Digitized 2007 AES.Includes bibliographical references (page 10)

Affsprung, H. E. (Harold E.)

Gehrke, Charles W.

Lee, Yung C.

University of Missouri: MOspace

RESEARCH BULLETIN 570 OcrOBER, 1954 UNIVERSITY OF MISSOURI COLLEGE OF AGRICULTURE AGRICULTURAL EXP ERIMENT STATION J. H. Longwell, Db'mar Direct Ethylenediaminetetraacetate Titration Procedure for Calcium In Biological Substances Anion Exchange Separation of Phosphate C HARLES W. G EH RK E, H AROLD E A ffSPLUNG AND Y UNC C. 1.£E (Publication authorized Ckrober 14, 19)4) COLU~{8IA, MISSOURI TABLE OF CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3 Apparatus and Reagents . .... .. .. .... .. .. ............... . . .. . 4 Analytical Procedure ... .......... . . . . . .. . . . . . . . • . ... . . . ... . 4 Preparation of Sample .... . .. . ............................. . 4 Removal of Phosphate . . ... .. . . . .. . .. . . . .... . . . . . . . . . ..... 5 T itration of Calcium ............. . ... . .•. . ...... . ... . ..... 5 Calculations .. ... . . ....... .... ........... . ... . . . ... .. . Results .......... .. ..... .. . ... . ... ... . . ... . . .. ....... . . .... . , 6 Discussion .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 9 Literature Cited . ....... ..... . ... . ... . . . ... . . .• .. . ... . . . . •...... 10 Report 00 Depanment of Agricultural Chemistry Research Project 132 , " AoalyriCll Services" Direct Ethylenediaminetetraacetate Titration Procedure for Calcium In Biological Substances Anion Exchange Separation of Phosphate CHARLES w. GEHRKE, HAROLD E. AFFSPR UNG AND Y UNG C. LEE IN T RODUCTION Methods have been devdoped, based upon the original work ofSch-warzenbach and co-workers (6, 16, 17), for the analysis of calcium and magnesium in water (4, 5, 9,10), limestone and soils (2, 7, 8). Banks (3) determined the calcium and magnesium coment of coal ash and ceramic materials, and Mattock and Hernandez (15) used disodiurn dihyclrogen ethyl-enediaminetecraacetate (ETA) for the dcrermination of calcium in pharma-ceutical products. Stephens (18) used an excess of a much scronger solution of ETA chan most investigators and then back-titrated with standard cal-cium and magnesium solutions. He applied the method for the analysis of calcium and magnesium in pharmaceuticals, calcium gluconatc, calcium sulfate, and phosphate. Attempts have been made to apply the methods for the determination of calcium in plant materials (7, 11, 19). In nearly all cases, the procedures developed require tWO tinations, a titration for cal-cium plus magnesium using Eriochrome Black T as the indicator, and a titration for calcium with murexide as the indicator, the magnesium being calculated as the difference between the twO titrations. Difficulties are en-countered in the calcium titration when orthophosphate ions are present. A direct method has been developed for calcium in milk and milk fractions (13). A method (12) has also been developed for the direct determination of magnesium in limestone, employing a separation of calciu m as calcium sulfite; this method is also applicable to some biological materials. Mason (14) worked out a method in which calcium was separated from phosphate by a cation exchanger. The calcium was then eluted from the exchanger lnd titrated with ETA in the effiuenc. A definite need was realized for a direct t itration procedure for calcium in biological materials which would be free of interference due to phosphate ions. This bulletin describes a rapid and accurate "anion-exchange Separa-tion-direct titration procedure." APPARATUS AND REAGENTS Artificial tight source, such aJ Precision Scientific Co., Titra tite. Cat. No. 9555. pHydrion indicator paper. MicroburetJ, .s and 10 mi. Voillmetric jia.rkJ and pipetJ. Anion exchange min. Amberiite IR-4B, an anion exchanger with a high capacity for phosphate was used. The resin particles are 0.4 to 0.6 mm. in size, and approximately 20-50 mesh. This resin can be obtained from the Rohm and Haas Co., Philadelphia, Pennsylvania. Anion exchange column. Regenerate the resin ro (he chloride form by the batch process. First exhaust it with three separate portions of 5% sodi-um carbonate or sodium hydroxide. Wash until all excess base is removed. Then treat the resin with at least three separate portions of5% hydrochloric acid with stirring. Rinse the resin with distilled water until thete is no further color throw. Prepare a glass column approximately 23 cm. long and 2 cm. in diameter. Divide the column 5 cm. from the bottom by sealing in a coarse porosity sintered glass plate. Attach a 2mm. two-way stopcock to the bottom of the column for regulating the flow of solution through the column. Place 30-50 grams of IR-4B-Cl in the column. The column of resin is about 9 cm. in height. Stir the resin to remove air bubbles and maintain the water level above the resin surface. Calcium indicator. Intimately mix 40 grams of C. P. potassium sulfate and 0.2 gram of murexide powder (Eastman Kodak Product) in a mOrtar (') . PofaJJillfJI hydroxide Jollltion, 10%. Dissolve 10 grams of reagent grade potassium hydroxide in 100 m!. of distilled water. Standard calcillm Joilltion. Dissolve 2.4972 grams of reagent grade cal-cium carbonate, previously dried at 110 0 c., in dilute hydrochloric acid. D ilute to 1 liter with double distilled water. This solution contains 1 mg. of calcium per milliliter. Standard Jolutions of disodillm dihydrogen ethylenediaminmtraacttate (ETA), 0.4 and 0.1%. Dissolve 20 grams, or 5 grams of the reagent in doub-le distilled water, and dilute to 5 liters. Standardize the solution against a standard calcium solution. T he titers are approximately 0.44 and 0.11 mg. of calcium p<::r milliliter, respectively. Stmldard phosphate Jotlltion. Dissolve 2.8658 grams of C. P. potassium dihydrogen phosphate in double distilled water, and make ro tliter. This solution contains 2.00 mg. of phosphate ion (PO.) per milliliter. ANALYTICAL PROCEDURE Preparation of Sample Ash a 10 gram sample of the plant materia! overnight in a quartz cru-cible at 5500 C. Place the sample in a cold furnace and gradually increase RESEARCH BULlETIN 570 , the temperature to 5500 C. Add 75 ml. of 1:1 hydrochloric acid and 2 drops of concentrated nitric acid. Digest the sample on a hot plate or steam bath for 2 hours and finally take to dryness. Add about 3 ml. of 1:1 hydrochloric acid to the residue and approximately 100 ml. of distilled water and digest for about 1 hour. Allow the solution to cool and transfer quandtatively to a 250 ml. volumettic flask. Fill to the mark with distilled water and mix thoroughly. Removal of Phosphate Transfer a 25 mt. aliquot of the sample solution to a 250 ml. beaker. The aliquot should contain between 10 and 15 mg. of calcium. Neutralize the solution to a pH of 3 to 4 with a 10% solution of potassium hydroxide using pHydrion indicatOr paper. Pass the sample solution through the col-umn of anion exchanger in the chloride form, and collect the effluent in a 300 mI. Erlenmeyer flask. Control the rate of flow to abom 2 to 3 mL per minme. Wash the resin column thoroughly with 150 ml. of distilled water in thtee separate porcions. Pass the first portion of 50 m I. through at the same rate as the sample. Pass the second portion through at a rate of about 9 ml. per minute. Allow the final porcion to pass through the column freely . Twelve samples can be handled at one time using a bank of 12 columns. The exchange capacity for phosphate of one column containing 30 grams of resin is about 1800 mg. Thus it is possible to pass a large number of ali-guots through a column before regeneration of the resin is necessary. T it ration of Calcium Add 5 ml. of 10 per cent potassium hydroxide and about 50 mg. of murexide indicator to the effluent and washings in the Erlenmeyer flask. Titrate the sample, with swirling, using a standard solution of ETA of the desired ticer. The end point is from salmon pink to a deep pu rple. At the end point no pink coloration is observed upon viewing the soll,lcion against a background of artificial light. Ie is always preferable to titrate with an artificial light, and a blank is used for comparison. Calculations T he calcium Content may be calculated as follows when a lO-gram s~mp.le is ashcd, made to 250 mL volume, and a 25 mr. aliquot is used for tltranon. AxB % I ' _:- = ca c!Urn 10 A = milligrams of calcium/ml. of titrant B = milliliters of titrant 6 MISSOURI AGRICULTURAL EXPE RIMENT STATION RESULTS The results of a series of determinations in which increasing amouncs of phosphate were added to known amOunts of calcium are shown in Table 1. These known solutions were prepared co give ratios of phosphate to cal-cium from less than 1:1 to 20:1. After the solutions were passed through the anion exchange column, the calci um was titrated with ETA, using murexide as the indicator. The effluents were also analyzed for phosphorus by the volumetric A. O. A. C. method (1); none was found. T he recovery of calcium was excellent for the solutions containing a small amount as well as a large amount of phosphate. The standard deviation was found to be ±O.017 for 12 samples containing 10 mg. of calcium and different amountS of phosphate ranging from 6 to 100 mg. TABLE 1 - - RECOVERY OF CALCIUM FROM STANDARD SOLUTIONS ~~6.~OO CL.---~1O.00===~~~~:;;~~ 10.00 10.00 10.02 +0.02 16.00 10.00 10.02 +0.02 20.00 10.00 10.02 +0.02 26.00 10.00 10.02 +0.02 36.00 10.00 9.99 - 0.01 40.00 10.00 9.99 - 0.01 46.00 10.00 9.97 -0.03 50.00 10.00 9.98 -0.02 56.00 10.00 9.99 - 0.01 60.00 10.00 10.00 0.00 80.00 10.00 9.98 - 0.02 70.00 5.00 5.00 0.00 90.00 5.00 5.00 0.00 100.00 5.00 4.99 _0.01 Stand:a.rd devlaUon • + 0.017 a Phospllate added as potassium dlhydrogen phosphate. The data obtained upon analyzing various forage and feed samples are presented in Table 2. The samples were chosen to have a range in phos-phate coment, as well as of calcium. T he determinations were conducted by both the classical oxalate method, as described in the O fficial Agricul-tural Chemists (I), and the proposed" ion-exchange - ET A titration pro-cedure." For samples 1,2, 3,4, and :5, three independent ashings were made at different times. For each ashing, cwo or three replicate determinations were made by both methods. For sample No.6 only one ashing was made and a triplicate determinacion was conducted by boch methods. The phos-phate (Po., ) content of these samples ranged from 0.:54 per cent to 6.48 per cent. It was observed chat the phosphare ions were effectively removed by TABLE 2 __ DETERMINATION OF CALCIUM IN FORAGES BY THE CLASSICAL 0.200 0.244 -.023 ... 002 0.227 0.240 ... 004 -.002 (Forage) 0.268 0.242 ... 045 0.000 0.2 13 0.240 - .010 - .002 0.222 0.240 -.001 _.002 0.209 0.244 -.014 .... 002 0.207 0.240 -.016 -.002 0.234 0.242 .... 011 0.00<1 0.244 ... 002 Average .. 0.223 0.242 ... 0.019 standard Deviation" 0.020 0.0017 2 O.SO 0.847 0.836 0.000 •. 009 0.633 0.818 _.014 - .009 (Fouge) 0.847 0.826 0.000 _.001 0.861 0.827 ... 014 0.000 0.864 0.820 .... 017 -.007 0.650 0.822 ... 003 _.005 0.840 0.836 -.007 .... 009 0.830 0.827 -.017 0.000 0.833 ... 006 Average .. 0.847 0.827 - 0.020 standard Devlatlon .. 0.0111 0.OOS3 3 0.54 1.177 1.106 ... 012 ... . 003 1.170 1.102 ... 005 -.001 (Forage) 1.177 1.104 ... 012 ... 001 1.154 1.102 -.Oll - .001 1.156 1.104 -. DOg +.001 1.158 1.098 -.009 -.005 Average" 1.165 1.103 - 0.082 standard Deviatlon .. 0.0100 0.0025 • 6.48 1.693 1.787 _.059 ...029 1.693 1.787 _.059 ... 029 (small 1.644 1. 778 -.108 .... 020 animal l.'l93 1. 747 +.041 _.011 ration) 1.814 1. 747 ... 062 -.011 1.809 1.742 .. . 057 -.016 1.783 1.744 ... 031 - .014 1.784 1.747 ... . 032 -.011 1.747 -.011 Average .. 1.752 1.758 ...0.006 standard Deviation .. 0.0605 O.018S 5 1.47 0.812 0.879 _.055 _.001 0.840 0.889 _.027 • • 009 (Dehydrated 0.848 0.880 -.019 0.000 spinach) 0.898 0.880 ... 031 0.000 0.902 0.880 .... 035 0.000 0.883 0.882 .... 016 •• 002 0.888 0.880 ... 021 0.000 0.871 - .009 0.880 0.000 Average .. 0.867 0.880 .. 0.013 standard DeViation .. 0.0318 0._ 6 0.86 1.280 1.251 ... . 004 -.001 1.280 1.253 .. . 004 ... 001 (Forage) 1.287 1.251 - .009 - .001 Average .. 1.276 1.252 -0.024 standard Deviation .. 0.0053 0.001 Average Standard Deviation ., .. 0.023 +0.0057 a Phosphate expressed as pereent P04 8 MISSOURI AGRICULTUR .... L E XPERIMENT STATION the anion exchanger. The difference in the aver:ages of the per cent calcium between the oxalate and ETA methods ranged trom -0.062 to +0.019 pcr cent, which is within the analytical tolerance usually :l.llowed for the classi-cal method of analysis for C:l.icium. The precision of the ETA method was excellent and much better than for the oxai2rc method; however, the preci-sion of the oxai:uc method was also considered good. Refer to T:l.ble 2 for compansons. Recovery experiments were made using various types of forages, such as ladino dover, orchard grass, red clover, soybeans, and alfalfa. Known amounts of calciu m were added as the chloride to aliquots of the original ash solution of the s:amples. The final concentntion of calcium in the :1liquoc was between 10 and 16 mg., as chis quanticy gave the smallest titration error. The solutions were adjusted to a p H of 4 to 5, then passed through [he anion exchange column. The recoveries of calcium ranged from 97.S to 100.6 per cent, with an average recovery of99.3 per eeoc. These recoveries were considered s'l:(isfactory. A study was made to determine the extent of the titration error when using ETA as the titrant and murexide as the indicator (Table 3). All of TABLE 3 -- RECOVERY OF CALCIUM FROM A STANDARD SOLUTION OF 2.00 5.00 10.00 15.00 20.00 25. 00 30.00 40.00 50.00 2.00 5.02 10.04 15.04 20.40 25.61 30. 76 40.80 50.98 ERROR a Each value is all; average of two independent determlnatlons. .0.02 .0.04 .0. 04 .0.40 . 0.61 .0.'16 .0.80 .0.98 the volumetric glusware was alibrated and corrections applied when neces· sary. The resulcs show chat a significant positive error occurred when che quantity of calcium exceeded 20 mg. in the solution being titrated. No significant error was found when smaller amounts of calciu m were ti trared. The end point was usually elusive, due pardy to the competition of the ETA and the murexide for the calcium and also perhaps to air oxidation of the indicator. The pink colof returns to the solution a few seconds after the stoichiomeuic point is reached. The end point used was that point at which . a complete disch:arge of the pink color was observed. W ith experience and a good artificial light source one can become proficien t in detecting the purple end point. To avoid a titration erro r in routine application of the RESEARCH BULLETIN no 9 method, the aliguots were always chosen so as to comain IS mg. or less of calcium. DISCUSSION The "anion-exchange separation-ETA titration method" is well suited to the routine analysis of all types of biological materials. At the Missouri Agricultural Experiment Station during the past twO years, hundreds of anal-yses have been made by this method on plant materials, milk, urine, serum, experimental rations, grains, and ocher substances in which phosphate in-terferes with the titration. The results obtained by the proposed method are superior in accuracy to those for the accepted classical oxalate method, and the procedure is much less time consuming. In this study, the number of analyses made by each method on forages and rations was approximately 50. The average differ-ence in per cent calcium was found to be 0.024 for al! the results by the pro-posed method, compared to the result by the oxalate method. In an overall evaluarion, the ETA values were slightly lower than the oxalate values. This average difference, 0.024, was nearly egual to the average standard deviation of the oxalate method which was + 0.023. T hese data show that the re-sults of the proposed method are in good agreement with those of the classi-cal oxalate procedure. The small average srandard deviation of + 0.OOS7 for the proposed procedure indicates that the reproducibility is excellent. If only a small amount of sample solution is available, the ion exchange column can be made proportionally smaller. It is important to keep the guantity of calcium below 20 mg. in the sample solution being titrated, as a significant titration error occurs when attempts are made to titrate larger amounts. Large amounts of potassium chloride were found to have no effect on the analysis for calcium. Dry ashing was preferred over the various wet ashing procedures for the removal of organic material in the forage samples. The dry ashing pro-cedure is rapid and convenient. When wet ashing is used the anion ex· change columns become exhausted more guickly as a result of the removal of sulfate, perchlorate, and nitrate anions used in the wet ashing method. As a result more time is consumed in regenerating the resin and refilling the columns. Several hundred samples of feces, foods, and urine were wet ashed usir,g nireic, sulfuric, and perchloric acids. When aJiguots of th ese solu· tions were passed through (he columns for phosphate removal, a more fre-guent change of resin was needed as the aliquots contained large quantities of anions. Such difficulties were not experienced when the same samples were dry ashed. When a column containing 30 grams of resin is used, a large number of aliquocs can be passed before regeneration is necessary. 10 M ISSOURI A G RI CULTURAL EXPERIMENT STATION LITERATURE CITED I . Assoc. of Official .... grie. Chemists, "Book of Methods of ""n,lysis," 7th Ed., ( 19~O). Box '40, Benjamin Franklin Sluion Washington". D. C. 2. &new!!,).).. 2nd Kenner, C. T. , Detcrminalion of Calcium and Magnesium in Limestones and DolomiteS, An"L Chern., 2~, 1186 ( 19'2). 3. &nJcs.).. The Volumeuic Detcrmin:nion of Calcium and Magnesium by the Ethyl. enedianUner(Il":l1Cet:lIC Method, Analyst , 77. 484 (19'2). 4. Ben,). D., and Noll, C. A., Toul·Hardness Determination by Direct Colorimetric Titntion, j. Am. Water Works Assoc., 42, 49 (19'0). '. 5. Ibid., p. 749. I 6. Biedermann, W " and Schw..nenbach, G .. CompJexons XI. The Complexomcnic Titration of Alkaline Ean:iu and Some Other Metab with Eriochrome Black T., Olinu. PN.gue, 2, Xi (19-48). 7. Cheng, K. L. , :and Bray, R. H., Determination of Cdcium and Magnesium in Soil and. Plant Material, Soil Sci .• 72. 449 (19~1 ). 8. OIeng, K. L. , Kurtz , T .. and Bray, R. H., Determinacion of Calcium, Magnesium, and Iron in Limestone. Anal. Chem., 24, 1640 ( 1~2) . 9. Connon.).)., Advances in Chemical and Colotimetric Methods,). Am. Water Works Assoc., 42, H (1950). 10. Diehl, H., Goetz, C. A .• and Hach. C. C . The Vel"$C:n1te Titration for Totd Hard. ness. Ibid. , p. 40 (1950). 11. GascJer, G. F., Proc. S. Dakota Acad. Sci., 28, 77 (1949). 12. Gehrke, C W .. Alfsprung. H. E., and lLc, Y. C, A Dir~t Disodium Dihydrogen EthylenediaminctetJUCeme Titration Procedure fot Magnesium in Limestone, AIuI. Chern .• Submitted for publication (1953). 13. Jenness. R., Titration of Calcium and Magnesium in Milk and Milk Fractions with Ethylenediamineterrucetate. Anal. Chern., 25. 966 (19H). 14. Mason, A. S., The Det(:rmination of Small Amounts of Calcium in Plant Materia ls, Analyst, 77, 529 (1952). 1'. Mattocks, A. M., and Hernandez, H. R., ASsay of Official Calcium Products by the SchWUleninch Method, ). Am . Pharm. Ass'n., Sci. Ed., 39, 519 (1950). 16. So:hWUlenbach, G ., and AeJ.:ct"mann, H., ComplotOni V. EthylenediaminC"te1:r:a:Ketic Acid., Hd v. Chim. Acta ,;m, 1798 (19047). 17. So:h"'"arzenbach, G. , Biedermann, W., and Bangerter. F., Complexons VI. New Simple Titrating Methods for Determining the Hardness of Water, Ibid., 29, 811 (1946). 18. Stephens, R. L , The Volumetric Determination of Calcium and Magnesium,). of Plurm. and Pharmacology. 5, 709 (19H). 19. Willson. A. E., Volumetric Determination of Cakium and Magnesium in Leaf Tis-sue, Anal. Chern. , 22, 1'71 (19'0). 

Direct ethylenediaminetetraacetate titration procedure for calcium in biological substances : anion exchange separation of phosphate

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