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Determination of tin in poly(vinyl chloride) by atomic-absorption spectroscopy
0039-9140/82/100869-021>03.00/0 Copyright © 1982 Perga mon Press Ltd
Talallta. VoL 29. pp. 869 to 870. 1982 Printed in Great Britain. Ali rights reserved
DETERMINATION OF TIN IN POL Y(VINYL CHLORIDE) BY ATOMIC-ABSORPTION SPECTROSCOPY JAMIL ANWAR and L L MARR Chemistry Department, Aberdeen University, Old Aberdeen, Scot land
(Received 22 February 1982. Accepted 8 May 1982)
Summary-A simple procedure is described for the determination of tin in PVC by atomie-absorption speetroscopy with an air-hydrogen flame, after wet digestion of the sample with sulphuric acid and hydrogen peroxide.
Organotin compounds have been used as stabilizers in poly(vinyl chio ride) for several years and are generally present to the extent of 1-2%. A significant amount of work has been done on the analysis of these stabilizers but mostly in relation to qualitative aspects 1 •2 or to the determination of specific organotin compounds. 3 •4 On the other hand, not much work has been published on the determination of total tin in poly(vinyl chloride). Bergnes et al. 5 determined tin in PVC by titration with EDTA after dissolution in hot tetrahydrofuran and dilution with ethanol to precipitate the PVC, which was filtered off. Olivier 6 adopted a similar approach for extracting the tin into an aqueous phase but then back-extracted it into methyl isobutyl ketone (MIBK) from the aqueous solution at pH 4-5 containing ammonium pyrrolidine dithiocarbamate. The MIBK extract was then aspirated into an air-acetylene fiame. Fassy and Lalet 7 determined tin in PVC at a level of 300 Jlg/g by ashing the sample, converting tin into its acetate and nebulizing the solution into an air-acetylene fiame for atomic-absorption measurement at 224.6 nm. In attempts to determine tin in PVC samples in this laboratory none of the above-mentioned methods gave adequately precise results. Therefore a comparatively simple procedure based on decomposition with sulphuric acid and hydrogen peroxide 8 and aspiration of the aqueous solution into an air-hydrogen flame to measure the tin absorption, has been adopted. The method is easy, rapid and sufficiently precise.
EX PERIMENT AL
Instrument
A Perkin-Elmer 305 atomic-absorption spectrophotome ter was used with a lO-cm single-slot bumer and readout on a chart recorder. Tin absorption was measured at 224.6 nm; an air-hydrogen flame was used. Procedure
Weigh out 0.1 g of PVC sample and transfer il to a micro Kjeldahl tube. Add 1 ml of concentrated sulphuric 869
aeid and char by gentle heating. Complete the decomposition by heating for about 10 min with slow dropwise addition of 2 ml of 50% hydrogen peroxide. Boil the contents for another 5 min to remove ail the hydrogen pero xi de and water. The liquor should be eolourless. Let it cool and transfer it to a 25-ml standard flask. Rinse the Kjeldahl tube and make up to volume with water. Determine the atomic absorbance as for the standards (see below). Calibration
Prepare a 5OO-ppm solution of tin by dissolving 0.250 g of pure tin metal in 25 ml of eoncentrated hydroehloric acid and diluting aecurately to 500 ml with water. Transfer an aliquot containing 0.5-2.5 mg of tin into a lOO-ml standard flask followed by 3 ml of concentrated sul ph urie acid and 8 ml of hydrogen peroxide. Shake to mix and make up to the mark with water. Prepare a blank by taking the same volumes of sulphuric acid and hydrogen peroxide. Nebulize the calibration solutions and samples into a fuelrich air-hydrogen flame and measure the tin absorbance at 224.6nm.
RESULTS AND DISCUSSION
The effects of acids and the blank It has been shown 9 that the presence of concentrated hydrochloric acid, even up to 20% vIv, does not effect the atomic-absorption determination of tin, so the presence of less than 0.3% vIv hydrochloric acid in the calibration solutions but none in the samples was considered not to present any difficulties. Sulphuric acid does cause a slight depression in the tin absorption, however,9 so equal amounts of this acid should be present in both calibration and sample solutions. Sodium stannate is sometimes used as stabilizer to pre vent the slow decomposition of hydrogen peroxide. Though with analytical-reagent grade hydrogen peroxide the amounts of sodium and additional tin were so minute that in practice the blank (containing a volume of hydrogen peroxide equivalent to that used for the sample) did not give any noticeable absor ban ce, and was unnecessary, in cases where the hydrogen peroxide does contain considerable amounts of sodium stannate, the preparation of a blank would be essential.
870
SHORT COMMUNICATIONS
Table 1. Determination of tin by various methods in sam pie A containing but yi tin maleate as additive Extraction of Sn by precipitation followed by
Determination
H 2 S0 4 /H 2 0 2 decomposition followed by AAS, Sn, %
Complexometry of ethanolic solution Sn,%
AAS of aqueous solution Sn,%
0.147 0.155 0.154 0.153 0.156 0.003
0.141 0.112 0.092 0.130 0.082 0.024
0.110 0.095 0.065 0.087 0.105 0.017
1
2 3 4 5 S.D.
Table 2. Determination of tin in PVC sampi es Sn,% PVC Sample
A B C
Additive
No. of determinations
Dioctylthiotin Dibitylthiotin Butyltin maleate
Other l11ethods
The methods mentioned in the introduction were aU trie d, but none gave acceptable results, which led to the need for a new method. A dry-ashing procedure was tried in which up to 1 g of sample was ignited at 800 in a silica crucible, but nothing was left after half an ho ur. In another attempt 0.5 g of sample was heated with 2 g of magnesium nitrate at 800° for half an hour. The residue was dissolved by adding hot hydrochloric acid but no trace of tin could be detected in the solution. Extraction of tin by dissolving the PVC in an organic solvent and reprecipitating only the pol ymer by addition of water or ethanol was also tried. The ethanol extract was titrated with O.ooIM EDTA, with Catechol Violet as indicator, as described by Burgnes et al. s and also the aqueous extract was aspirated into an air-hydrogen fiame, but the results obtained in both cases, and given in Table 2, were poor and imprecise, which is most probably due to incomplete extraction of the tin or its adsorption on the precipitated polymer. Another approach suggested by Olivier 6 for the determination of trace metals in polymers is based on the direct aspiration of a 2% solution of the pol ymer in an organic solvent into the fiame. This method was also tried but in this case rapid blockage of the bumer assembly was indicated by a sharp decrease in the absorption with time. A comparison of results and standard deviations obtained by the different methods is shown in Table 1. 0
Analysis of pvc sal11ples
By courtesy of Messrs. Albright and Wilson three
3 3
5
Stated
Found
0.152 0.152 0.159
0.146 0.144 0.153
samples of PVC, each containing a different additive at around the 1-2% level, were made available: 100 mg of sample could be easily and completely decomposed with 1 ml of concentrated sulphuric acid and 2 ml of 50% hydrogen peroxide. The whole decomposition procedure took 15 min at the most. The air-hydrogen fiame was found adequately sensitive for the amounts of tin present in the samples and a linear calibration graph for tin in the range 5-20 jlg/ml could easily be obtained by use of a slightly fuel-rich fiame.! 0 The results obtained for the analysis of PVC samples containing various organotin additives are given in Table 2. A relative standard deviation of 2% was obtained for five successive de terminations of butyltin maleate in PVC. In conclusion, with the present method tin can be determined with more satisfactory results and with very little effort, compared to other methods given in the literature.
REFERENCES
1. J. Udris, Analyst, 1971,96, 130. 2. R. Sawyer, ibid., 1967,92,569. 3. H. Woidich and W. Pfannhauser Dtsch. Lebensm. Rundsch., 1978, 74, 231. ' 4. D. Simpson and B. R. Currell, Analyst, 1971,96, 515. 5. K. G. Bergnes, U. Rudt and O. Mack, Dtsch. Lebensm. Rundsch., 1967,63, 180. 6. M. Olivier, Z. Anal. Chem., 1969,248, 145. 7. H. Fassy and P. Lalet, Chim. Anal. (Paris), 1970, 52, 1281. 8. R. P. Taubinger and J. R. Wilson, Analyst, 1965, 90, 429. 9. L. Capacho-Delgado and D. C. Manning, Spectrochim. Acta, 1966,2, 1505. 10. J. L. Marr and J. Anwar, Analyst, 1982,107,260.
Talallta. VoL 29. pp. 869 to 870. 1982 Printed in Great Britain. Ali rights reserved
DETERMINATION OF TIN IN POL Y(VINYL CHLORIDE) BY ATOMIC-ABSORPTION SPECTROSCOPY JAMIL ANWAR and L L MARR Chemistry Department, Aberdeen University, Old Aberdeen, Scot land
(Received 22 February 1982. Accepted 8 May 1982)
Summary-A simple procedure is described for the determination of tin in PVC by atomie-absorption speetroscopy with an air-hydrogen flame, after wet digestion of the sample with sulphuric acid and hydrogen peroxide.
Organotin compounds have been used as stabilizers in poly(vinyl chio ride) for several years and are generally present to the extent of 1-2%. A significant amount of work has been done on the analysis of these stabilizers but mostly in relation to qualitative aspects 1 •2 or to the determination of specific organotin compounds. 3 •4 On the other hand, not much work has been published on the determination of total tin in poly(vinyl chloride). Bergnes et al. 5 determined tin in PVC by titration with EDTA after dissolution in hot tetrahydrofuran and dilution with ethanol to precipitate the PVC, which was filtered off. Olivier 6 adopted a similar approach for extracting the tin into an aqueous phase but then back-extracted it into methyl isobutyl ketone (MIBK) from the aqueous solution at pH 4-5 containing ammonium pyrrolidine dithiocarbamate. The MIBK extract was then aspirated into an air-acetylene fiame. Fassy and Lalet 7 determined tin in PVC at a level of 300 Jlg/g by ashing the sample, converting tin into its acetate and nebulizing the solution into an air-acetylene fiame for atomic-absorption measurement at 224.6 nm. In attempts to determine tin in PVC samples in this laboratory none of the above-mentioned methods gave adequately precise results. Therefore a comparatively simple procedure based on decomposition with sulphuric acid and hydrogen peroxide 8 and aspiration of the aqueous solution into an air-hydrogen flame to measure the tin absorption, has been adopted. The method is easy, rapid and sufficiently precise.
EX PERIMENT AL
Instrument
A Perkin-Elmer 305 atomic-absorption spectrophotome ter was used with a lO-cm single-slot bumer and readout on a chart recorder. Tin absorption was measured at 224.6 nm; an air-hydrogen flame was used. Procedure
Weigh out 0.1 g of PVC sample and transfer il to a micro Kjeldahl tube. Add 1 ml of concentrated sulphuric 869
aeid and char by gentle heating. Complete the decomposition by heating for about 10 min with slow dropwise addition of 2 ml of 50% hydrogen peroxide. Boil the contents for another 5 min to remove ail the hydrogen pero xi de and water. The liquor should be eolourless. Let it cool and transfer it to a 25-ml standard flask. Rinse the Kjeldahl tube and make up to volume with water. Determine the atomic absorbance as for the standards (see below). Calibration
Prepare a 5OO-ppm solution of tin by dissolving 0.250 g of pure tin metal in 25 ml of eoncentrated hydroehloric acid and diluting aecurately to 500 ml with water. Transfer an aliquot containing 0.5-2.5 mg of tin into a lOO-ml standard flask followed by 3 ml of concentrated sul ph urie acid and 8 ml of hydrogen peroxide. Shake to mix and make up to the mark with water. Prepare a blank by taking the same volumes of sulphuric acid and hydrogen peroxide. Nebulize the calibration solutions and samples into a fuelrich air-hydrogen flame and measure the tin absorbance at 224.6nm.
RESULTS AND DISCUSSION
The effects of acids and the blank It has been shown 9 that the presence of concentrated hydrochloric acid, even up to 20% vIv, does not effect the atomic-absorption determination of tin, so the presence of less than 0.3% vIv hydrochloric acid in the calibration solutions but none in the samples was considered not to present any difficulties. Sulphuric acid does cause a slight depression in the tin absorption, however,9 so equal amounts of this acid should be present in both calibration and sample solutions. Sodium stannate is sometimes used as stabilizer to pre vent the slow decomposition of hydrogen peroxide. Though with analytical-reagent grade hydrogen peroxide the amounts of sodium and additional tin were so minute that in practice the blank (containing a volume of hydrogen peroxide equivalent to that used for the sample) did not give any noticeable absor ban ce, and was unnecessary, in cases where the hydrogen peroxide does contain considerable amounts of sodium stannate, the preparation of a blank would be essential.
870
SHORT COMMUNICATIONS
Table 1. Determination of tin by various methods in sam pie A containing but yi tin maleate as additive Extraction of Sn by precipitation followed by
Determination
H 2 S0 4 /H 2 0 2 decomposition followed by AAS, Sn, %
Complexometry of ethanolic solution Sn,%
AAS of aqueous solution Sn,%
0.147 0.155 0.154 0.153 0.156 0.003
0.141 0.112 0.092 0.130 0.082 0.024
0.110 0.095 0.065 0.087 0.105 0.017
1
2 3 4 5 S.D.
Table 2. Determination of tin in PVC sampi es Sn,% PVC Sample
A B C
Additive
No. of determinations
Dioctylthiotin Dibitylthiotin Butyltin maleate
Other l11ethods
The methods mentioned in the introduction were aU trie d, but none gave acceptable results, which led to the need for a new method. A dry-ashing procedure was tried in which up to 1 g of sample was ignited at 800 in a silica crucible, but nothing was left after half an ho ur. In another attempt 0.5 g of sample was heated with 2 g of magnesium nitrate at 800° for half an hour. The residue was dissolved by adding hot hydrochloric acid but no trace of tin could be detected in the solution. Extraction of tin by dissolving the PVC in an organic solvent and reprecipitating only the pol ymer by addition of water or ethanol was also tried. The ethanol extract was titrated with O.ooIM EDTA, with Catechol Violet as indicator, as described by Burgnes et al. s and also the aqueous extract was aspirated into an air-hydrogen fiame, but the results obtained in both cases, and given in Table 2, were poor and imprecise, which is most probably due to incomplete extraction of the tin or its adsorption on the precipitated polymer. Another approach suggested by Olivier 6 for the determination of trace metals in polymers is based on the direct aspiration of a 2% solution of the pol ymer in an organic solvent into the fiame. This method was also tried but in this case rapid blockage of the bumer assembly was indicated by a sharp decrease in the absorption with time. A comparison of results and standard deviations obtained by the different methods is shown in Table 1. 0
Analysis of pvc sal11ples
By courtesy of Messrs. Albright and Wilson three
3 3
5
Stated
Found
0.152 0.152 0.159
0.146 0.144 0.153
samples of PVC, each containing a different additive at around the 1-2% level, were made available: 100 mg of sample could be easily and completely decomposed with 1 ml of concentrated sulphuric acid and 2 ml of 50% hydrogen peroxide. The whole decomposition procedure took 15 min at the most. The air-hydrogen fiame was found adequately sensitive for the amounts of tin present in the samples and a linear calibration graph for tin in the range 5-20 jlg/ml could easily be obtained by use of a slightly fuel-rich fiame.! 0 The results obtained for the analysis of PVC samples containing various organotin additives are given in Table 2. A relative standard deviation of 2% was obtained for five successive de terminations of butyltin maleate in PVC. In conclusion, with the present method tin can be determined with more satisfactory results and with very little effort, compared to other methods given in the literature.
REFERENCES
1. J. Udris, Analyst, 1971,96, 130. 2. R. Sawyer, ibid., 1967,92,569. 3. H. Woidich and W. Pfannhauser Dtsch. Lebensm. Rundsch., 1978, 74, 231. ' 4. D. Simpson and B. R. Currell, Analyst, 1971,96, 515. 5. K. G. Bergnes, U. Rudt and O. Mack, Dtsch. Lebensm. Rundsch., 1967,63, 180. 6. M. Olivier, Z. Anal. Chem., 1969,248, 145. 7. H. Fassy and P. Lalet, Chim. Anal. (Paris), 1970, 52, 1281. 8. R. P. Taubinger and J. R. Wilson, Analyst, 1965, 90, 429. 9. L. Capacho-Delgado and D. C. Manning, Spectrochim. Acta, 1966,2, 1505. 10. J. L. Marr and J. Anwar, Analyst, 1982,107,260.