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Use of precision colorimetry for elemental microanalysis
Tahta,
Vol. 25, pp. 183-184.
PergamonPress,1978.
Fmntcd
USE OF PRECISION
m Great
Brain.
COLORIMETRY MICROANALYSIS*
FOR ELEMENTAL
E. DEBAL and R. LEVY Service Central de Microanalyse du CNRS, 2 rue Henry Dunant, 94320 Thiais, France (Received
20 June 1977. Accepted
25 August 1977)
Summary-Spectrophotometric techniques are useful for microdeterminations of many elements in organic compounds and some inorganic compounds. It is possible to determine most metals and some non-metals (Al, Co, F, Fe, MO, P, Pd, Pt, Si, Sn, Ti...) by applying spectrophotometric techniques described in the literature (most often for trace analysis) and using commercial reagents.
The increasing variety and number of elements found in organic compounds is leading microanalysts towards choosing techniques which they can work out rapidly, which require a single type of laboratory equipment for the determination of several elements, and make it possible to change rapidly from one element to another in the analysis. Precision calorimetry meets such requirements. It is possible to determine most metals and some nonmetals by applying spectrophotometric techniques described in the literature (most often for trace analysis) and using commercial classical reagents. ADAPTINGPRECISIONCOLORIMETRY TO ELEMENTAL ANALYSIS For application of these techniques in elemental microanalysis, it is necessary that the accuracy, precision and other characteristics should be similar to those for other microanalytical techniques for determination of common elements in organic compounds, viz. the absolute error should be lower than 0.2-0.3x, interfering elements can be masked or corrected for, and the duration of an analysis should be not too long. In order to achieve the desired accuracy and precision it is not only necessary to use appropriate balances for weighing samples and a precision spectrophotometer reading absorbances to 0.001, but also to conduct the analysis in a strictly reproducible manner. Thus “once for all” calibration curves should not be used. Instead, the standard solutions and sample solutions should be prepared by the same procedure at the same time, with identical quantities of identical reagents at the same temperature, the only difference being the presence of known volumes of standard solution in the former and of the solutions of the microanalytical samples in the latter. The solutions should be left standing for the same period and undergo the same possible temperature changes before their absorbances are measured; air-conditioning, though advisable, is not absolutely necessary, but it should be pointed out that some calorimetric techniques, for instance tin or molybdenum determination, give erratic results when the ambient temperature reaches the 28-30” level. The weight W mg of element contained in the unknown solution of absorbance A, is calculated from the average absorbance a of standard solutions containing w mg of this element : W = wA/Si
There is a limiting value for the difference between w and W which should not be exceeded and which depends on * Presented at the International Symposium on Microchemical Techniques, Davos, May 1977.
the element being determined and on any divergence from the Lambert-Beer law found by testing the technique. The preliminary destruction of an organic sample, generally achieved by the wet process (original or modified micro Lorenz technique) or by combustion in oxygen or with sodium peroxide, introduces reagents which may change the light-absorption properties of the final solution. A study must be made to find whether some reagents have to be rejected or one spectrophotometric technique changed for another. However, it is often the case that though the effect of some reagents is not negligible it is reproducible when a definite and precise procedure is used; it is then convenient to prepare blank solutions in the same way as the sample solutions and to apply the whole procedure to them. This is not necessary when the blank effect is proved to be negligible, but this must be established by a careful study because the blank effect is not always manifested by an absorbance change which can be seen immediately. For instance, in determination of iron with tiron after a mineralization involving hydrogen peroxide, the colour of the complex is less stable than it is when the complex is prepared from a pure iron solution. It is therefore necessary to subject the blank to the decomposition procedure, and to make the measurements before the colour begins to, fade. A similar effect occurs in determination of molybdenum and the analytical results are improved if the entire procedure is applied to the blank. Most of the spectrophotometric techniques described in the literature have been developed for trace analysis or for determination of low contents of elements in rocks, alloys, waters, etc. Appropriate choice of capacities of the standard flasks utilized and the use of appropriate aliquots make it possible to adapt many of these techniques for the determination of major or minor elemental components of both organic and inorganic samples. The precision of the analytical results is not lowered by use of aliquots as much as might be expected, provided two or three replicates from the same solution are analysed and the mean is taken as the correct value. As far as interfering elements are concerned the data found in the literature are generally of little or no use for application to microanalytical work. This is because organic compounds will contain relatively few elements, the natore of which will nearly always be known, and which are generally present in small atomic ratios, in contrast to the case with alloys, rocks or trace analysis. Moreover, the elements which interfere with a calorimetric determination usually belong to the same family as the one being determined, a situation seldom encountered in our case. For these reasons it is possible to use commercial reagents for elemental calorimetric determinations even when other elements interfere to some degree. It is then
183
possible to correct the calorimetric results for the effect of interfering elements as a function of their concentration (as in phosphorus determination’ in the presence of palladium, platinum or rhodium) or slightly to alter the procedure for the better (as in phosphorus determination’ in presence of silicon or arsenic) or to mask interfering elements by means of complexation etc. In the dete~ination of fluoride there are many interfering elements and it is necessary to eliminate them by distilling hexafluorosilicic acid by the Willard and Winter or other suitable method. This is of general use but very time-consuming; that is why for fluorine contents lower than 20% and in the presence of sulphur or phosphorus the direct alizarin complexone technique should be used instead of one of the bleaching methods such as decolouration of the ferric sulphosalicylate complex. In comparison with the automatic analyser techmques for carbon, hydrogen, nitrogen and oxygen microdeterminations, the calorimetric techniques together with the necessary sample preparation at first sight appear to be much more time-consuming. However, they have compensating advantages, such as the possibility of determining various elements one after another without the need for separate specialized pieces of apparatus, most of which will not be in operation the whole time. In addition, there is no time-wasting conditioning of apparatus after a shutdown, and assistants already trained in one determination can quickly adapt to others. Furthermore, the use of flowcells makes it possible to avoid the tedious handling of ordinary cells, but only if the reagents used do not dirty the walls of the cells and tubing, because flow-cells are much more difficult to clean; for instance they should not be used for calorimetric determination of tin with phenylfluorone as complexing agent. All things considered, the average time taken for an elemental determination by a calorimetric technique is very reasonable, and even more so if a large batch is dealt with at the same time. Calorimetric techniques are used in our laboratory: (a) together with prior combustion in an oxygen flask for fluorine determination; (b) together with a wet decomposition process for determination of aluminium, cobalt, iron, molybdenum, phosphorus, palladium, platinum, tin and titanium; (c) together with previous d~mposition with sodium peroxide in a W&s&mitt bomb for silicon determination. Techniques are also being developed for the microdetermination of elements such as boron, rhenium, tellurium, vanadium and zirconium. Calorimetric methods have not been developed for metals such as nickel or copper, for which other techniques (atomic absorption, pol~o~aphy, etc.) are already in use.
METHODS
IN
USE
The calorimetric determinations used in our laboratory are briefly outlined below. Aluminium is determined at 490 nm by means of its redorange complex with Alizarin Red S and calcium in buffered solution (pH 4.4-4.7).‘-’ Cobalt is determined at 34Omn by means of its greenyellow complex with tiron in solution buffered at pH 9.6.‘~” Tin is determined at 52Onm by means of its orange complex with phenylfluorone in acidic aqueous metbanolic medium in the presence of glycine and gelatine; under our conditions the presence of sulpbate is necessary for obtaining correct results.z~6*7 Iron is determined at 480nm by means of its red complex with tiron in buffered medium (pH 9.5); some interferences may be avoided by working at pH 35-4.5, though the sensitivity is then lower.2*s*9
Depending on the presence of interfering elements and its content in the sample, JIuoride may be determined in two ways: (a) in buffered medium (pH 2.8-2.9) by measurement of the decrease in absorbance of the ferric sulphosalycilate complex’at 510 nm;‘*-i4 (b) by means of its purplish-blue complex with alizarin complexone and Ce in aqueous acetone medium (buffered at pH 4.6), at 625 nm.i5*i6 Molybdenum is determined at 475 nm by means of the red complex obtained by reduction with potassium iodide in the presence of thiocyanate in 3 M hydrochloric acid medium.’ Pull~i~~ is determined at 408 nm by means of the brownish pink complex [PdIe]*- obtained by the reaction of potassium iodide with Pd * ’ in 0 .25 M hydrochloric acid medium. 1 Phosphorus is determined at 410nm by means of the yellow phosphovanadomolybdate obtained in 0.26 it4 sulphuric acid medium.‘~” Plafiff~ is determined at 406 nm by means of the yellow [PtCl.$obtained by reduction of @tCl,]‘- by stannous chloride in 1.8 M hvdrochloric acid medium.’ Silicon is determined at 400 nm by means of yellow silicomolybdic acid; this determination is more time-consuming and more difficult than the others because the sample is decomposed with sodium peroxide in a Wilrzschmitt bomb and the large bulk of hydrogen peroxide generated in the process solution has to be destroyed so that it cannot interfere in the calorimetric measurements.” Titanium is determined at 410 nm by means of its yellow complex formed with tiron in the presence of oxalic acid (in order to avoid the hydrolysis of Ti4+ salts)in buffered solution (pH 4.7)’ B, Re, Rh, Te, V and Zr are also determined,” but these techniques are stilf under development in order to solve problems related to extension of their field of application and the precision of the analytical results.
REFERENCE5
1. E. Debal, R. Chassin and S. Peynot, ~~~~~~~ 1976, 23, 35. 2. E. Debal, R. Chassin, S. Peynot and 0. Poliakoff, ibid., 1977. 24. 491. 3. C. A. Parker and A. P. Goddard, Anal. Chim. Acta, 1950, 4, 517. 4. J. A. Corbett and B. D. Guerin, Analyst, 1966, 91, 490. 5. C. K. Bhaskare and S. K. Deshmukh, Z. Anal. Chem., 1975, 277, 127. 6. A. M. Leblond and R. Bouhn, Chim. Anaf. (Pa&), 1968, 50, 171. 7. A. M. Dymov, I. G. Ivanov and T. I. Romantseva, Zh. Analit. Khim.. 1971, 26. 2360. 8. J. H. Yoe and A. ‘L. Jones, 2nd. Eng. C&m., Anal. Ed., 1944, 16, 111. 9. G. V. Potter and C. E. Armstrong, Anal. Chem., 1948, 20, 1208. 10. R. Levy and E. Debd, Mikrochim. Acta, 1962, 224. 11. E. Debal, Chim. Anal. (Paris), 1963, 45, 66. 12. E. Debal, S. Peynot and S. Raveau, Bull. Sot. Chim. France, 1969, 2919. 13. E. Debal, G. Nabias and S. Peynot, ibid., 1971, 3373. 14. M. Poirier, ?‘aluntu,1975, 22, 607. 15. G. Cbarlot, Chimie Analytique Quantitative, Vol. I, betides Ch~miques et Physicoch~iq~s, 6th Ed., p. 421. Masson, Paris, 1974. 16. E. Debal, G. Madelmont, S. Peynot and 0. Poliakoff, unpublished. 17. E. Debal and M. Riocreux, Z’alanta,1972, 19, 15. 18. E. Debal, unpublished work.
Vol. 25, pp. 183-184.
PergamonPress,1978.
Fmntcd
USE OF PRECISION
m Great
Brain.
COLORIMETRY MICROANALYSIS*
FOR ELEMENTAL
E. DEBAL and R. LEVY Service Central de Microanalyse du CNRS, 2 rue Henry Dunant, 94320 Thiais, France (Received
20 June 1977. Accepted
25 August 1977)
Summary-Spectrophotometric techniques are useful for microdeterminations of many elements in organic compounds and some inorganic compounds. It is possible to determine most metals and some non-metals (Al, Co, F, Fe, MO, P, Pd, Pt, Si, Sn, Ti...) by applying spectrophotometric techniques described in the literature (most often for trace analysis) and using commercial reagents.
The increasing variety and number of elements found in organic compounds is leading microanalysts towards choosing techniques which they can work out rapidly, which require a single type of laboratory equipment for the determination of several elements, and make it possible to change rapidly from one element to another in the analysis. Precision calorimetry meets such requirements. It is possible to determine most metals and some nonmetals by applying spectrophotometric techniques described in the literature (most often for trace analysis) and using commercial classical reagents. ADAPTINGPRECISIONCOLORIMETRY TO ELEMENTAL ANALYSIS For application of these techniques in elemental microanalysis, it is necessary that the accuracy, precision and other characteristics should be similar to those for other microanalytical techniques for determination of common elements in organic compounds, viz. the absolute error should be lower than 0.2-0.3x, interfering elements can be masked or corrected for, and the duration of an analysis should be not too long. In order to achieve the desired accuracy and precision it is not only necessary to use appropriate balances for weighing samples and a precision spectrophotometer reading absorbances to 0.001, but also to conduct the analysis in a strictly reproducible manner. Thus “once for all” calibration curves should not be used. Instead, the standard solutions and sample solutions should be prepared by the same procedure at the same time, with identical quantities of identical reagents at the same temperature, the only difference being the presence of known volumes of standard solution in the former and of the solutions of the microanalytical samples in the latter. The solutions should be left standing for the same period and undergo the same possible temperature changes before their absorbances are measured; air-conditioning, though advisable, is not absolutely necessary, but it should be pointed out that some calorimetric techniques, for instance tin or molybdenum determination, give erratic results when the ambient temperature reaches the 28-30” level. The weight W mg of element contained in the unknown solution of absorbance A, is calculated from the average absorbance a of standard solutions containing w mg of this element : W = wA/Si
There is a limiting value for the difference between w and W which should not be exceeded and which depends on * Presented at the International Symposium on Microchemical Techniques, Davos, May 1977.
the element being determined and on any divergence from the Lambert-Beer law found by testing the technique. The preliminary destruction of an organic sample, generally achieved by the wet process (original or modified micro Lorenz technique) or by combustion in oxygen or with sodium peroxide, introduces reagents which may change the light-absorption properties of the final solution. A study must be made to find whether some reagents have to be rejected or one spectrophotometric technique changed for another. However, it is often the case that though the effect of some reagents is not negligible it is reproducible when a definite and precise procedure is used; it is then convenient to prepare blank solutions in the same way as the sample solutions and to apply the whole procedure to them. This is not necessary when the blank effect is proved to be negligible, but this must be established by a careful study because the blank effect is not always manifested by an absorbance change which can be seen immediately. For instance, in determination of iron with tiron after a mineralization involving hydrogen peroxide, the colour of the complex is less stable than it is when the complex is prepared from a pure iron solution. It is therefore necessary to subject the blank to the decomposition procedure, and to make the measurements before the colour begins to, fade. A similar effect occurs in determination of molybdenum and the analytical results are improved if the entire procedure is applied to the blank. Most of the spectrophotometric techniques described in the literature have been developed for trace analysis or for determination of low contents of elements in rocks, alloys, waters, etc. Appropriate choice of capacities of the standard flasks utilized and the use of appropriate aliquots make it possible to adapt many of these techniques for the determination of major or minor elemental components of both organic and inorganic samples. The precision of the analytical results is not lowered by use of aliquots as much as might be expected, provided two or three replicates from the same solution are analysed and the mean is taken as the correct value. As far as interfering elements are concerned the data found in the literature are generally of little or no use for application to microanalytical work. This is because organic compounds will contain relatively few elements, the natore of which will nearly always be known, and which are generally present in small atomic ratios, in contrast to the case with alloys, rocks or trace analysis. Moreover, the elements which interfere with a calorimetric determination usually belong to the same family as the one being determined, a situation seldom encountered in our case. For these reasons it is possible to use commercial reagents for elemental calorimetric determinations even when other elements interfere to some degree. It is then
183
possible to correct the calorimetric results for the effect of interfering elements as a function of their concentration (as in phosphorus determination’ in the presence of palladium, platinum or rhodium) or slightly to alter the procedure for the better (as in phosphorus determination’ in presence of silicon or arsenic) or to mask interfering elements by means of complexation etc. In the dete~ination of fluoride there are many interfering elements and it is necessary to eliminate them by distilling hexafluorosilicic acid by the Willard and Winter or other suitable method. This is of general use but very time-consuming; that is why for fluorine contents lower than 20% and in the presence of sulphur or phosphorus the direct alizarin complexone technique should be used instead of one of the bleaching methods such as decolouration of the ferric sulphosalicylate complex. In comparison with the automatic analyser techmques for carbon, hydrogen, nitrogen and oxygen microdeterminations, the calorimetric techniques together with the necessary sample preparation at first sight appear to be much more time-consuming. However, they have compensating advantages, such as the possibility of determining various elements one after another without the need for separate specialized pieces of apparatus, most of which will not be in operation the whole time. In addition, there is no time-wasting conditioning of apparatus after a shutdown, and assistants already trained in one determination can quickly adapt to others. Furthermore, the use of flowcells makes it possible to avoid the tedious handling of ordinary cells, but only if the reagents used do not dirty the walls of the cells and tubing, because flow-cells are much more difficult to clean; for instance they should not be used for calorimetric determination of tin with phenylfluorone as complexing agent. All things considered, the average time taken for an elemental determination by a calorimetric technique is very reasonable, and even more so if a large batch is dealt with at the same time. Calorimetric techniques are used in our laboratory: (a) together with prior combustion in an oxygen flask for fluorine determination; (b) together with a wet decomposition process for determination of aluminium, cobalt, iron, molybdenum, phosphorus, palladium, platinum, tin and titanium; (c) together with previous d~mposition with sodium peroxide in a W&s&mitt bomb for silicon determination. Techniques are also being developed for the microdetermination of elements such as boron, rhenium, tellurium, vanadium and zirconium. Calorimetric methods have not been developed for metals such as nickel or copper, for which other techniques (atomic absorption, pol~o~aphy, etc.) are already in use.
METHODS
IN
USE
The calorimetric determinations used in our laboratory are briefly outlined below. Aluminium is determined at 490 nm by means of its redorange complex with Alizarin Red S and calcium in buffered solution (pH 4.4-4.7).‘-’ Cobalt is determined at 34Omn by means of its greenyellow complex with tiron in solution buffered at pH 9.6.‘~” Tin is determined at 52Onm by means of its orange complex with phenylfluorone in acidic aqueous metbanolic medium in the presence of glycine and gelatine; under our conditions the presence of sulpbate is necessary for obtaining correct results.z~6*7 Iron is determined at 480nm by means of its red complex with tiron in buffered medium (pH 9.5); some interferences may be avoided by working at pH 35-4.5, though the sensitivity is then lower.2*s*9
Depending on the presence of interfering elements and its content in the sample, JIuoride may be determined in two ways: (a) in buffered medium (pH 2.8-2.9) by measurement of the decrease in absorbance of the ferric sulphosalycilate complex’at 510 nm;‘*-i4 (b) by means of its purplish-blue complex with alizarin complexone and Ce in aqueous acetone medium (buffered at pH 4.6), at 625 nm.i5*i6 Molybdenum is determined at 475 nm by means of the red complex obtained by reduction with potassium iodide in the presence of thiocyanate in 3 M hydrochloric acid medium.’ Pull~i~~ is determined at 408 nm by means of the brownish pink complex [PdIe]*- obtained by the reaction of potassium iodide with Pd * ’ in 0 .25 M hydrochloric acid medium. 1 Phosphorus is determined at 410nm by means of the yellow phosphovanadomolybdate obtained in 0.26 it4 sulphuric acid medium.‘~” Plafiff~ is determined at 406 nm by means of the yellow [PtCl.$obtained by reduction of @tCl,]‘- by stannous chloride in 1.8 M hvdrochloric acid medium.’ Silicon is determined at 400 nm by means of yellow silicomolybdic acid; this determination is more time-consuming and more difficult than the others because the sample is decomposed with sodium peroxide in a Wilrzschmitt bomb and the large bulk of hydrogen peroxide generated in the process solution has to be destroyed so that it cannot interfere in the calorimetric measurements.” Titanium is determined at 410 nm by means of its yellow complex formed with tiron in the presence of oxalic acid (in order to avoid the hydrolysis of Ti4+ salts)in buffered solution (pH 4.7)’ B, Re, Rh, Te, V and Zr are also determined,” but these techniques are stilf under development in order to solve problems related to extension of their field of application and the precision of the analytical results.
REFERENCE5
1. E. Debal, R. Chassin and S. Peynot, ~~~~~~~ 1976, 23, 35. 2. E. Debal, R. Chassin, S. Peynot and 0. Poliakoff, ibid., 1977. 24. 491. 3. C. A. Parker and A. P. Goddard, Anal. Chim. Acta, 1950, 4, 517. 4. J. A. Corbett and B. D. Guerin, Analyst, 1966, 91, 490. 5. C. K. Bhaskare and S. K. Deshmukh, Z. Anal. Chem., 1975, 277, 127. 6. A. M. Leblond and R. Bouhn, Chim. Anaf. (Pa&), 1968, 50, 171. 7. A. M. Dymov, I. G. Ivanov and T. I. Romantseva, Zh. Analit. Khim.. 1971, 26. 2360. 8. J. H. Yoe and A. ‘L. Jones, 2nd. Eng. C&m., Anal. Ed., 1944, 16, 111. 9. G. V. Potter and C. E. Armstrong, Anal. Chem., 1948, 20, 1208. 10. R. Levy and E. Debd, Mikrochim. Acta, 1962, 224. 11. E. Debal, Chim. Anal. (Paris), 1963, 45, 66. 12. E. Debal, S. Peynot and S. Raveau, Bull. Sot. Chim. France, 1969, 2919. 13. E. Debal, G. Nabias and S. Peynot, ibid., 1971, 3373. 14. M. Poirier, ?‘aluntu,1975, 22, 607. 15. G. Cbarlot, Chimie Analytique Quantitative, Vol. I, betides Ch~miques et Physicoch~iq~s, 6th Ed., p. 421. Masson, Paris, 1974. 16. E. Debal, G. Madelmont, S. Peynot and 0. Poliakoff, unpublished. 17. E. Debal and M. Riocreux, Z’alanta,1972, 19, 15. 18. E. Debal, unpublished work.