In the Laboratory edited by
Green Chemistry
Mary M. Kirchhoff ACS Education Division Washington, DC 20036
Greener Alternative to Qualitative Analysis for Cations without H2S and Other Sulfur-Containing Compounds
Indu Tucker Sidhwani* and Sushmita Chowdhury Department of Chemistry, Gargi College, University of Delhi, Delhi-110049, India; *
[email protected] Throughout the world, and in particular in developing countries such as India, there is an increasing concern about environmental pollution and a desire to secure a healthy earth for future generations. To ensure progress and sustainable development, it is essential to address the toxic and hazardous effects of chemicals on life around us. Hence, there is a marked increase in interest in green chemistry. In comparison to developed countries where there is greater focus on instrumentation, the developing countries, particularly South Asia, are still relying more on classical methods. There are about 700 universities and approximately 5000 colleges in India offering chemistry as a discipline. In addition, there are innumerable high schools where chemistry is taught. In all cases, qualitative inorganic analysis is carried out; however, the level of complexity varies. A science student, irrespective of his or her major subject, has chemistry labs for five years in which hydrogen sulfide is handled for at least three years. In high schools, there is one lab class a week for two hours duration. A maximum of eight such classes per year are allotted for qualitative analysis. In college, the students work for five hours a week for about thirty weeks doing such experiments. The average number of students doing qualitative analysis in a college is about 150. The H2S scheme given by Fresenius (1) is a universally accepted scheme. H2S has a great pedagogical importance, gives detailed understanding of solubility products, ionic and complexation equilibria, Le Châtelier’s principle, and the common-ion effect (2). It allows the study of theory to be correlated in the best way with analytical practice. It is inexpensive, can be generated easily, and has stood the test of time for over a century. Chemists have probably developed such an immunity to the odor of H2S that it is not recognized as a deadly poison. In concentration as low as 0.05 percent, by volume in air, H2S may be fatal (2) and may kill faster than HCN (3). By virtue of an excellent immune system, an individual may escape death but physiological disturbances are not uncommon. Apart from air pollution, H2S has appreciable solubility in water, enhanced if the medium is alkaline, and binds to toxic heavy metals (soft acids in the HSAB concept) (4) forming stable soluble compounds and rendering water purification difficult. H2S is generated using Kipp’s apparatus through the following reaction:
FeS(s) H2SO4(aq)
FeSO4(aq) H2S(g)
For a class of 40 students working for ~3 h almost 40 dm3 of H2S is generated using two Kipp’s apparatuses. Some H2S
escapes to the atmosphere, causing air pollution, while the rest contaminates water. We have been conducting labs at the undergraduate level for over 25 years and have noted many problems with the H2S scheme. While the scheme works effectively for analysis of a single salt, difficulties arise in mixture analysis (three cations and three anions). Preparing a mixture for analysis is a tedious task and care should be taken in choosing various combinations of salts to ensure smooth analysis. For example, excess of nitrate and Fe3+ salts are to be avoided. These salts oxidize H2S to S and sometimes SO42–, thereby rendering detection of group II difficult because colloidal sulfur obscures group II precipitates and Ba2+, Sr2+, Ca2+ are pre-precipitated in group II as sparingly soluble sulfates. People are aware of the difficulties associated with the H2S scheme (5–9). A shift from macro to semi-micro analysis reduces consumption of H2S. In situ generation of H2S using thioacetamide, potassium trithiocarbonate (5), ammonium sulfide, and sodium sulfide (7) have been reported but all suffer from inherent disadvantages. Adjustment of pH is crucial, water-pollution is not eliminated, and starting materials for these reagents are not green. In many schools, students use water saturated with H2S, but air and water-pollution are not eliminated; in fact, more H2S is generated to saturate water than what is needed otherwise. Experimental Development To eliminate the use of toxic H2S and to increase efficiency and ensure certainty of analytical results, a green scheme was developed. The analysis of arsenic and mercury compounds is not done because they are highly toxic, but they can be successfully analyzed by this scheme. The group reagents are hydrochloric acid, sodium sulfate, sodium hydroxide, and ammonia solution. Eco-friendly spot tests (10) have increased the speed of analysis although green conventional tests (6) have also been undertaken. A survey of literature shows few non-sulfide schemes that are foolproof (7). In one case sodium acetate is used as a group reagent (9); however, formation of basic acetate is doubtful and the author mentions extremely rigorous control of pH is needed to avoid precipitation of other hydroxides, acetates, and basic acetates. Most references pertain to detection of few cations (11, 12) or separation of specific metals such as copper and cadmium (13). In one case the flame test is used for identification of few metals (14).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
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In the Laboratory
The present scheme ensures separation on the basis of common chemical properties and requires fewer steps as the number of groups is reduced from six to four (Table 1). In this method, heating is needed in only a few steps. Another advantage is the early detection of K+ and Mg2+. In the H2S scheme, these are detected in group VI (6). The solution becomes very dilute and these cations are difficult to detect and frequently missed.
Table 1. Classification of the Cations in the Present Scheme Group
Proposed Greener Scheme for Analysis of Cations The present scheme of analysis has been carried out in a mixture in which interfering anions such as fluoride, oxalate, phosphate, and borate are not present. We are in the process of modifying and rendering this scheme applicable in the presence of interfering anions. (As3+ and Hg2+ are not included due to high level of toxicity.)
Cation
Precipitated as
Zero
NH4+, K+
Directly detected in water extract
I
Pb2+, Ag+
II
Ba2+,
Sr2+,
Chlorides Ca2+,
Pb2+
IIIA
Fe3+, Mn2+, Mg2+
IIIB
Cu2+,
IV
Cr3+, Al3+, Zn2+, Sn2+
Cd2+,
Ni2+,
Sulfates Hydroxides
Co2+
Soluble ammine complexes Present as soluble hydroxo complexes; (Cr3+ as CrO42¯ )
Note: The lead ion may occur in group II if it is not completely precipitated in group I.
Hazards There are very few hazards associated with this experiment. The corrosive H2SO4 used earlier (8) has been replaced by Na2SO4. Concentrated HNO3 (6) used to oxidize Fe2+ to Fe3+ has been replaced by 6% H2O2, which is less corrosive. Heating is considerably reduced thereby lowering cases of liquid spurting. Owing to its toxic nature, a very dilute solution of Nessler’s reagent is used. The concentration may further be reduced by using it as a spot reagent. A list of the reagents and CAS numbers are available in the online material. Results and Discussion In the presence of 2 M HCl, Pb2+ and Ag+ are precipitated as the sparingly soluble chlorides (Ksp values: PbCl2, 1.6 × 10–5; AgCl, 1.78 × 10–10). The analysis of mercury is eliminated due to its extreme toxicity, however if Hg22+ is present it will also be precipitated as chloride. The Ksp of PbCl2 is higher than that of AgCl and some Pb2+ may be left behind (as in H2S scheme). This will be precipitated in group II as PbSO4(s). Ba2+, Sr2+, Ca2+, and Pb2+ are precipitated as sulfates. The solubility of CaSO4 is higher than the others (Ksp, values: CaSO4, 1.4 × 10–5; BaSO4, 1.1 × 10–10; SrSO4, 2.8 × 10–7; PbSO4, 1.6 × 10–8) and alcohol is added to reduce the solubility of CaSO4 and ensure complete precipitation. The precipitating agent is sodium sulfate. Solid Na2SO4 can be added but the solution should be cooled to avoid volatilization of alcohol. An added advantage over the other scheme is the use of sodium rhodizonate, a single reagent, for detection of Ba2+, Sr2+, and Ca2+. However the reagent should be freshly prepared. Group III has been subdivided into groups IIIA and IIIB. The commonly available and inexpensive NaOH is used as a precipitating agent. Advantage is taken of the fact that hydroxides of Al3+, Zn2+, and Sn2+ are amphoteric and dissolve as hydroxo complexes. Here the student is familiarized with the concept of complexation and the effect of complexation on solubility. The conventional oxidizing agents, conc HNO3 and bromine water, to oxidize Fe2+ to Fe3+ (used in other scheme) have been replaced by the greener reagent, 6% hydrogen peroxide, a single reagent that oxidizes Fe2+and Cr3+. A major advantage is early detection of Mg2+ in group III, instead of group VI as in the H2S scheme.
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The cations of group IIIB are present as ammine complexes and are detected in the presence of each other by use of masking agents. Tedious separations are eliminated, thereby making the analysis simple and quick. Cd2+ is easily detected, the test is highly specific and rigid pH control and dilution are not needed. The cations of group IV are detected in a different portion of the same solution because the tests are highly selective. Selectivity in some cases is enhanced by use of masking agents. The proposed scheme has the following advantages:
• Green analysis at the student level.
• Simple with fewer steps because the number of groups is reduced thus saving on heat energy and ensuring greater speed of analysis.
• Students learn about solubility product, various equilibria (solubility, complexation, reversible, etc.), and the concept of amphoterism.
• The separation is sharp and effective. The tests are highly sensitive and selective, so various cations can be detected in different portions of the same solution. The students are also taught spot tests (first attempt in undergraduate and high school levels).
Conclusion The present scheme was tried along with the H2S scheme on selected undergraduate students and a thorough study was made by three post-graduate students. The results were comparable. The students were more comfortable with the present scheme and found it easy and quick. Acknowledgment We gratefully acknowledge the approval and funding of this project by the University Grants Commission. We also thank the undergraduate and post-graduate students who tried this scheme of analysis.
Journal of Chemical Education • Vol. 85 No. 8 August 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Literature Cited 1. Fresenius, H. Z. Analyt. Chem. 1897, 30, 10. 2. Brockman, C. J. J. Chem. Educ. 1939, 16, 133. 3. Manahan, S. E. Environmental Chemistry, 7th ed.; Lewis Publishers: Boca Raton, FL, 1999; p 728. 4. Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 3rd ed.; Pearson Education: New Delhi, 2005. 5. Johri, K. N. Macro and Semimicro Analysis without H2 S Using Potassium Trithiocarbonate (PTC) Reagent, 2nd ed.; Asia Publishing House: Bombay, 1971. 6. Svehla, G. Qualitative Inorganic Analysis, 7th ed.; Pearson Education: New Delhi, 2006. 7. Weisz, H. J. J. Chem. Educ. 1956, 33, 334. 8. Prokopov, T. S. Mikrochimica Acta (Wien) 1970, 697–701. 9. Rathnamma, D. V. J. Chem. Educ. 1980, 57, 287. 10. Feigl, F.; Anger, V. Spot Tests in Inorganic Analysis, 6th ed.; Elsevier: New York, 2006.
11. 12. 13. 14.
Kilner, C. J. Chem. Educ. 1985, 62, 80. Grenda, S. C. J. Chem. Educ. 1986, 63, 720. Hayden, D. W.; Hunt, R. L. J. Chem. Educ. 1982, 59, 59. Blitz, J. P.; Sheeran, D. J.; Becker, T. L. J Chem. Educ. 2006, 83, 277.
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