A New Method of the Qualitative Chemical Analysis of Common Cations

Transcription

1 A New Method of the Qualitative Chemical Analysis of Common Cations by Keiichiro MATS UOKA* Introductory A qualitative analysis of inorganic substances is concerned with the methods of determining the species of base-forming and acid-forming constituents that are present in a substance. A qualitative analysis admits of twofold classification ; one is chemical and the other is physical method. Recent progress has introduced into the physical method many useful instruments and, for this reason, it is possible for the physical method to perform a very minute analysis in the accurate manner. Yet these instruments are regrettably too high to be within the reach of an average chemical laboratory. In any qualitative chemical analysis important procedures are separation, detection and confirmatory test of the obtained result. With this method it is imperative that a very minute and accurate determination should be performed in less time. A chemical method is also divided into two kinds ; dry and wet methods. The former consists of heating test which contains blowpipe test as a sub-class and bead reaction including borax bead test, microscopic bead test, etc. Though the dry method is useful, its main value is derived from being used as a preliminary procedure. The latter method is further divided into four classes as follows. 1. Method by the use of H2S. 2. Method which uses substitutes for H2S. 3. Method which uses organic reagents instead of sulfide. 4. Method which detects each element on an individual basis. Methods in (1), (2) and (3) are systematic separating methods. And method in (1) and (2) are analyses based on the same principle. Though method using H2S in (1), referred here for short as H2S method, is a classical one, it has been improved by E. P. TREADWELL, W. T. HALL [1] and A. A. NOYES [2], etc. and application range of this method is rather extensive. It is a credit to this method that an analysis is carried out with simple common apparatus and common reagents in easy processes. This method is frequently used in a chemistry class to train the students of qualitative analysis in careful manipulation of these chemical experiments and also in its theoretical side. Although the educational value of this method is very large, it is the definite drawback that H2S, which is essential to this method, is a corrosive gas and injurious to health. Moreover, with this method a complete precipitation or separation of sulfides is very hard. Many attempts to remove the faults of H2S method have been Department of Chemistry, Nihon Univ. School of Dentistry (Chief : Prof. Ichiji Kondo)

2 made so far. The method (2) tries to use Na2S [3] or (NH4)2S, or NH4OH containing H2S [4] instead of H2S. There is no need for using Kipp's-(gas)-generator in this method and therefore it is better than H2S method. Yet here again the removal of gas injurious. to health is not complete. The method in (3) consists of an extraction with organic solvents, using special organic reagents [5]. With this method it is possible to analyse Ti, U, rare metal and rare earth metal, etc., as well as the common ions, which have been treated with classical methods such as H2S method. The processes of the method include colorimetry, complex compound formations and ions exchange, and they are rather complex. And also instruments and reagents which are used in the method are rather expensive. The method in (4)[6] has many merits but it is necessary to remove obstructing elements. Moreover the processes used in the method are rather complex. In view of these considerations, though H2S method has many faults, it can be said to be the. best method for the primary instruction in a qualitative analysis. The author has attempted to remove the faults in H2S method, and arrived at a new systematic qualitative chemical analysis. Most of the procedures in the new method hardly emit any gas or fume injurious to health. The present paper describes experimental procedures performed by the author. Procedures of New Method A. Detection of Sodium. As sodium came into the test-solution from reagents, which separated cations into groups, detection of sodium had to be carried out in the first place. (Procedure1: Confirmatory Test for Sodium). A platinum wire having a small loop at one end was heated in a gas flame till it no longer colored the flame, dipped in HCl, touched to the unknown sample solution, and introduced into the flame again (yellow color: presence of sodium). TABLE1 Separation of Cations into Groups

3 B. Separation into Groups. For detecting the base-forming constituents (cations), a systematic method was employed in which a test solution was successively added to hydrochloric acid, sodium hydroxide and sodium carbonate, sodium bicarbonate. The detailed way in which the base-forming constituents were thus separated into groups is shown in Table 1. C. Precipitation of the I-Group and Analyses of Them. (Procedure 11 : Precipitation of the I-Group). About 15 ml of the cold solution containing cations was put into a conical flask, and to it 2-normal HCl was added drop by drop so long as the precipitate increased, whose presence indicated I-group. Then the mixture was left standing for 3 or 4 minutes before filtration. The precipitate was treated as in the procedure 12, and the filtrate as in the procedure 21. Analysis TABLE 2 of the I-Group * The precipitate forms only when large quantity of lead is present. (Procedure 12 : Extraction and Detection of Lead). Through the filter containingthe HCl precipitate was repeatedly poured 10 ml of boiling water and the precipitate was thoroughly washed with hot water for subsequent treatment. The residue insoluble in hot water was treated as in Proc. 14. To the aqueous extract was added 10 ml 6-normal H2SO4. White precipitate indicated the presence of lead. After filtration the precipitate was treated as in Proc. 13 and the filtrate was rejected. (Procedure 13 : Confirmatory Test for Lead). Through the filter containing the H2SO4 precipitate 20 ml portion of CH3COONH4 solution was successively added and also to the filtrate added a few drops of K2CrO4 solution and 2-3 ml CH3COOH. Yellow precipitate indicated the presence of lead. (Procedure 14 : Detection of Silver and Mercury). Through the filter containing the residue insoluble in hot water was poured 5-10 ml portion of NH4OH. (Black residue on the filter indicated the presence of mercurous mercury). The filtrate was acidified by means of HNO3. White precipitate indicated the presence of silver. D. Precipitation and Separation of the II-A-Group and II-B-Group. (Procedure 21 : Precipitation of II-Group and Confirmatory Test of Ammonium). The filtrate from the HCl precipitate was neutralized with 2-normal NaOH, and was further mixed with 10 ml 2-normal Na2CO3 and 20 ml 2-normal NaOH. After the solution came to a boil, a piece of moist red litmus paper was inserted. Blue coloration of the litmus paper meant the presence of ammonium. After boiling the mixture for 5 to 10 minutes, it was subjected to filtration. If there was a lot of ammonium in

4 TABLE 3 Separation of II-A-Group and II-B-Group the test-solution, it had to be expelled completely from the solution by means of more boiling. The filtrate was treated as in the procedure 61. (Procedure 22 : Treatment of Precipitate with Hydrochloric Acid). To the precipitate in Proc. 21 was added ml normal HCl solution and the mixture was stirred. It was left standing for 3 or 4 minutes, then it was filtered. (Procedure 23 : Confirmatory Test for Antimony). The residue from the HCl treatment in the preceding procedure was dissolved in a 6-normal HCl. Beneath the solution was inserted a piece of tin-foil and the mixture was left standing for 3 or 4 minutes. Black deposit on the tin-foil meant the presence of antimony. (Procedure 24 : Separation of II-A-Group and II-B-Group). To the solution from the HCl treatment in the procedure 22 in a beaker was added 1-2 ml 2-normal HNO3 and heated to boiling by gently heating for 5 minutes. The solution was left standing for minutes and neutralized it with 2-normal NH4OH. 10 ml 2-normal NH4OH and 10 ml NH4Cl solution was further added. The mixture was stirred and precipitated. The precipitate was treated as in the procedure 31, and filtrate as in the procedure 41. E. Analysis of the ILA-Group. (Procedure 31 : Separation of Chromium from Another). Dissolve the precipitate from (NH3 & NH4+) solution in the procedure 24 was dissolved in 5 ml 2-normal HCl. To the solution was added 10 ml 2-normal NaOH dropwise. It was stirred and filtered. (Procedure 32 : Detection of Chromium). The filtrate from the NaOH precipitate was boiled gently for 3 or 4 minutes. Filtration should be made while the solution was hot. (Procedure 33) : Confirmatory Test for Chromium). A small parts of the precipitate in the preceding procedure was transferred into porcelain crucible, it was added a little solid KNO3 and Na2CO3. And the mixture was fused. When it cooled at the room temperature, 2-normal CH3CO2H was used for acidification and the mixture was filtered. The filtrate was treated with 2 ml 2-normal (CH3CO2)2Pb. Yellow precipitate meant the presence of chromium. (Procedure 34 : Separation of Mercury from Bismus and Iron). The NaOH precipitate in Proc. 3l was dissolved in 3 ml 2-normal HCl. To the solution was added 5 ml 4-normal NH4OH and 5 ml 4-normal NH4Cl solution and heated to boiling

5 161 gently for a minute or two. It was filtered and the precipitate was washed. (Procedure 35 : Precipitation of Mercury). The filtrate from the hot NH4Cl precipitate in Proc. 34 was boiled and concentrated to about 5 ml. Then the solution was cooled at the room temperature, and filtered. (Procedure 36 : Confirmatory Test for Mercury). The precipitate in the preceding procedure was dissolved in 2 ml 2-normal HCl, and to it SnCl2 solution was added drop by drop. White precipitate or gray precipitate indicated the presence of mercury. (Procedure 37 : Confirmatory Test for Iron). The hot NH4Cl precipitate in Proc. 34 was dissolved in 5 ml normal HNO3. To one fifth of the solution were added a few drops of K4Fe(CN)6 solution. Prussian blue color meant the presence of iron. (Procedure 38 : Detection of Bismuth). To the remainder of the HNO3 solution in the above procedure was added Na2HPO4 solution drop by drop so long as the white precipitate increased. After this, it was filtered. (Procedure 39 : Confirmatory Test for Bismuth). The Na2HPO4 precipitate in Proc. 38 was dissolved in about 3 ml 2-normal HC1, and to it was added a cold freshly prepared solution of sodium stannite. Black precipitate meant the presence of bismuth. TABLE 4 Analysis of the II-A-Group Note : The solution of sodium stannite (Na2SnO2) is prepared when it is needed by adding NaOH solution, a few drops at a time, to one ml SnCl2 reagent diluted with 10 ml water. The mixture is cooled in running water after each addition until the large precipitate of Sn(OH)2 first formed is dissolved and a clear or slightly turbid liquid results. The solution must be kept cold while it is prepared, and it must be freshly prepared, because the stannite decomposes spontaneously into stannate (Na2SnOa) and metalic tin, and because it oxidizes in contact with air to sodium stannate. F. Analyses of the Alkalin-Earth Group and Manganese. (Procedure 41 : Reprecipitation of the Alkalin-Earth Group and Manganese). The filtrate from the (NH3 and NH4+) precipitate was evaporated to a volume of 10 ml and the solution was added by 3-normal (NH4)2CO3 drop by drop as long as the

6 162 TABLE 5 Analyses of the Alkaline-Earth Group and Manganese Note : Manganese ammine complex is unstable [7] and the solubility product of manganese carbonate is very minute. Thus most of the manganese are precipitated with ammonium carbonate. precipitate increased. The mixture was warmed on the water bath prior to being filtered. The precipitate was treated as in the procedure 42, and the filtrate in the procedure 51. (Procedure 42 : Extraction and Confirmatory Test of Manganese). The filter containing the (NH4)2CO3 precipitate a 10 ml portion of 2 -normal NH4OH saturated with NH4Cl was repeatedly poured. After the treatment of the residue, the extract was added by the solution of 2-normal Na2HPO4 in one ml or two. White precipitate indicated the presence of manganese. (Procedure 43 : Detection of Barium). The residue insoluble in NH4OH and NH4Cl in Proc. 42 was washed with a little of water and it was dissolved in a hot 10 ml portion of 2-normal CH3CO2H. The CH3CO2H solution was reduced by evaporation to a volume of 2 ml. To the solution was added 10 ml hot 2-normal CH3CO2H, 10 ml hot 3-normal CH3CO2NH4, and 10 ml hot water and the solution was heated to, boiling. With the addition of normal K2Cr04 solution to the solution, it was heated and shaken of ter each addition so long as the precipitate increased. Finally the mixture was heated at Ž for a or two minutes, shaking at the same time. The precipitate was filtered and washed thoroughly with cold water. Pale yellow precipitate meant the presence of barium. (Procedure 44 : Confirmatory Test of Barium). The K2CrO4 precipitate was dissolved in a 5 ml portion of 2-normal HCl. A platinum wire having a small loop at one end was heated in a gas flame till it no longer colored the flame, dipped in HCl, touched to the HCl solution, and it was introduced again into the flame. Green color

7 indicated the presence of barium. When this flame test was concluded, one ml 6-normal H2SO4 was added to the solution. White precipitate indicated the presence of barium. (Procedure 45 : Reprecipitation of Strontium and Calcium). To the filtrate from the K2CrO4 precipitate in a small beaker was added 2-normal NH4OH slowly until the filtrate became neutral, and then 5 ml more was added. After boiling, it was mixed with 2-normal (NH4)2CO3, drop by drop so long as the precipitate increased. The precipitate was filtered and washed until the wash-water would no longer give yellow color. (Procedure 46 : Detection of Strontium). The (NH4)2CO3 precipitate was dissolved in hot 5 ml 2-normal CH3CO2H, with the addition of water. To it was added 2-normal NH4OH until it became neutral, and then 2 ml more. To this basic solution was further added 5 ml 2-normal NH4Cl solution and 10 ml saturated (NH4)2SO4 solution, and the mixture was stirred, boiled gently for 3 minutes, and filtered, and the precipitate was w ashed afterward. (Procedure 47: Confirmatory Test of Strontium). The (NH4)2SO4 precipitate was dissolved in a hot 6-normal HCl. At the same time, a platinum wire having a small loop at one end was heated in a gas flame till it no longer colored the flame. After dipping it in HCl and touching to the HCl solution, it was introduced again into the flame. Red color indicated the presence of strontium. (Procedure 48: Confirmatory Test of Calcium). To the filtrate from the (NH4)2SO4 precipitate in the preceding procedure was added 3 ml 2-normal NH4OH and then heated to a boiling. Following this, it was slowly mixed with 10 ml boiling hot (NH4)2C204 solution, to be kept boiling for 5 minutes. White precipitate meant the presence of calcium. G. Analyses of Copper, Cobalt, Nickel, and Cadmium. (Procedure 51 : Detection of Copper). The filtrate from (NH4)2CO3 precipitate in the procedure 41 was acidified with 2-normal CH3CO2H and further treated with 5 ml of 2-normal NaHSO3 and 5 ml 2-normal NH4SCN. The solution was stirred and boiled for a minute or two. White precipitate meant the presence of copper. 163 TABLE 6 Analysis of Copper Group

8 164 (Procedure 52 : Confirmatory Test for Copper). The white precipitate was dissolved in 2 ml 2-normal NH4OH and the resultant NH4OH solution was acidified with 2-normal CH3CO2H, and K4Fe(CN)6 added. Red precipitate indicated the presence of copper. (Procedure 53 : Concentration of the Solution containing Co, Ni, Cd, and Mg). The filtrate from the NH4SCN precipitate was evaporated to a volume of 10 ml. And then the concentrated solution was treated as in the procedures 54 and 56. (Procedure 54 : Detection of Cobalt). To a part of the concentrated solution was added 5 ml saturated KNO2 solution and 3 ml 2-normal CH3CO2H. Yellow precipitate meant the presence of cobalt. The mixture was left standing for 10 minutes and then filtered. (Procedure 55 : Confirmatory Test of Nickel). To the filtrate from the KNO2 precipitate was added NH4OH until it became basic. Then a few drops of dimethylglyoxime alcoholic solution were further added and heated to boil. Red precipitate indicated the presence of nickel. (Procedure 56 : Removal of Manganese). To the remainder of the concentrated solution in the preceding procedure was added 6-normal NH4OH until it became basic. It was then treated with one gr. of (NH4)2S208 and heated to boiling gently for 3 minutes. The precipitate was filtered. (Procedure 57 : Confirmatory Test of Calcium). To a small part of the filtrate from the (NH4)25208 precipitate were added a few drops of diphenyl carbazide alcoholic solution. Blue purple color pointed out to the presence of cadmium. (Procedure 58 : Confirmatory Test for Manganese). To the remainder of the filtrate from the (NH4)2S208 precipitate was added 2 ml 2-normal Na2HPO4 solution. White precipitate indicated the presence of manganese. H. Separation of III-A-Group and III-B-Group, and Analysis of III-A-Group. (Procedure 61 : Precipitation of III-A-Group). The filtrate from the NaOH and TABLE 7 Precipitation and Analysis of III-A-Group

9 Na2CO3 precipitate in the procedure 21 was neutralized with 2-normal HCl, boiled and. evaporated to a volume of about 10 ml. Then the solution was cooled at room temperature and to it was further added 10 ml normal NaHCO3 solution drop dy drop, stirring the mixture after each addition. The precipitate was finally filtered. The filtrate was treated as in the procedure 71. (Procedure 62 : Removal of Lead). The NaHCO3 precipitate (P. 61) was treated with 3 ml 4-normal H2SO4. The solution was left standing for a few minutes. White precipitate pointed out to the presence of lead. The precipitate was to be filtered. (Procedure 63 : Precipitations of Antimony, Tin and Aluminium). To the filtrate from the H2SO4 precipitate (P. 62) was added 15 ml 2-normal NH4OH saturated with NH4Cl. The precipitate was filtered. (Procedure 64 : Confirmatory Test of Zinc). The filtrate from the NH4OH and NH4Cl precipitate (P. 63) was treated with K4Fe(CN)6 solution. White precipitate indicated the presence of zinc. (Procedure 65 : Extraction of Aluminium). Through the filter containing the NH4OH and NH4Cl precipitate (P. 63), a 5 ml portion of normal CH3CO2H was repeatedly poured. (Procedure 66 : Confirmatory Test of Aluminium). To a small part of the CH3CO2H solution was added normal CH3CO2Na together with aluminon reagent. Then the mixture was left standing for a few minutes. Red colloid was indicative of the presence of aluminium. (Procedure 67 : Dissolving of Antimony and Tin). The residue undissolved by CH3CO2H (P. 65) was dissolved in 5 ml 2-normal HCl. A part of the HCl solution was treated as in the procedure 68, the remainder being treated as in the procedure 69. (Procedure 68: Confirmatory Test of Antimony). Beneath the HCl solution (P. 67) a piece of tin-foil was placed and the mixture was exposed for a few minutes.. Black deposit on the tin-foil meant the presence of antimony. (Procedure 69 : Confirmatory Test of Tin). To the remainder of the HCl solution (P. 67) was added an iron nail, and heated to boiling for 5 minutes, then filtered. The filtrate was further mixed with 0.2-normal HgCl2 solution. White or gray pre cipitate pointed out to the presence of tin. I. Analysis of III-B-Group. (Procedure 71 : Confirmatory Test of Potassium). The filtrate from the NaHCO3 precipitate in the procedure 61 was concentrated through evaporation to a volume of 3-5 ml. If a precipitate formed itself, it was filtered and the precipitate was discarded. A platinum wire having a small loop at one end was heated in a gas flame till it no TABLE Analysis of III-B-Group

10 166 longer colored the flame and after dipping it in HCl and touching the concentrated solution, it was introduced again into the flame. The flame was observed through a cobalt glass and violet color was indicative of potassium. (Procedure 72 : Confirmatory Test of As+++++) The half of the concentrated solution (P. 71) was treated with 3 ml 2-normal NH4OH saturated with NH4Cl and one ml magnesia mixture. If no precipitate appeared, the wall of the test-tube was gently rubbed with a glass rod for a minute or two. White crystalline precipitate meant the presence of As (Procedure 73 : Confirmatory Test of As+++) To the remainder of the concentrated solution (P. 71) was added 2 ml 2-normal NaOH and a piece of aluminiumfoil. A piece of paper dipped in AgNO3 solution was inserted and the mixture was heated to near boiling. Yellow or black spots on the paper pointed out to the presence of As+++. Discussion As mentioned above, with the new method described no hydrogen sulfide or other sulfides or concentrated acids are used. So with this method little gas or fume injurious to health is emitted. It can be said to be a better method for reasons of sanitation than H2S method.and the processes in the method are systematic and simple, and therefore, it is easy for the average chemical student to acquire mastery of the method. Then it is fit for primary courses in analytical chemistry. As in H2S method, however, the presence of phosphate, oxalate or organic substance stands in the way of the new method. And also the coexistence of arsenic and what make precipitates with arsenic in basic solution constitute difficulties a full use of this method. Research to remove these obstructions is now under study by the author. References [ 1 ] E. P. TREADwELL and W. T. HALL : Analytical Chemistry. Vol. I. Qualitative, 9 th. English Ed., [ 2 ] A. A. NOYES and E. H. SWIFT : Qualitative Chemical Analysis, Macmillan, N. Y., [ 3 ] S. TAKAGI : Qualitative Analytical Chemistry. Vol. II, Nankodo, Tokyo, [ 4 ] M. ISHIDATE : Micro-Qualitative Analysis. Nanzando, Tokyo, [ 5 ] Shinbunsekikagaku k za, Yukishiyaku niyoru bunribunsekiho. (New lectures on analytical chemistry) Ky ritsu Shuppan, Tokyo, [ 6 ] G. CHARLOT : L'Analyse Qualitative et Les Reactions en Solution. Saint-Germain, Paris, [ 7 ] D. P. MELLOR and L. E. MALEY : Nature, 159, 370 (1947); 161, 436 (1948). [ 8 ] P. W. WEST : Comprehensive Analytical Chemistry. Vol. 1A, 332, Elsevier Co.. N. Y., 1959.