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 569 OCTOBER, 1954 UNIVERSITY OF MISSOURI COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION J. H. Longv.-ell, Dil'«t(n" A Direct Disodium Dihydrogen Eth y lenediaminetetraacetate Titration Procedure for Magnesium in Limestone c. w. GEHRKE, H. E. AFFSPRUNG AND YUNG C. LEE (Publica.tion authotized October 14, 1954) COLUMBIA, MISSOURI TABLE OF CONTENTS Introduction . ... . . . . ...... ....... .... . . . . .. . ... . . . .. . . • ... ... .. 3 Appantus and Reagents . . .. . ... . . .. . .. . .. ..... . . .. . ..•. . .. . . .. .. 4 Analytial. Procedure . . . . . . . . . . . . • . • . . . . . . . . . . . . . . . . . . • . . . . . . . . . .. 4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . • . . . . . . . . . . ~ Discussion .. .. .... ..... . . . . . . . ... . .. ... .. ... ... .. . .•. ... . . ... . 8 litef2rure Cited .. . . . .• .. . . . . • . . .• . . .• . . . . . .... . •. . . . • . . .. . . .... 10 Report o n Department of Agricuhural Chcoilitcy Research Project No. 132, "Analytieal Services" A Direct Disodium Dihydrogen Eth y lenediaminetetraacetate Titration Procedure for Magnesium in Limestone CHARLES W. GEHRKE, H AROLD E. A FFSPRUNG AND Y UNG C. LEE INTRODUCTION Beause of the large number of limestone samples and plam materials which are analyzed each year in comrollaboratories, any procedure whic~ will allow (he npid determination of calcium and magnesium will ~ of great value. In the past few years magnesium has received more :mention as a major plant nutrient and many reguests are now made for its determina-ti on, especially in Ii mescone. T he classical standard methods of analysis involve time-consuming precipiutions and separadons. Sch~zenbach and co-workers (5, 13, 14) introduced ethylenedi'-!TIine-tctrOlllCetic acid as a com plexing agent for calcium, magnesium, iron, lind other metals. Shoedy thereafter new analytical methods, employing disod-ium dihydrogen ethylenediaminetecraacecate (ETA) as the tim,nt for these elementS, were developed which were rapid and accurate. Many applica-tions of these methods have been made in analyses of soils and limestones (6,7, S), water (3, 4, 9, lO), and plant materials (6, 11, 15). ~nerally chese methods involve twO eitrations, a titration using Eriochrome Black-T as indicator for both calcium :md m:otgnesium, and a titration using murexide as indic2tor for calcium. The following investigators have developed a direct timtion procedure for magnesium in limestone (I) and water (3, 10) using oxalate as the pre-cipitating agent for calcium. Banewicz and Kenner (1) encountered diffi-culties in detecting the F-241 end point when titrating for magnesium after separating the calcium as oxalate. They stated th:lt the end point in the titra-rion was :lffected by the excess oxalate present and was indistinct and uns:nis-factory. However, a satisfaclOry reproducible end point was obtained by these investigators after the addition of hydrogen peroxide. Diehl, GO("tz, and Hach (10) coupled the versenate method with the calcium oxalate sepa-ration and obtained fairly s:lrisfactory results for calcium and magnesium in water. Betz and Noll (3) determined calcium in water directly with the ETA titration and murexide indicator. It was reported that this method was 4 MISSOURI AGRICULTURAL E XPERIMENT STATION more rapid and convenient and subject to fewer technique errors than the method of precipitating calcium as the oxalate and ti tradng the magnesium in the filtrate. The present paper describes a simple, "direct disoclium dihydrogen ethylenediaminetetraacecate citr:l.tion procedure" for magnesium in lime-s~one. The method is rapid and accurate. APPARATUS AND REAGENTS Artificial light SOUret, such as Precision Scientific Company Ticra-li te. Cat. No. 9555. Filter paper, Whatman N o. 5 or equival=nt. pHydrion indicator paper. Buffer solution 1,pH 10. Dissolve 67.5 g rams of C. P. ammonium chloride in 200 mi. of distilled water, add 570 ml. of reagent grade concentrated ammonium hydroxide, dilute to 1 liter (10). Buffer solution II,pH 8. Dissolve 80 grams of C. p , ammonium chloride in 200 mi. of distilled water, add 60 ro!. of reagent grade concentrated ammonium hydroxide, dilute to 1 liter. Eriochrome Black-T indicator (P-24 1)' D issolve 0.2 gram of the indicator powder (Eastman Kodak, P6361) in 50 mi. of A. R. methyl alco hol containing 2 grams of hydroxylamine hydrochloride (6). Potassium cyanide, 2% aqueous solution. Use reagent grade potassium cyan-ide. Standard solution of disodium dihydrogen ethylenediaminttetraamate (ET /1) 0.4 and 0.1%. Dissolve 20 grams or 5 grams of the reagent in double distilled water, and dilute to '5 liters. Standardize the solution against a standard magnesium solution. The ti ter is approximately 0.27 or 0.068 mg. of magnesium per mI., res~ctively. Standard magnesium solution. D issolve 1 gram of A. R. magnesium turnings in dilute hydrochloric acid, dilute to 1 liter with double distilled water. This solution contains 1 mg. of magnesium per mI. Sodium sulfite solution. Dissolve 20 grams of C. P. sodium or ammonium sulfite in 100 mL of double distilled water. Prepare this solution JUSt before use. ANALYTICAL PROCEDU RE W eigh a 0.5 gram limeStOne sample of 60-80 mesh directly into a 12'5 mI . Erlenmeyer flask. Add '5 ml. of 1 to 1 hydrochloric acid, warm gently, and evaporate to dryness on a hot plate or steam bach. Redissolve the residue in 1 m!. of 1 to 1 hydrochloric acid. Add about 2'5 ml. of water. Adjust the p H co 5 to 7 by adding buffer solution II. D etermine the correct pH using pH ydrion paper. Add, dropwise with swirling, 10 ml. of a freshly prepared RESEARCH B ULLETIN 569 , 20% solution of sodium o r ammonium sulfite. Allow the solution to stand for about a half hour, then filter through a Whatman No.5 or other hard and retentive filter paper. W ash the residue thoroughly with 30 to 50 mI. of a di lute, 1%, slightly basic solu tion of ammonium or sodium sulfite. Collect the filctate and washings in a 100 mL volumetric flask, dilute to the mark. Pipet a representative aliquot of the fil trate into a 300 ml. Erlenm~'er flask. Add 50 mi. of d isdlled water, :> ml. of buffer solution I, 2 ml. of po. tassium cyanide solution to com plex: traces of heavy metals, and 10 drops of F·241 indicator. Titrate the sample with ETA until the solution changes color permanencly from wine· red to pure blue. The color change approaching the end point is gradual; therefore, a blank has been prepared fo r comparison purposes, although its use is not necessary. It was also found preferable to ti tute using an artificial light. Calculation; A x B x Factor = Percent magnesium carbonate. A = mg. of magnesium/ mt. o f titrant. B = ml. of titt:m t. F lO at.33 actor = O .!IOOO ~l000"ft.H"IOO . $.94, when a 0.5000 gram sample and a 10 ml. :l. liquot :l.re used. RESULTS Standard samples of limestOne were selected with percentage of mag· nesium oxide ranging from 0.85 to 21.82. Some of these samples contained more than 1 percent of iron. The data. in Table 1 show that the results ob· TABLE 1 -- DIRECT DETERMINATION OF MAGNESIUM IN STANDARD LIMESTONES Magnesium OJdde, Percent Sample Nos. Hach Reported Deter mined Chem. Co. Value Valuea OUference 1000 0.85 0.89 +0.04. .001 21.82 21.54 _0.28 .00' 0.81 0.89 +0.08 . 00. 1.67 1.85 +0.18 ' 007 2.24 2.46 +0.22 1015 4. 53 4. 51 -0.02 "140 13.38 13 .33 -0.05 "47 15. 38 15.10 _0.28 "., 19.08 19. 05 -0.03 NBSb 88 21.48 21.31 -0.17 Average a Each va lue ill an average of three Independent determinat ions. b National Bureau of Standard, Standard Deviation ""~. Method 0.05 0.1 4 0.01 0.00 0.01 0.00 0.02 0.04 0.01 0.05 :Q.ri'S 6 MISSOURI AGRICULTURAL E XPERIMENT STATION rained by the "direct ticr:ation" procedure art in agreement with the reported values. The average sranclard deviation was found CO be ± 0.03. Six co rn-merciallimestones were selected which varied in composition from 0.5 to 20 percent magnesium oxide. These samples were analyzed by the "direct titration" and the gravimetric oxinate (12) methods. The results obtained by the twO methods agreed closely. T he dara are presented in Table 2. TABLE 2 -- COMPARATIVE VALUES OF MAGNESIUM IN COMMERCIAL LIMESTONES BY -OffiECT TITRATlON- AND OXlSATE METHODS , 1.00 7.19 +0.19 • 9.70 9.47 -0.23 5 16.07 15.87 -0.20 , \9.61 19.58 -0.03 Average Standard Devl1t1on ~O.04 .. 0.05 a Each val",e Is an average of three Independent deterlllinations. A recovery study was made, the dau. of which are given in T2.ble 3. Ten st2ndard s2.mples oflimesrone obt2.ined from the H2.ch Chemical Company and the National Bureau of Standards were weighed :md dissolved in 1 to 1 hydrochloric acid, warmed gencly, then evaporated to dryness. The follow. ing were added co the residue; an aliquot of a standard magnesium chloride solution containing 50 mg. of magnesium, 1 m!. of 1 to 1 hydrochloric acid, 2nd 10 ml. of water. T ho.": sample solutions were then buffero.":d to a pH of 5 to 7, the R20 1 precipitated, and the calcium separated as calcium sulfiee. The filtrates were diluted co 100 ml. , and 10 m!. aliq uocs were taken for the magnesium rit rat ions. Th o.": average recovery of magnesium was found to be 99.04 percent. T he recovery of 99.04 percent of the magnesium added co standard li mesrones is slightly low but is reasonably satisfactory. T he reo covery range was 98.07 to 99.84 percent. T he percentages o f magnesiu m oxide in the standard samples uSo.":d in the recovery studies ranged from 0.85 to 21.82 perco.":nt. The amount of magno.":sium addo.":d to these samples rango.":d from a facror of 2 times (he magnesium preso.":nt to a factor of about 25. Thae was no correlation between low recovery values fo r magnesium and the chemical composition of the samples, that is, the content of calcium oxide, phosphorus pentoxide, magnesiu m oxide, silicon dioxide, or the A20, group. To determine the titration error, s..-:verai samples of a standard solution of magnesium chloride were titrated using Eriochrome Black-T as the in· RESEARCH B ULLETIN 569 TABLE 3 - _ RECOVERY OF MAGNESIUM ADDED TO STANDARD LIMESTONE SAMPLES Rich Chemical 1001 1002 100' 100'1 1015 1040 1047 1000 NBS' 88 MUUgrams of Magnesium In Totala in 65.61 115.61 114.97 114.70 114.90 2.44 52.44 52.01 52.01 52.01 5.04 55.04 54.81 54.95 54.95 6.76 56.76 55.75 55. 75 55. 75 13.66 63.66 62.57 62. 43 62.57 40.35 90.35 69.97 69.97 90.11 46.38 96.38 95.59 95.59 95.59 57.54 107.54 106.95 106.95 106.95 64.76 114.78 11 3. 10 113.36 113.36 a Fifty milligrams of magnesium were added to each sample. b National Bureau of standards. c Calculated from reported values. Percent 99.27 99.04 99.21 99.16 99.16 99.18 99.58 99.84 99.84 96.22 98.22 98.22 98.29 98.07 98.29 99.58 99.58 99.73 99.18 99.18 99.18 99.45 99.45 99. 45 98 .54 98.76 98.76 7 8 MISSOURI AGRICULTURAL EXPERIMENT STATION diator (Table 4). The end point and the stoichiometric point corce5ponded quite well when the amOunt of magnesium in the aliquot was from 1 to 20 mg. At a higher concentration a positive error was introduced due to the color ringe of the indicator at the end poine. However, in routine limestOne analysis, the aliquot taken for the tit ration by the proposed "direct method" seldom contains morc than 20 mg. of magnesium. A blank with the blue end point color was prepared for comparison purposes, although this W:lS not necess:l.ry. It was found desirable co dt~te using ~ arcificilll Auorescem ligh t 25 a background light. In chis way all of the analyrica.! work was con-dueted with the same background illumin2tion. The COIOI change approach-ing the end point was gradual, but definite, and a blank correction was nOt found to be necessary when the aliquot contained 20 mg. or less of mag-nesium. TABLE 4 -- RECOVERY OF MAGNESIUM FROM A STANDARD SOLUTION OF MAQNESIUM CHLORIDE =======~ c====== 2.00 2.00 5.00 5.00 10.00 10.01 +0.01 15.00 15.00 20.00 20.01 +0.01 25.00 25.09 .. 0.09 an aver age DISCUSSION The "direct titration" of magnesium in limestones is simple, accunte, and rapid. Several hund red commercial limestone samples have been as-sayed by this method in the Missouri Ex periment Snuion laboratories and the results have been very satisfactory. An average standard deviation of ± 0.Q3 was obtained by the "direct method" on 16 samples of standard and commercial limestones rangi ng from low to high (0.89 - 21.:54) in percent-age of magnesium oxide. At least 3 independent determinations were made on each sample. These data show that the method has a good degree of pre-cision. Most of the previously published methods for the analysis of mag-nesium in limestone or in plant and animal materials. using ETA as the titrant, involve twO citrations and the magnesim is calculated as a difference. Any errors inhetent in these twO determinations will be reflected in the cal-culated value for magnesium. The proposed "direct method" involves a sing-le titr;uion for magnesium only. The "direct titration procedure" for mag-RESEARCH BULLETIN )69 9 nesium requires less time th;tn the oxinate method or the "indirect ETA dtration method." The end point in the "direct method" is excellent. Also, the pure blue color is more pronounced wd stable. and does not &de as is commonly ex-perienced in the absence of sulfite. The end point color remains unch1nged for 24 hours Ot more. This improvement in end poim color is due to the ex-cess of sulfite ion present in the solution, 2S in the absence of sulfite the nor-mally observed f1ding occurs. Bray'S group (8) reporced that if a sample con-uins more th;tn 1 percent of iron, (he solution may turn from a blue to a brown color in;t few minutcs. This change in color did not occur in the pro-posed procedure, even when the samples conuined 4 percent or more of R:O •. In this meThod, the R, O, is largely removed when the soludon is buffered to a pH of) to 7 previous to the rcmovaJ. of Cl.icium by the addi rion of sodium sulfite. A rather large difference exists in the solubili ty of calcium and mag-nesium sulfites, (uSO,.2 H2 0 , 0.0043, and MgSO, . 6H%O 1.2) partS per hundred paIlS of Mter). The separation of calcium from magnesium is based on rhis observ;ttion. The sulfite soludon must be ;tdded slowly and at room temper;tture, for when the sulfite solution is added rapidly, or;tt elevated temperatures, excessively low results are obtained for magnesium. A quantitative spectrographic study was made on commercial and Standard limestone samples to determine: rhe extent of coprecipintion of magnesium sulfite with rhe R: O, group 1nd the calcium sulfite residue. The magnesium content of the residue was of the order of 1 to )000 to 1 to 2(l(X). A satisfaCTory sep:u-:a.tion of the magnesium from the a!cium was thus obtained by means of a single precipitation of the calcium 2S calcium sulfite. The recovery data seems to show considerable coprecipitation, but :1.S these d2t;t were obtained from s;tmples to which rnher large amounts of mag-nesium lud been added and the resulting solutions were quite concentrated, it is felt that these results represent the maximum error to be expected. The solutions used for the spectrographic study were much less concentrated than those used for the recovery study, by;t factor of 2 to 2) , and these data show that excesslve copredpitation does noc occur unless {he concenrruion of magnesium in the solutlons used is high. 10 MISSOURI AGRICULTURAL EXPERIMENT STATION LITERATURE CITED 1. &newic:t,j. j., and Kenner, C. T., Dcccrminuion of Calcium and Magnesium in Limestones and Dolomites, Anal. Chern., 24, 1lS6-7 (19~2). 2. Banks, ].. The Volumetric Determination of G.lcium and Magnesium by the Ethyl-ienediaminetetn:u:(t:atc Method, Analyst, 77,484-8 (19'2). 3. Bc:tt.j. D ., and Noll, C. A., TonJ Hardness Determina!ion by Direct Colorimetric Ticr2cion, j. Am. Water Works Assoc., 42, 49·56 (lsryO). 4. Ibid., pp. 749-H. 5. Biedermann, W., and Schwuzcnbach, G., Complc:xons XI. The: Complexomc:tric TiUlltion of Alk2.1ine: E.mhs and Some Other Metals With Eriochromc Bla<:k T., Chimia Pugue, 2,)6.9 (190'8). 6. OIeng, K . L., and Bny. R. H ., Detcrminuion of Calcium and Magnesium in Soil and Plant Materill , Soil .xi., 72, 449-'8 (1951). 7. Oicng, K . L., Bray, R. H md KUHz, T., Determinuion of Toal Iron in Soils by Oisodium Oihydrogen Ethylenediamineteuaaceme Titmion. Anal. Chern .• 25. }47-8 (1953). 8. Cheng, K. L . Kum, T., and Bray. R. H. , Determination of Calcium. Magnesium. and Iron in Limestone, Anal. Chern., 24, 1640-1 (1952). 9. Connors. J]', Advances in Chemieal and Colorimetric Methods.]. .... m. Water Works .... ssoc., 42, H·39 (1950). 10. Diehl, H .• GoetZ, C. A. , and H11.ch. C. C. T he Venenate Titration for Toal Hard· ness, J Am. W11.ter Works Assoc., 42. 40·48 (1950). It. G;utler, G . F .• Proc. S. Dakota .... cad. Sci.. 28. 77-81 (1949). 12. I lilkbnnd, W . F., and Lundell, G . E. F .• Applied Inorg:lOic Analysi5. John Wiley 11.nd Sons. Inc .• New York, 1929. 13. Schwuzenbach, G., and .... kermann. H., Cornplexons V. Ethylenediamineteuncetic Add, Helv. Chim. Am. 30, 1798·1804 (1947). 14. Schwarzenbach, G ., Biedermann, W., and Bangerter, F., Complexons VI. New Simple Titrating Methods for Determining the H udness of Water, Ibid., 29, 811·18 (1946). 15. Willson, A. E., Volumetric Determination of Calcium 11.nd Magnesium in l.eafTis· sue, .... n11.1. Chern., 22, 1571·2 (1950). 

A direct disodium dihydrogen ethylenediaminetetraacetate titration procedure for magnesium in limestone

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