A New Method of the Qualitative Chemical Analysis of Common Cations

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 sul- fides is very hard. Many attempts to remove the faults of H2S method have been * 松岡敬一郎: Department of Chemistry, Nihon Univ. School of Dentistry (Chief : Prof. Ichiji Kondo) 157 158 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 159 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. TABLE 2 Analysis 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 containing- the 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 160 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 10-15 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 10-15 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.

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