrf Cloudiness which settled in 12 hours Cloudiness which settled in 12 hours Curdy. yellow particles of A d % Curdy, gellow particles of AsSs Curdy, yellow particles of A*&
MODIFIED QUALITATIVE PROCEDURE FOR THE ARSENIC GROUP
The sulfides of Group I1 have been extracted with warm, yellow ammonium saEde. The filtrate contains (NH4)sSnSa, (NH&As&, (NH&SbSp, and an excess of (NH4)&. This solution is evaporated until it becomes cloudy and then 20 ml. of a saturated solution of HzC20, and 5 ml. of 3% Hz02 solution are added. Boil 5 minutes, dilute to 25 ml., saturate with H2S. Filter and wash.
beaker of hot water (nearly bailing). A bright yellow precipitate of iNHdaAs0. .12M00. indieltps A,.
