THE DETERMINATION OF SERUM PHOSPHATE BY THE MOLYBDIVANADATE METHOD

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1 THE DETERMINATION OF SERUM PHOSPHATE BY THE MOLYBDIVANADATE METHOD BY DAISY G. SIMONSEN, MAXINE WERTMAN, LEOLA M. WESTOVER, AND JOHN W. MEHL (From the Departments of Medicine and Biochemistry, School of Medicine, University of Southern California, and the Los Angeles County General Hospital, Los Angeles) (Received for publication, August 22, 1946) One of the commonly used methods for the calorimetric determination of phosphorus is based upon the reduction of molybdic acid to molybdenum blue and subsequent estimation of the intensity of the blue color formed.. An excellent discussion of the molybdenum blue reaction and its limitations is given by Woods and Mellon (1). In using this reaction over a period of years the authors have been increasingly aware of its shortcomings. Acid stannous chloride was used as the reducing agent and the procedures of Kuttner and Cohen (2), Woods and Mellon (l), and Fontaine (3) were carefully followed. In addition, modifications in concentration and type of both acid and molybdate have been studied. The results obtained were in accord with those of Woods and Mellon (1) who found that the relative amounts of acid and molybdate were important since, if the ratio of acid to molybdate was too low, some of the molybdate was reduced along with the heteropoly acid, and if too high, there was a marked decrease in the intensity of the color. In general, the difficulties encountered in our laboratory with the use of chlorostannous acid as the reducing agent were (1) control of concentration of acid and molybdate, (2) instability of the color after a short time interval, (3) non-conformity with Beer s law when the concentration of phosphorus was above 4 parts per million, (4) rapid deterioration of the stannous chloride solution, particularly of the dilute reagent, and (5) inability to obtain duplicate results from day to day when standard solutions of phosphate were used. Three recent methods for the determination of phosphate have made use of ammonium vanadate with subsequent calorimetric estimation of the yellow molybdivanadophosphoric acid formed. Willard and Center (4) described a method for the determination of phosphorus in iron ore, Koenig and Johnson (5) one for the determination of phosphorus in foods and biological material, and Kitson and Mellon (6) a method used for the determination of phosphorus in carbon and low alloy steels. Since the procedure seemed to offer distinct advantages from the standpoint of stability of the final color and simplicity, the application of the method of Kitson and Mellon to the determination of serum phosphate has been investigated. 747

2 748 DETERMINATION OF SERUM PHOSPHATE Procedure Reagents- 1. Redistilled water (from an all-glass still) or distilled water known to be phosphate-free. This water was used for the preparation of all reagents. 2. Trichloroacetic acid, 7.5 and 10 per cent solutions. 3. Ammonium molybdate, 5 per cent aqueous solution. 4. Ammonium vanadate,l 0.25 per cent solution. Dissolve 2.5 gm. of ammonium vanadate in approximately 500 ml. of boiling water, cool slightly, and then add 20 ml. of concentrated nitric acid. Allow the mixture to cool to room temperature and make up to a volume of 1 liter. 5. Nitric acid (1: 2) ;21 volume of concentrated nitric acid plus 2 volumes of water. 6. Phosphate standard. Dissolve gm. of reagent grade potassium dihydrogen phosphate (previously dried to constant weight) in 7.5 per cent trichloroacetic acid and make to a volume of 1 liter (with the same strength acid). 1 ml. of this solution contains 100 y of phosphorus. Suitable dilute standards (in 7.5 per cent trichloroacetic acid) are prepared from the stock solution to give a range of 5 to 80 y of phosphorus in the final 5 ml., which was adopted as the standard volume for this procedure. Procedure for Klett-Summerson Photoelectric Calorimeter-Measure into a calibrated Klett tube (preferably graduated at 5 ml.) 3 ml. of a suitable dilute standard or sample. Add 0.5 ml. of 1: 2 nitric acid and mix by thorough shaking. Then add 0.5 ml. of 0.25 per cent ammonium vanadate solution and mix by shaking. Finally add 0.5 ml. of 5 per cent ammonium molybdate, make to a volume of 5 ml. with water, and mix by inversion. After the mixture has stood 5 minutes, read in the calorimeter with the No. 42 (blue) filter. A blank is prepared with 3 ml. of 7.5 per cent trichloroacetic acid and the reagents added in the same amounts and order as for the standard. In every case the calculations are based upon the reading after subtraction of the blank reading. Procedure for Serum-To 6 ml. of 10 per cent trichloroacetic acid in a 15 ml. centrifuge tube add 2 ml. of serum. Mix by inversion, allow to stand 15 minutes, and then centrifuge for 7 to 10 minutes at 2500 to 3500 R.P.M. or until the supernatant liquid is clear. Measure 3 ml. of the supernatant 1 May be obtained from Eimer and Amend, New York. * After the larger part of the experiment.s had been completed, it was found that the vanadate and nitric acid could be combined by dissolving the ammonium vanadate as directed, but adding 350 ml. of concentrated nitric acid instead of 20 ml., and diluting to 1 liter. 0.5 ml. oi this combined reagent is used in place of 0.5 ml. each of the original ammonium vanadate and 1:2 nitric acid. The strongly acid vanadate solution has given the same results as those obtained with the original reagents, and has been stable for the 2 months that it has been in use.

3 SIMONSEN, WERTMAN, WESTOVER, AND MEHL 749 fluid (equivalent to 0.75 ml. of serum) into a calibrated Klett tube, and proceed in the same manner described for the standard solutions. The number of micrograms of phosphorus, taken from the calorimeter reading of the standard curve, multiplied by equals the number of mg. of phosphorus per 100 ml. of serum. Inasmuch as the nitric acid and vanadate could be combined as one reagent, the procedure for the determination of serum phosphorus was altered as follows: To 4 ml. of the trichloroacetic acid filtrate add 0.5 ml. of the combined vanadate-nitric acid reagent, then 0.5 ml. of ammonium molybdate, and mix by inversion. Read in the calorimeter after a 5 minute interval. It is advisable to run one or two standards with each set of unknowns in order to check for contamination and stability of reagents and any variability of the calorimeter. A blank determination is also made each time. Spectrophotometric Determinations-Spectrophotometric measurements were also made in certain instances. The determinations were made in the Beckman quartz spectrophotometer, with 1.00 cm. Corex cells. The results are reported as the observed optical densities of the solutions. Results Phosphate Standards-The phosphate standards were routinely made up in trichloroacetic acid of the same concentration as that in the serum filtrates, since some effect of trichloroacetic acid was noted in the higher eoncentrations of phosphate. The calibration curves are given in Fig. 1. The curve of corrected readings against concentration is not strictly linear when the amount of phosphate corresponds to 5 to 80 y of P in the final 5 ml. This deviation from a straight line is doubtless attributable to the spectral characteristics of the filter and the response of the photocell. The optical densities of these same solutions, measured at 440, 420, 400, and 380 rnp, are given in Fig. 2. In each case the absorption of the blank at the corresponding wave-length has been subtracted. The solutions will be seen to follow Beer s law. It is also evident that the absorption increases with decreasing wavelength. Since it was not clear from previous reports whether there might be an absorption peak which could be used for spectrophotometric studies, the values of the optical densities of the blank and of solutions containing 0.1 and 0.2 mg. of P per 100 ml. were determined at several wave-lengths and are given in Fig. 3. It was not possible to determine the position of the maximum because of the very great absorption of the blank at the shorter wave-lengths, but it is evident that there is little to be gained by using wave-lengths below about 380 rnp, since the blank is increasing much more rapidly than the value of E:?,..

4 750 DETERMINATION OF SERUM PHOSPHATE I I I I I I I POqP, Y FIG. 1. Curves of Klett-Summerson calorimeter readings (corrected for the blank) for various amounts of phosphate P in the final 5 ml. of solution. 0, phosphate solu- tions in water; 0, phosphate solutions in 7.5 per cent trichloroacetic acid. PO4-P, Y FIG. 2. Optical densities of solutions (corrected for the blank) for various amounts of phosphate in the final 5 ml. of solution, at 440,420,400, and 330 ~UL. All phosphate solutions were made in 7.5 per cent trichloroacetic acid.

5 SIMONSEN, WERTMAN, WESTOVER, AND MEHL 751 Some attention was given to the possibility of reducing the blank by decreasing the concentration of the reagents employed. This attempt was not successful, since the high blank is largely due to the molybdate, and color development does not take place in the absence of the large excess of molybdate specified in the procedure. Although greater sensitivity can be obtained with the spectrophotometer, the results to be presented were obtained with the calorimeter, since this instrument is more likely to be used in routine procedures. 12! 1 I I I I I I mu FIG, 3. Optical densities of the blank (Curve A), a solution containing 0.1 mg. of Pod-P per 100 ml. (Curve B), and of a solution containing 0.2 mg. of Pod-P per 100 ml. (Curve C), as a function df the wave-length. The color of the blanks and the test solutions is entirely reproducible and is stable for periods of at least 24 hours. Duplicate determinations did not vary more than 3 divisions on the calorimeter scale, and were usually in perfect agreement. The blank gives a reading of 10 to 11 divisions, and the increase over the blank is 26 to 30 divisions per microgram of P per ml., depending upon the concentration. After the color has been developed, it is not possible to dilute the samples with water or 7.5 per cent trichloroacetic acid in order to obtain a lower calorimeter reading. Readings of the diluted samples give a positive deviation, of as much as 12 per cent in the higher concentrations, when the dilution factor is taken into account. Thus, if the unknown solution contains too high a concentration of phosphorus to be calculated from the calibration curve, one must repeat the procedure with a smaller sample.

6 752 DETERMINATION OF SERUM PHOSPHATE TABLE Recovery of Phosphate - I Added to Serum - or _. - AveraReerror... Patient No. TABLE II Serum Phosphate Values of Normal Adults I Females w Y -- per cent o M&S m ho.5 Average

7 SIMONSEN, WERTMAN, WESTOVER, AND MEIIL 753 Recovery of Phosphate Added to Serum-Various phosphate standards were added to 3 ml. samples of pooled serum so that the added phosphorus ranged from 10 to 100 y per 3 ml. of serum. Sufficient water and 20 per cent trichloroacetic acid were then added so that the final volume and acidity were identical with those used in the determination of serum phosphate. %P sex F. M. I I F. M. L F. - TABLE III Serum Phosphate Values of Children P per loo I ml. senm yrr. w Diagnosis Controlled diabetes Probable infectious mononucleosis Acute rheumatic fever I Streptococcus throat Sickle cell anemia Probable I subacute glomerulonephritis Acute glomerulonephritis Undiagnosed but possible anemia Cellulitis Possible 1 rheumatic fever Mumps or recurrent parotitia Upper respiratory infection Tonsillitis Lobar pneumonia Acute glomerulonephritis Rheumatic fever Average The phosphorus was then determined in 3 ml. of the acid filtrate according to the standard procedure. The recoveries are given in Table I. The results show an excellent recovery of added phosphorus. The errors are no greater than those that would be obtained by setting up and reading a series of different sera or a set of standards, and give an average deviation of ~0.5 per cent from the calculated value. Serum Phosphate of Normal Adults-Blood samples were obtained from members of the laboratory staff which included chemists, physicians, technicians, medical students, and general laboratory assistants. There were

8 754 DETERMINATION OF SERUM PHOSPHATE forty adults in the group, ranging in age from 20 to 60 years. Sex distribution showed nineteen females and twenty-one males. The values obtained ranged from 2.8 to 4.9 mg. of phosphorus per 100 ml. of serum. The results are given in Table II. Serum Phosphate of Children-A group of twenty-three hospitalized children was used for the study of phosphorus values of children. Patients with bone disease were excluded; otherwise the cases were selected at random. Since the majority were near the end of convalescence and ready to be dismissed from the hospital, the values approximate those of normal children. The values obtained range from 4.6 to 7.2 mg. of phosphorus per 100 ml. of serum. The difference between the lowest values of children and adults was 1.8 mg. and that of the highest values was 2.3 mg. of P per 100 ml. of serum. Based on the average values of the two groups, the children had a serum phosphate higher by 1.8 mg. of P per 100 ml. of serum than the adults. The complete results are given in Table III. DISCUSSION The method is based upon the yellow color formed when an excess of molybdate is added to an acidified solution of an orthophosphate and a vanadate. The exact nature of the reaction is uncertain, but presumably depends upon the formation of a heteropoly complex, (NH&*POa. NH,VOs- 16Mo03, such as that formulated by Mission (7). This method is less sensitive than the molybdenum blue methods, but is adequate for most biological applications, and has the great advantage of providing a stable solution for calorimetry. We feel that this advantage, together with a lower sensitivity to changes in final acid concentration, outweighs the advantages of the molybdenum blue methods for orthophosphate determinations. SUMMARY 1. The molybdivanadate method for orthophosphate, as described by Kitson and Mellon, is suitable for the determination of serum phosphate. 2. The solutions obey Beer s law, and the method may be used spectrophotometrically for concentrations of PO,-P between 0.5 and 16 y per ml. 3. The increase in calorimeter readings was not found to be strictly proportional to the concentration of phosphate, but this appeared to be due to the characteristics of the instrument. The method is applicable to the photoelectric calorimeter for the range of concentrations from 2 to 16 y of P per ml. 4. The serum phosphate in twenty-one normal, adult males was found to average 3.8 mg. of P per 100 ml., with an extreme range of 2.8 to 4.8 mg. per 109 ml. The values in ninetcen females averaged 4.0 mg. per 100 ml., with a range from 3.5 to 4.9 mg. per 100 ml.

9 SIMONSEN, WERTMAN, WESTOVER, AND MEHL The serum phosphate in twenty-three children had an average value of 5.7 mg. of P per 100 ml., and varied from 4.6 to 7.2 mg. per 100 ml. BIBLIOGRAPHY 1. Woods, J. T., and Mellon, M. G., Ind. and Eng. Chem., Anal. Ed., 13,760 (1941). 2. Kuttner, T., and Cohen, H. R., J. Biol. Chem., 76,517 (1927). 3. Fontaine, T. D., Innd. and Eng. Chem., Anal Ed., 14,77 (1942). 4. Willard, H. H., and Center, E. J., Ind. and Eng. Chem., Anal. Ed., 13,81 (1941). 5. Koenig, R. A., and Johnson, C. R., Ind. and Eng. Chem., Anal. Ed., 14,155 (1942). 6. Kitson, R. E., and Mellon, M. G., Ind. and Eng. Chem., Anal. Ed., 16,379 (1944). 7. Mission, G., Chem. Ztg., 32,633 (1908).

10 THE DETERMINATION OF SERUM PHOSPHATE BY THE MOLYBDIVANADATE METHOD Daisy G. Simonsen, Maxine Wertman, Leola M. Westover and John W. Mehl J. Biol. Chem. 1946, 166: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at tml#ref-list-1