ESGilreath (1954) Cation Group 1 PDF

## Experimental Procedures ### Transferring of Precipitates Occasionally it is necessary to transfer a precipitate from a test tube to another receptacle. This is often a difficult operation, especially if the precipitate is gelatinous in nature. Customarily such a transfer is accomplished with th...

## Experimental Procedures ### Transferring of Precipitates Occasionally it is necessary to transfer a precipitate from a test tube to another receptacle. This is often a difficult operation, especially if the precipitate is gelatinous in nature. Customarily such a transfer is accomplished with the use of a nickel microspatula. However, this instrument cannot be used on precipitate which is wet with a solution that reacts with nickel. ### Directions for laboratory work The total time and number of operations allotted to laboratory work in qualitative analysis vary widely among different colleges and instructors. Complete directions for the analyses of four known solutions and ten unknown samples are given in the Appendix (Secs. A-1 to A-9) under the heading Special Laboratory Directions. A suggested schedule of laboratory work is also given in Sec. A-11. ### The Known A student should be familiar with the procedures of an analytical scheme and be able to recognize the identifying tests for the ions within the scheme before he tackles the analyses of unknown mixtures. This practice is obtained by analyzing solutions containing known mixtures of ions. Known solutions are prepared from test solutions according to the directions given in Secs. A-1 to A.4. ### The Unknown Samples for analysis are issued to the student as solutions and solids. Specific directions for beginning the analysis of unknowns are given in Secs. A-1 to A-9. The analyses of these unknowns increase in complexity from No. 1 through No. 9. In addition to these special directions, Chap. 22 gives a detailed explanation of the generally accepted methods for the examination and complete analysis of an unknown sample. A record of the analysis of the unknown should be kept, using the same form as was used in the analysis of a known. ### General Laboratory Directions #### Introduction to the Systematic Analysis of the Cations The 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. An analysis of a mixture containing all the known cations would be quite an involved process and certainly beyond the scope of an elementary course in qualitative analysis. The Fresenius scheme and those schemes modified from it attempt to identify only those cations of common occurrence. These cations, some 23 in number, are systematically separated into groups by the use of group reagents which precipitate chemically related ions. Thus the cations are divided into five major groups, four of which are precipitated by group reagents, whereas the fifth constitutes a soluble group. After group separation has been effected, it is necessary to separate, whenever feasible, the individual ions within a group for purposes of identification. In the Fresenius scheme, two groups of cations are precipitated with hydrogen sulfide, one in an acid solution and the other in an ammoniacal solution. The greatest objection to the use of hydrogen sulfide is the poisonous nature of the gas when improperly used. The production and use of hydrogen sulfide, by any method, also limit the rapidity of the analysis. In spite of this undesirable feature of the Fresenius separation of the cations, the proponents of nonhydrogen sulfide schemes have not produced a satisfactory substitute during a period of more than 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. Although the Fresenius scheme of analysis for the cations has survived for more than a hundred years, its usefulness in technical analysis has been fairly well outmoded. In the chemical industry, far more accurate and rapid methods are utilized for cation analyses. The value of this scheme of qualitative analysis is its pedagogical success in imparting to the student a considerable knowledge of fundamental and descriptive inorganic chemistry. Students are intrigued by the analyses of unknowns, and therefore willingly acquire skills and knowledge in matching wits against unknown mixtures. No procedure of qualitative analysis is without faults. All texts vary to some extent in the separation of cations for identification. The scheme presented in this text is the result of constant revisions over years of laboratory instruction. Two objectives have ever been in mind: to present a scheme in which the student will have confidence in analyzing his unknowns, and to present useful and meaningful chemistry from a teaching standpoint. The use of organic reagents has been avoided as much as possible since such use adds very little practical chemistry to the sum of knowledge that may be assimilated by a student in elementary qualitative analysis. ### Systematic Separation of Cations into Groups 1. Test for NH4+ on a portion of the original solution. Add 3 F HCl and centrifuge. - **Residue contains chlorides of Group I:** PbCl2, AgCl, and Hg2Cl2. - **Centrifugate contains cations of Groups II-V.** Add HNO3 to oxidize stannous ions, adjust acidity, and saturate with H2S. Centrifuge. - **Residue contains sulfides of Group II.** Treat with 3 F KOH. Centrifuge. - **Undissolved residue contains sulfides of Group IIA, which are:** HgS, PbS, Bi2S3, CuS, and CdS. - **Centrifugate contains cations of Group IIB in the form of soluble anions:** AsO3- and AsS2-, Sb(OH)4- and SbS2-, Sn(OH)62- and SnS32-. - **Centrifugate contains cations of Groups III-V.** Add saturated NH4Cl solution, 3 F NH3, and H2S. Centrifuge. - **Residue contains sulfides and hydroxides of Group III.** Dissolve in aqua regia, and treat the solution with Na2O2 and 3 F KOH. Centrifuge. - **Centrifugate contains cations of Group IIIΑ as soluble anions:** CrO42-, Al(OH)4- and Zn(OH)4-. - **Residue contains hydroxides of Group IIIB:** MnO(OH)2, Fe(OH)3, Co(OH)3, and Ni(OH)3. - **Centrifugate contains cations of Groups IV and V.** Add 0.5 F (NH4)2HPO4 and concentrated NH3. Centrifuge. - **Residue contains phosphates of:** Ba3(PO4)2, Sr3(PO4)2, Ca3(PO4)2, and MgNH4PO4. - **Centrifugate contains soluble cations, Na+ and K+, which did not precipitate in the first four groups.** ### Chapter 12, Group I Cations The systematic analysis of the common cations is based upon the successive precipitation of groups of ions, so that the total number of ions can be broken down into a small number of groups, each containing a number of related cations. The first of these separations is the precipitation of Group I, which is composed of those common cations whose chlorides are relatively insoluble in dilute acids. #### Theoretical Discussion The successful separation of a group of cations is determined by the relative solubility products of the compounds formed by the cations with the precipitating anion. The insoluble chlorides of Group I are lead chloride, mercurous chloride, and silver chloride. The solubility products of these compounds are 1 × 10-4 for PbCl2, 2 × 10-18 for Hg2Cl2, and 1.56 × 10-10 for AgCl. To the beginning student, the term solubility product may not be as significant as that of solubility. From the above figures the solubility of lead chloride is calculated to be approximately 0.04 F, that of mercurous chloride to be 7.5 × 10-7 F, and that of silver chloride as 1.3 × 10-5 F. A rough comparison of these solubilities indicates that lead chloride is one thousand times more soluble than silver chloride, and one hundred thousand times more soluble than mercurous chloride. Since the precipitating anion is in excess, the above solubilities are further decreased by common-ion effect. The values for the solubilities of Group I chlorides indicate that silver chloride and mercurous chloride are almost completely precipitated, whereas lead chloride is always incompletely precipitated and in low concentrations may not be precipitated at all. After the group separation is made, the ions within the group must be separated to the extent necessary for individual identification. Again, in the case of PbCl2 there is a poor separation. Lead chloride is partially separated from mercurous chloride and silver chloride because of an increase in solubility of lead chloride in hot water. The solubility of PbCl2 is 0.673 g/100 ml of water at 0°C and 3.34 g/100 ml at 100°C. In other words, the solubility of lead chloride increases approximately five times in going through this temperature range. Therefore, some PbCl2 may be leached from the group precipitate by treating it with hot water, provided that the water stays hot, but all the PbCl2 is seldom removed. A portion of the undissolved PbCl2 may remain as a white residue and cause some confusion in making the identification tests for silver and mercurous ions. Silver chloride is separated from mercurous chloride by its solubility in ammonia, in which the soluble complex ion, Ag(NH3)2+, is formed. The efficiency of this separation depends upon the concentration of the ammonia and the amount of AgCl. For most analytical concentrations this separation is clean, but the solubility of AgCl in ammonia water to form the soluble complex ion has a limiting value, and if the amount of AgCl is very large some of it may not be dissolved. Ammonia water not only reacts with AgCl, but it also serves as a medium for the auto-redox action of Hg2Cl2 to produce mercury and mercuric aminochloride, both of which are insoluble. Metallic mercury, in a finely divided state, is black in color; therefore, a blackening at this point proves the presence of Hg2Cl2. #### Analysis of Group I Cations The analysis of this group is relatively simple. Three principal steps make up the procedures: 1. **Precipitation of Group I Cations.** Place 10 drops of the solution to be analyzed (1) in a 10-ml test tube and add 4 drops of 3 F HCl. (2) Mix thoroughly and centrifuge. Test for completeness of precipitation by adding another drop of 3 F HCl to the supernatant liquid. Centrifuge and remove centrifugate with a dropping tube. This centrifuge is saved for analysis of Groups II-V. Precipitate remaining in the test tube is washed with 10 drops of cold water containing 1 drop of 3 F HCl. (3) Discard wash water. 2. **Separation of Lead Chloride.** White precipitate obtained in Procedure 1 may be PbCl2, AgCl, and Hg2Cl2. Add 6-7 drops of water and heat, with stirring, for 3 min in water bath. Centrifuge quickly (4) and immediately remove centrifugate, while keeping mixture hot in a steam bath. - **Centrifugate may contain Pb++.** Add 4 drops of 1 F KaCrO4. Yellow precipitate confirms presence of LEAD ION. - **Residue may contain AgCl and Hg2Cl2.** Treat with 10 drops of 3 ammonia, stir thoroughly, and centrifuge. (5) A blackening of the residue indicates the presence of the mercurous ion. - **Centrifugate may contain Ag(NH3)2+.** Acidify centrifugate with 3 F HNO. (6) Formation of a white precipitate confirms presence of SILVER ION. - **Residue may contain mercury.** (7) Wash with 10 drops of water and discard washings. Dissolve precipitate in 2 drops of concentrated HNO3. Dilute with 5 drops of water. (If solution is not clear, centrifuge and retain centrifugate.) Add 1-2 drops of SnCl2 solution. White or gray precipitate confirms presence of MERCUROUS ION. 3. **Treatment of Residue in Group I Analysis with Ammonia.** When a mixture of AgCl and Hg2Cl2 is treated with ammonia, the AgCl dissolves, leaving a black residue composed of mercury and mercurio aminochloride. - AgCl + 2NH3 = Ag(NH3)2+ + Cl- Ammonia acts upon mercurous chloride to produce an internal redox reaction in which one mercurous ion is reduced to mercury and the other is oxidized to the mercuric state. - Hg2Cl2 + 2NH3 = HgNH2Cl + Hg + NH4+ + Cl- Mercuric aminochloride is white and the finely divided mercury is black; the resulting mixture is black. This blackening is an identification test for the mercurous ion. #### Confirmation of Presence of Silver Ion The centrifugate from the treatment of the Group I residue with ammonia contains Ag(NH3)2+ and Cl- ions. If this solution is made acid with nitric acid, the complex is destroyed and AgCl is reprecipitated. - Ag(NH3)2+ + Cl- + 2H3O+ = AgCl + 2NH4+ + 2H2O #### Identification of Mercurous Ion The residue from the treatment of mercurous chloride with ammonia is a mixture of Hg and HgNH2Cl. Although the production of this black mixture is sufficient to indicate the presence of the mercurous ion, additional confirmation is obtained by dissolving the mixture in nitric acid and testing with stannous chloride solution. Nitric acid dissolves Hg and HgNH2Cl with the following reactions: - 3Hg + 2NO3- + 8H3O+ = 3Hg++ + 2NO +12H2O - 2HgNH2Cl + 2NO3- + 4H3O+ = 2Hg++ + N2 + 2NO + 2Cl- + 6H2O In the presence of chloride ions, the mercuric ions will tend to form the slightly dissociated mercuric chloride molecule, HgCl2, or the complex ion, HgCl42-. - Hg++ + 2Cl- = HgCl2 - HgCl2 + 2Cl- = HgCl42- (low concentration of Cl- ions) - HgCl2 + 2Cl- = HgCl42- (high concentration of Cl- ions) Acid solutions containing HgCl2 and HgCl42- give precipitates with stannous ions. These precipitates may be white, gray, or black, depending upon the relative concentrations of the reactants. - 2HgCl2 + SnCl4 = Hg2Cl2 + SnCl62- - 2HgCl42- + SnCl4 = Hg2Cl2 + SnCl62- + 4Cl- Further addition of stannous chloride reduces white Hg2Cl2 to black, finely divided mercury. - Hg2Cl2 + SnCl4 = 2Hg + SnCl62- Usually a gray mixture of Hg2Cl2 and Hg is obtained. **Pertinent Chemical Reactions Involved in the Separation and Identification of Cations of Group I** The purpose of this section is to give some explanation of the procedures followed in the analysis of Group I cations. As far as possible these explanations and equations for chemical reactions follow the same sequence as the successive steps in the analytical procedures. **Group Precipitation** Lead, silver, and mercurous ions give white precipitates with the chloride ion in an acid solution. - Pb++ + 2Cl- = PbCl2 - Ag+ + Cl- = AgCl - Hg2++ + 2Cl- = Hg2Cl2 **Separation and Identification of Lead Ion** An incomplete separation of PbCl2 from the group precipitate is effected with hot water. PbCl2 is soluble to the extent of approximately 1 g/100 ml at room temperature, and 3.34 g/100 ml at the temperature of boiling water. The threefold increase in solubility is small, but sufficient to give a lead-ion concentration that can be detected with chromate ions. PbCrO4 is much less soluble than PbCl2 (approximately 2 × 10-5 g/100 ml). - Pb++ + CrO42- = PbCrO4 **Treatment of Residue in Group I Analysis with Ammonia** When a mixture of AgCl and Hg2Cl2 is treated with ammonia, the AgCl dissolves, leaving a black residue composed of mercury and mercurio aminochloride. - AgCl + 2NH3 = Ag(NH3)2+ + Cl- Ammonia acts upon mercurous chloride to produce an internal redox reaction in which one mercurous ion is reduced to mercury and the other is oxidized to the mercuric state. - Hg2Cl2 + 2NH3 = HgNH2Cl + Hg + NH4+ + Cl- Mercuric aminochloride is white and the finely divided mercury is black; the resulting mixture is black. This blackening is an identification test for the mercurous ion. **Confirmation of Presence of Silver Ion** The centrifugate from the treatment of the Group I residue with ammonia contains Ag(NH3)2+ and Cl- ions. If this solution is made acid with nitric acid, the complex is destroyed and AgCl is reprecipitated. - Ag(NH3)2+ + Cl- + 2H3O+ = AgCl + 2NH4+ + 2H2O **Identification of Mercurous Ion** The residue from the treatment of mercurous chloride with ammonia is a mixture of Hg and HgNH2Cl. Although the production of this black mixture is sufficient to indicate the presence of the mercurous ion, additional confirmation is obtained by dissolving the mixture in nitric acid and testing with stannous chloride solution. Nitric acid dissolves Hg and HgNH2Cl with the following reactions: - 3Hg + 2NO3- + 8H3O+ = 3Hg++ + 2NO + 12H2O - 2HgNH2Cl + 2NO3- + 4H3O+ = 2Hg++ + N2 + 2NO + 2Cl- + 6H2O In the presence of chloride ions, the mercuric ions will tend to form the slightly dissociated mercuric chloride molecule, HgCl2, or the complex ion, HgCl42-. - Hg++ + 2Cl- = HgCl2 - HgCl2 + 2Cl- = HgCl42- (low concentration of Cl- ions) - HgCl2 + 2Cl- = HgCl42- (high concentration of Cl- ions) Acid solutions containing HgCl2 and HgCl42- give precipitates with stannous ions. These precipitates may be white, gray, or black, depending upon the relative concentrations of the reactants. - 2HgCl2 + SnCl4 = Hg2Cl2 + SnCl62- - 2HgCl42- + SnCl4 = Hg2Cl2 + SnCl62- + 4Cl- Further addition of stannous chloride reduces white Hg2Cl2 to black, finely divided mercury. - Hg2Cl2 + SnCl4 = 2Hg + SnCl62- Usually a gray mixture of Hg2Cl2 and Hg is obtained.