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Interference in inorganic phosphorus analysis of membrane-filtered solutions
Water Research Vol. 15, pp. 511 to 512, 1981 Printed in Great Britain
0043-1354/81/040511.02502.00/0 Pergamon Press Ltd
TECHNICAL NOTE INTERFERENCE IN INORGANIC PHOSPHORUS ANALYSIS OF MEMBRANE-FILTERED SOLUTIONS G. P. BICKFORD and I. R. WILLETF CSIRO Division of Soils, P.O. Box 639, Canberra City, Australia 2601
(Received September 1980) AMtraet--The filtration of solutions through a membrane which contained wetting agent, caused interference in two widely used colorimetric methods for inorganic phosphorus. This effect was due to the release of some substance from the membrane. It is recommended that "low extractables" membrane filters be used when the filtrate is required for inorganic phosphorus analyses by the methods used here.
INTRODUCTION
Prior to the analysis of natural waters, and soil or sediment extracts, it is usually necessary to remove any solid component. Filtration using membranes of 0.2 or 0.45 #m pore size has been widely adopted for this purpose. Membrane filtration is advantageous because it does not contaminate the filtrate with fibres, and rapid filtration is possible because of high flow rates. Recently, we found that determinations of inorganic phosphorus by the widely-used colorimetric methods of Fogg & Wilkinson (1958), and Murphy & Riley (1962) were irreproducible in a membrane filtrate. On the other hand, results for Mn 2. and Fe 2+ determined by atomic absorption spectrophotometry were satisfactory. In this note, we present evidence that some substance released from a membrane filter (Gelman membrane GA6) causes severe interference in the colorimetric determination of inorganic phosphorus. EXPERIMENTAL
Experiments were conducted with a 0.01 M CaCI2 suspension of a sample of an alluvial soil which contained 11~ organic matter and to which had been added 500 mg P kg- t. Previous results had shown that this quantity of P would maintain approx. 0.1 mg P 1-1 in solution. The suspension was prepared by continuously stirring 333 g of soil with 2 10.01 M CaCI2 for 17 h. The CaCI2 was used to prevent dispersion of the soil and facilitate filtering. In other experiments, 0.01 M CaCI2 containing 0.1 mg P 1-1, (as KH2PO4), or no added P, were used.
The soil suspension and CaCI2 solutions were then treated in four ways, viz: (a) centrifugation (45 min at 2250 rpm), (b) filtering through Whatman No. 42 paper, (c) filtering through Gelman TCM-450 membrane, and (d) filtering through Gelman GA6 membrane. The membrane filters were both of 0.45 pm pore size, and were made of cellulose triacetate. They differ only in that TCM-450 is manufactured without wetting .agent (<1~o extractables in boiling water), whereas GA6 has a wetting agent added during manufacturing ( < 2 ~ extractables), (Gelman Instruments Company, 1977). In each filtration method, 20cm 3 of suspension, or CaCI2 solution, was filtered through one filter, and the first few drops of filtrate discarded. The clear solutions were analysed for inorganic phosphorous by the methods of Murphy and Riley, and Fogg and Wilkinson adapted for use with an autoanalyser. The main differences in these methods is that colour development is at 25 ° with added antimony in the former method, and at 95 ° with no antimony in the latter method. Three determinations for phosphorus were made for each filtrate. In each analytical run, filtrates of GA6 were analysed last as it caused suppression of the standard curve and subsequent analyses (see Results and Discussion). Results for this filter were based on a preceding set of standards.
RESULTS AND DISCUSSION
Results for inorganic phosphorus in soil and CaCI2 solutions obtained by the two analytical methods are recorded in Table I. In each case, the results for GA6
Table 1. Influence of method of filtration on P analytical results. Means of 3 determinations, mgP1-1 Solution Analytical method Centrifugate Whatman No. 42 Membrane TCM-450 Membrane GA6 Least significant difference (P < 5Vo)
Soil extract Murphy & Fogg & Riley (1962) Wilkinson (1958) 0.118 0.118 0.121 0.051 0.017
0.162 0.153 0.159 0.128 0.009
511
CaCI2 solution (0.1 mg P 1- t ) Murphy & Fogg & Riley (1962) Wilkinson (1958) 0.103 0.115 0.101 0.037 0.007
0.102 0.116 0.102 0.079 0.009
512
G.P. BICKFORDand i. R WlLLJ~T~
filtrates were significantly lower than those obtained in other filtrates, or in the centrifugate. In the CaC12 solution, the results for Whatman No. 42 filtrates were higher than those for the TCM-450 filtrate, or the centrifugate, which were near to the known value of 0.1 mg P 1 ~. The results for the soil extract were greater when determined by the method of Fogg and Wilkinson than by the method of Murphy and Riley, but this effect was not shown in the CaC12 solutions. One possible explanation of the apparent low P concentration in the GA6 filtrate, is that of retention of P by this particular filter. However, on every occasion this filtrate was analysed, it was found that the succeeding standard curve was markedly suppressed. In another trial, a standard curve was obtained and then followed by a sample of CaCI2 (with no added P) which had been filtered through a GA6 membrane. A standard curve obtained after the filtrate showed a sensitivity about half that of the standard curve obtained before the filtrate with the Murphy and Riley method, and about 3/4 in the Fogg and Wilkinson method. Furthermore, after attempting to analyse GA6 filtrates, the autoanalyser required thorough washing before sensitivity returned to its original level. It therefore appears that some substance released from the GA6 filter depressed the sensitivity of the P analytical methods. Since no effect was recorded with the TCM-450 filtrates, and because the GA6 only differs from the TCM-450 in that it contains more extractable wetting agent, this would appear to be the cause of the interference. The greater values for P obtained in the soil extract by the method of Fogg and Wilkinson, compared to those of the Murphy and Riley method, may be caused by the hydrolysis of organic P because of the heating employed in the former method. This effect has been reported previously for soil extracts (Salt, 1968), sea-water (Strickland & Austin, 1959), and lake water (Chamberlain & Shapiro, 1973). Since the CaC12 did not contain any organic matter, both methods gave similar results. The higher results obtained for Whatman No. 42 filtrates
in the CaCI2 solution, compared to those from the TCM-450 filtrate, and the centrifugate, may be caused by the release of P from the filter paper. On the other hand. results for these three methods of separation were similar in the soil extract. However, previous results in our laboratory have shown that P release from filter papers is variable. CONCLUSIONS (a) Interference to P determination in GA6 filtrates was caused by the release of some substance, probably wetting agent added during manufacture. (b) When membrane filtration of samples is required to obtain clear solutions for inorganic phosphorus analyses by the methods used here, "low extractables" membrane filters such as TCM-450 should be used. REFERENCES
Chamberlain W. & Shapiro J. (1973) Phosphate measurements in natural water---a critique. Environmental Phosphorus Handbook, pp. 355-366. Wiley, New York. Fogg D. N. & Wilkinson N. T, (1958) The colorimetric determination of phosphorus. Analyst 83, 406-414. Gelman Instruments Company (1977) Gelman Filtration Products. Ann Arbor, MI. Murphy J. & Riley J. P. (1962) A modified single solution method for the determination of phosphate in natural waters. Analyt: chim. acta 27, 31-36. Salt P. D. (1968) The automatic determination of phosphorus in extracts of soil made with 0.5 M sodium hydrogen carbonate and 0.01 M calcium chloride. Chemy Ind. 18, 584-586. Strickland J. D. H. & Austin K. H. (1959) The direct estimation of ammonia in sea water with notes on reactive iron, nitrate, and inorganic phosphorus. J. Cons. perm. int. Explor. Mer. 2,4, 446-451.
0043-1354/81/040511.02502.00/0 Pergamon Press Ltd
TECHNICAL NOTE INTERFERENCE IN INORGANIC PHOSPHORUS ANALYSIS OF MEMBRANE-FILTERED SOLUTIONS G. P. BICKFORD and I. R. WILLETF CSIRO Division of Soils, P.O. Box 639, Canberra City, Australia 2601
(Received September 1980) AMtraet--The filtration of solutions through a membrane which contained wetting agent, caused interference in two widely used colorimetric methods for inorganic phosphorus. This effect was due to the release of some substance from the membrane. It is recommended that "low extractables" membrane filters be used when the filtrate is required for inorganic phosphorus analyses by the methods used here.
INTRODUCTION
Prior to the analysis of natural waters, and soil or sediment extracts, it is usually necessary to remove any solid component. Filtration using membranes of 0.2 or 0.45 #m pore size has been widely adopted for this purpose. Membrane filtration is advantageous because it does not contaminate the filtrate with fibres, and rapid filtration is possible because of high flow rates. Recently, we found that determinations of inorganic phosphorus by the widely-used colorimetric methods of Fogg & Wilkinson (1958), and Murphy & Riley (1962) were irreproducible in a membrane filtrate. On the other hand, results for Mn 2. and Fe 2+ determined by atomic absorption spectrophotometry were satisfactory. In this note, we present evidence that some substance released from a membrane filter (Gelman membrane GA6) causes severe interference in the colorimetric determination of inorganic phosphorus. EXPERIMENTAL
Experiments were conducted with a 0.01 M CaCI2 suspension of a sample of an alluvial soil which contained 11~ organic matter and to which had been added 500 mg P kg- t. Previous results had shown that this quantity of P would maintain approx. 0.1 mg P 1-1 in solution. The suspension was prepared by continuously stirring 333 g of soil with 2 10.01 M CaCI2 for 17 h. The CaCI2 was used to prevent dispersion of the soil and facilitate filtering. In other experiments, 0.01 M CaCI2 containing 0.1 mg P 1-1, (as KH2PO4), or no added P, were used.
The soil suspension and CaCI2 solutions were then treated in four ways, viz: (a) centrifugation (45 min at 2250 rpm), (b) filtering through Whatman No. 42 paper, (c) filtering through Gelman TCM-450 membrane, and (d) filtering through Gelman GA6 membrane. The membrane filters were both of 0.45 pm pore size, and were made of cellulose triacetate. They differ only in that TCM-450 is manufactured without wetting .agent (<1~o extractables in boiling water), whereas GA6 has a wetting agent added during manufacturing ( < 2 ~ extractables), (Gelman Instruments Company, 1977). In each filtration method, 20cm 3 of suspension, or CaCI2 solution, was filtered through one filter, and the first few drops of filtrate discarded. The clear solutions were analysed for inorganic phosphorous by the methods of Murphy and Riley, and Fogg and Wilkinson adapted for use with an autoanalyser. The main differences in these methods is that colour development is at 25 ° with added antimony in the former method, and at 95 ° with no antimony in the latter method. Three determinations for phosphorus were made for each filtrate. In each analytical run, filtrates of GA6 were analysed last as it caused suppression of the standard curve and subsequent analyses (see Results and Discussion). Results for this filter were based on a preceding set of standards.
RESULTS AND DISCUSSION
Results for inorganic phosphorus in soil and CaCI2 solutions obtained by the two analytical methods are recorded in Table I. In each case, the results for GA6
Table 1. Influence of method of filtration on P analytical results. Means of 3 determinations, mgP1-1 Solution Analytical method Centrifugate Whatman No. 42 Membrane TCM-450 Membrane GA6 Least significant difference (P < 5Vo)
Soil extract Murphy & Fogg & Riley (1962) Wilkinson (1958) 0.118 0.118 0.121 0.051 0.017
0.162 0.153 0.159 0.128 0.009
511
CaCI2 solution (0.1 mg P 1- t ) Murphy & Fogg & Riley (1962) Wilkinson (1958) 0.103 0.115 0.101 0.037 0.007
0.102 0.116 0.102 0.079 0.009
512
G.P. BICKFORDand i. R WlLLJ~T~
filtrates were significantly lower than those obtained in other filtrates, or in the centrifugate. In the CaC12 solution, the results for Whatman No. 42 filtrates were higher than those for the TCM-450 filtrate, or the centrifugate, which were near to the known value of 0.1 mg P 1 ~. The results for the soil extract were greater when determined by the method of Fogg and Wilkinson than by the method of Murphy and Riley, but this effect was not shown in the CaC12 solutions. One possible explanation of the apparent low P concentration in the GA6 filtrate, is that of retention of P by this particular filter. However, on every occasion this filtrate was analysed, it was found that the succeeding standard curve was markedly suppressed. In another trial, a standard curve was obtained and then followed by a sample of CaCI2 (with no added P) which had been filtered through a GA6 membrane. A standard curve obtained after the filtrate showed a sensitivity about half that of the standard curve obtained before the filtrate with the Murphy and Riley method, and about 3/4 in the Fogg and Wilkinson method. Furthermore, after attempting to analyse GA6 filtrates, the autoanalyser required thorough washing before sensitivity returned to its original level. It therefore appears that some substance released from the GA6 filter depressed the sensitivity of the P analytical methods. Since no effect was recorded with the TCM-450 filtrates, and because the GA6 only differs from the TCM-450 in that it contains more extractable wetting agent, this would appear to be the cause of the interference. The greater values for P obtained in the soil extract by the method of Fogg and Wilkinson, compared to those of the Murphy and Riley method, may be caused by the hydrolysis of organic P because of the heating employed in the former method. This effect has been reported previously for soil extracts (Salt, 1968), sea-water (Strickland & Austin, 1959), and lake water (Chamberlain & Shapiro, 1973). Since the CaC12 did not contain any organic matter, both methods gave similar results. The higher results obtained for Whatman No. 42 filtrates
in the CaCI2 solution, compared to those from the TCM-450 filtrate, and the centrifugate, may be caused by the release of P from the filter paper. On the other hand. results for these three methods of separation were similar in the soil extract. However, previous results in our laboratory have shown that P release from filter papers is variable. CONCLUSIONS (a) Interference to P determination in GA6 filtrates was caused by the release of some substance, probably wetting agent added during manufacture. (b) When membrane filtration of samples is required to obtain clear solutions for inorganic phosphorus analyses by the methods used here, "low extractables" membrane filters such as TCM-450 should be used. REFERENCES
Chamberlain W. & Shapiro J. (1973) Phosphate measurements in natural water---a critique. Environmental Phosphorus Handbook, pp. 355-366. Wiley, New York. Fogg D. N. & Wilkinson N. T, (1958) The colorimetric determination of phosphorus. Analyst 83, 406-414. Gelman Instruments Company (1977) Gelman Filtration Products. Ann Arbor, MI. Murphy J. & Riley J. P. (1962) A modified single solution method for the determination of phosphate in natural waters. Analyt: chim. acta 27, 31-36. Salt P. D. (1968) The automatic determination of phosphorus in extracts of soil made with 0.5 M sodium hydrogen carbonate and 0.01 M calcium chloride. Chemy Ind. 18, 584-586. Strickland J. D. H. & Austin K. H. (1959) The direct estimation of ammonia in sea water with notes on reactive iron, nitrate, and inorganic phosphorus. J. Cons. perm. int. Explor. Mer. 2,4, 446-451.