New Indicator for Titration of Calcium with (Ethy Ienedin itriI0) Tet raacetate JAMES PATTON and WENDELL REEDER Campbell Taggart Research Corp., Dallas, r e x .
A new in'dicator for calcium, 2-hydroxy-l-(2-hydroxy-4sulfo-l-naphthylazo)-3-naphthoicacid, has been synthesized which permits an accurate titration of calcium in the presence of magnesium with (ethylenedinitrilo) tetraacetate. The indicator produces a sharp, stable color change from wine red to piire blue. Calcium and magnesium separations are not necessary. Titration procedures for the determination of calcium and magnesium in water, limestone, salt, and boiler scale are described. The usual ammonia-ammonium chloride buffer is replaced by an odorless monoethanolaminehydrochloric acid buffer containing coniplexed magnesium.
1'
H E determination of calcium and nidgnesium by titration with standard solutions of disodium dihydrogen (ethylenedinitrilo) tetraacetate using Eriochrome Black T as the indicator has been investigated thoroughly and widely accepted. Following the original work of Biedermann and Schwarzenbach ( d ) , Diehl, Goetz, and Hach ( 5 ) utilized the titration for the separate determination of calcium and magnesium by precipitating the calcium as the oxalate and then titrating the magnesium after filtration. Banewicz and Kenner (2) further studied the oxalate separation method for calcium and magnesium in dolomites and limestones. More recently, Gehrke, .Iffsprung, and Lee (8) adopted a sulfite separation method for the determination of magnesium r h e n calcium is present. Schwarzenbach, Biederniann, and Bangerter (13) showed that calcium can be titrated in the presence of magnesium when ammonium purpurate is used as the indicator. Betz and No11 (3) studied the calcium det,ermination, using ammonium purpurate as the indicator, and found the tit,rat,ion acceptable for use in m t e r analysis. Calcium has been determined in biological fluids with (ethylenedinitrilo) tetraacetate, using ammonium purpurate as the indicator (6, 7). The main objections to the determination of calcium or magnesium separately by the compleximetric methods offered to date are indistinct end points and/or the necessity of separating the metals prior t o titration. Hoffman and Shapiro (10) used a n automatic photometric tit,rator to obtain improved end points. T h e advantage of a method which gives a good visual end point' and permits a rapid titration of calcium in the presence of magIiesiiim is apparent'.
Table I.
Sample No.
Determination of Calcium and RIagnesiunl in Water Samples Magnesium! P.~P.I\l.as .IIgCOsy Calcium. P.P.M. a s CaCOa Oxalare Direct separation BY AOAC method method difference 29 140 141 07 08 69 52 68 68 81 31 29 61 8.56
0
83 32 30 61
854
Average of triplicate determinations
36 11 10 5 23
Tlirongh the synthesis of a new indicator, the authors have made possible a direct titration of calcium in the presence of magnesium. The new indicator produces a sharp color change from wine red to pure blue xhen calcium is titrated with (ethylenedinitrilo) tetraacetate a t p H values between 12 and 14. Accurate determinations of calcium with no interference from magnesium have been made on a variety of samples including water, limestone, salt, blood serum, milk, and urine. Calcium and magnesium separations were not necessary. The indicator is 2-hydroxy-l-(2-hydroxy-4-sulfo-l-naphthylazo)-3-naphthoic acid, which is synthesized by coupling diazotized 1-amino-2-naphthol-lsulfonic acid with 2-hydroxy-3naphthoic acid. This indicator is commercially available from Scientific Service Laboratories, P. 0. Box 175, Dallas 21, Tex. APPARATUS AND REAGESTS
A measuring spoon for dry indicators, approximately 0.1-gram capacity, is available from the Hach Chemical Co., Ames, Iowa. As a titration light an ordinary reflector-type lamp, fitted r i t h a 100-watt screw-base bulb, was used. Total Calcium and Magnesium Indicator. Triturate 0.2 gram of Eriochrome Black T (F 241) and 50 grams of reagent grade potassium chloride wit,h a mortar and pestle until all the dye is evenly distributed on the surface of the potassium chloride crystals. Calcium Indicator. Diazotized l-amino-2-naphthol-4-sulfonic acid is so readily decomposed by light that it has been used in photogra hic printing fapers ( 1 1 ) . The diazotization process describezby Woodwar (9) produces good results when the reaction takes place in the absence of light. A photographic dark room is an ideal place t o carry out the reaction. If a dark room is not available, the reaction may be carried out at, night or in a light-tight container. Stir a mixture of 23.9 grams of l-amino-2-naphthol-4-sulfonic acid and 100 ml. of water at 20" C. in a 400-ml. beaker. -4dd 0.2 gram of cupric sulfate pentahydrate and 23 ml. of a 307; solution of sodium nitrite to the mixture, Continue stirring the mixture for 45 minutes and filter the solution into a dropping funnel. .-Z dark room safe light may be used t o aid in the manipulations. Pre are a suspension of 18.8 grams of powdered 2-hydroxy-3naphtIoic acid and 50 ml. of water in a 600-ml. beaker. .kid with stirring 35 ml. of a SOYo solution of potassium hydroxide. When complete solution is obtained, cool the mixture to about 20" C. Add the diazotized l-an1ino-2-naphthol-4-s~1lfonic acid solution dropwise with constant stirring. When all of the diazonium salt has been added, stir for an additional 15 minutes. The mixture now may be exposed to ordinary light intensitj-. Add about 50 grams of cracked ice to the dark blue solution and acidify with 50 ml. of concentrated hydrochloric acid slowly added with constant stirring. Filter the solution by suction and wash the dye with lOy0hydrochloric acid ilntil the filtrate is light red in color. Dry the 2-h~-drox~-1-(2-hydrosy-4-srilfo-l-naphthylazo).3-naphthoic acid on the steam bath until it can be reduced to a fine powder. Dry the material in an oven at 110' C. for 3 to 4 hours. The yield is 30 to 32 grams or about 7 5 S of the theoretical amount. Triturate 0.5 gram of 2-hydroxy-l-(2-hydroxy-4-sulfo-lnaphthylazo)-3-naphthoic acid and 50 grams of reagent grade sodium sulfate until the dye is evenly distributed on the sodium sulfate. Standard Magnesium Chloride. Dissolve 2.03 grams of reagent grade magnesium chloride hexahydrate in a small volume of water and dilute to 1 liter. Standard Calcium Chloride. Transfer 1.000 gram of pure calcium carbonate (special lo^ in alkalies and heavy metals) to a 1-liter volumetric flask with the aid of a powder funnel. Rinse the powder funnel and the neck of the flask with distilled water. .4dd 250 ml. of 0.100N hydrochloric acid and swirl the contents 1026
V O L U M E 28, NO. 6, J U N E 1956 Table 11.
Recovery of Calcium from Known Solutions of Calcium and Magnesium Magnesium Present, P.P.?J. as hlgCOs
a
1027
Calcium, P.P.M. a s CaCOla Present
Found
Average of triplicate determinations.
Table IIT.
Determination of Calcium and Magnesium in Salt Samples Magnesium a s hIgS04, %" Calcium a s CaSOd, 7%" AOAC 0.80 0.19 0.89 0.19 0.26 0.14 0 19 0 31
Direct titration 0.80 0.19 0.88 0.19 0.26 0.14 0.17 0.31
AOAC 0.13 0.01 0.10 0.10 0.01 0.01 0.03 0.04
BY difference 0.13 0.01 0.11 0.10 0 01 0 01 0 03 0 04
-4verage of triplicate dctcrminations.
until solution is complete. Add 40 ml. of 0.1005 sodium hydroxide] cool bhe contents to room temperature] and dilute to the mark. Standard Disodium Dihydrogen (Ethylenedinitrilo) Tetraacetate Dihydrate. Dissolve 3.9 grams of the analyt,ical reagent material in distilled water and dilute to 1 liter. St'andardize against the standard calcium chloride solution using Eriochrome Black T as the indicator. Buffer (14). Add 55 ml. of concentrated hydrochloric acid to 400 ml. of distilled water and mix thoroughly. Slowly pour 310 nil. of redistilled monoethanolamine with stirring into the mixture. Cool the solution t o room temperature. Titrate 50.0 ml. of standard magnesium chloride solution n4th standard (ethylenedinitrilo) tetraacetate solution using 1 ml. of the monoethanolamine-hydrochloric acid solution as the buffer. Use the method described below for total calcium and magnesium determination. .Idd 50.0 ml. of magnesium chloride solution t o the amount of (ethylenedinitrilo) tetraacetate required t o sequester the magnesium exactly, pour the mixture into the monoethanolaminehydrochloric acid solution, and mix well. Dilute the mixture to 1 liter. Potassium cyanide, reagent grade. Hydroxylamine hydrochloride, reagent grade. Pot,assium hydroxide, 8 N , made with reagent grade potassium hydroxide. PREPAR.4TION OF SAMPLES
Salt Samples. Transfer 25 grams of a representative sample to a 250-ml.volumetric flask Tvith the aid of a powder funnel. Wash the powder funnel and the neck of the flask with small poTtions of distilled water. Add 100 ml. of 0.100N hydrochloric acid to the flask and warm the contents on the steam bath until complete solution is obtained. Add 50 ml. of 0.1OOiV sodium hydroxide to the flask, cool the contents t o room temperature, then dilute to the mark. Limestone and Scale Samples. Grind representative samples to a fine powder with a mortar and pest,le, then pass through a No. 80 sieve. Dry the samples to constant weight in an oven a t 100" to 105" C. Ignite 1-gram samples in platinum crucibles over the full heat of a RIeker-type burner. Bring the samples into solution as described by Banewicz and Kenner ( 2 ) and dilute to a final volume of 250 ml. Water Samples. Filter the samples through a retentive fluted filter paper such as Whatman S o . 12. ANALYTICAL PROCEDURE
Total Calcium and Magnesium. Pipet an appropriate aliquot of the prepared sample into a 250-ml. wide-mouthed Erlenmeyer flask and dilute with distilled water to about 50 ml. Add 5 ml. of buffer solution and mix by swirling. rldd about 30 mg. each of potassium cyanide and hydroxylamine hydrochloride and again mix. ildd one spoonful of the total calcium and magnesium
indicator and titrate to the pure blue end point with standard (ethylenedinitrilo) tetraacetate solution. Record the volume as the amount of standard solution required for total calcium and magnewum. Make a blank titration, replacing the sample with distilled water. Calcium. Pipet duplicate aliquots of the prepared sample into 250-ml. wide-mouthed Erlenmeyer flasks and dilute to about 50 ml. Add 4 ml. of 8N potassium hydroxide solution to the first flask and mix. (A precipitate of magnesium hydroxide may be noted here.) Allow the flask to stand for 3 to 5 minutes with occasional sn-irling. Add about,30 mg. each of potassium cyanide and hydroxylamine hydrochloride and swirl the flask until the reagents have dissolved. -4dd one spoonful of calcium indicator and titrate with standard (ethylenedinitrilo) tetraacetate solution to the pure blue end point. Refill the buret and put into the second flask a n amount of titrant e ual to the amount required to reach the end point less 1 ml. I d d 4 ml. of Si\- potassium hydroxide and mix the contents thoroughly. Continue the titration procedure as n-ith the first sample. Record the volume as the amount of standard (ethylenedinitrilo) tetraacetate required for calcium. Make a blank titration, replacing the sample with distilled water. One milliliter of 0.02005(ethylenedinitrilo j tet raacet ate solution is equivalent to 0.4008 mg. of calcium. Magnesium. The volume of standard (ethylenedinitrilo) tetraacetate solution equivalent to magnesium is obtained by subtracting the volume required for the calcium from the volume required for total calcium and magnesium on equal aliquots. One milliliter of 0.0200N (ethylenedinitrilo) tetraacetate is equivalent to 0.2432 mg. of magnesium. RESULTS
The results of analysis of water, common salt, limestone! and boiler scale samples showed good agreement between the values obtained for calcium by the direct titration, using 2-hydroxy-l(2-hydroxy-4-sulfo-l-naphthylazo)-3-naphthoic acid as the indicator, and the official .IOAC method (1). Indirect magnesium tieterminat,ions made by subtracting the direct calcium titration from the total calcium and magnesium titration were also in good agreement with the direct magnesium determinations volumetrically and gravimetrically, following an oxalate separation of calcium. Direct magnesium deterniinat,ions were made volumetrically with (ethylenedinitrilo) tetraacetate, using Eriochrome Black T as the indicator, and gravimetrically by official .40AC methods. The results of analyses of water samples are given in Table I. The maximum variation in the values obtained for calcium and magnesium is 2 p.p.ni. as calcium carbonate and 1 p.p.m. as magnesium carbonate.
Table IV.
Determination of Calcium and 3Iagncsium in Limestones and Boiler Scales
.~~ Calciiim Oxide?,C/o" Sample Present Detd. NBSlimestone l a !1.32 41.44 X B S dolomite 88 JO.49 :io 1 3 Boiler scale 1 3 7 .32 3 7 .'I 5 Boiler scale 2 4,j,Ci7!, -12 .i8 Average of trililicatc rlrtrrminations. b Determined hy .%O.-\C iiictliods. !J
Magnesium Oxide, Present Detd. 2.19 21.48 Il.68a 1.4Rh
2 13 21 37
11.79 1 .4.3
The determinations of calcium in solutions containing kr1on.n quantities of calciiini and magnesium are given in Table 11. The direct calcium titration gave good results in the presence of magnesium a t concentrations froin 0 to 839 p.p.ni. as magnesium carbonate. The results of comparative analyses of common salt samples are given in Table 111. The maximum deviation from the average of the determinations nas =k0.0107c for calcium sulfate and =!cO.O05%l,for magnesium sulfate. The results of analyses of National Bureau of Standards Dolomite 88 and limestone l a and of two typical carbonate boiler scales are given in Table IV. Calcium and magnesium were
ANALYTICAL CHEMISTRY
1028
in the titration of calcium when 2-hydrosy-l-(2-hydroxy-4sulfo-l-naphthylazo)-3-naphthoic acid is used as the indicator. Diehl, Goetz, and Hach ( 5 ) have made a rather complete study of the more common interfering ions and have indicated the proper measures to be taken to eliminate the interferences. When calcium is titrated directly a t high pH values, barium, strontium, lead, and zinc will titrate as calcium. Barium interference may be eliminated by adding exress sulfate ions to the hot slightly acid aliquot prior to the addition of potassium hydroxide. Strontium ions cannot be separated from calcium bjDISCUSSION simple means; therefore, any strontium present will increase The success of the titration of calcium with (ethylenedinitrilo) the calcium titration. The interference from small amounts tetraacetate using 2-hydroxy-l-(2-hydroxy-4-sulfo-l-naphthyl- of lead may be eliminated by adding an excess of sodium di azo)-3-naphthoic acid as the indicator is made possible by two ethyldithiocarbamate before the indicator is added. Small facts: (1) The p H of the solution to be titrated is sufficiently amounts of zinc, copper, cobalt, nickel, and manganese do not high to assure the quantitative precipitation of magnesium as interfere n-hen potassium cyanide and hydroxylamine hydromagnesium hydroxide; (2) calcium forms a stronger complex chloride are added as in the recommended procedure. with (ethylenedinitrilo) tetraacetate than does magnesium ( 1 2 ) . I n the presence of sufficient magnesium, small amounts of Although magnesium hydroxide precipitate is present in the phosphate and iron do not interfere. Fifty parts per million of titrating flask, the chelating solution nil1 not react with magmagnesium eliminates the interference from 40 p.p.m. of phos. nesium until all free calcium and calcium complexed by the phate and over 20 p.p.m, of iron. indicator have been chelated by (ethylenedinitrilo) tetraacetate. The manner in which magnesium removes the interference If the indicator is added before potassium hydroxide, an end point from iron and phosphate is not completely understood. The may not be reached because magnesium salts form a lake with the authors assume that the interfering ions are coprecipitated with indicator as the p H increases. The magnesium-indicator lake magnesium hydroxide in such a manner that the outer surface is coprecipitated with magnesium hydroxide. of the resulting precipitate is almost completely composed of the The colored complex formed with the indicator and calcium very insoluble magnesium hydroxide. If the iron and phosphate is absorbed on any magnesium hydroxide precipitate, b u t a sharp precipitates are coated with magnesium hydroxide, they are not end point is possible because the chelating agent has contact readily available to the chelating agent and indicator. It has with the colored complex. A sharper end point is obtained in been noted that when ferric ions are precipitated with potassium the presence of magnesium hydroxide precipitate. hydroxide in the presence of sufficient magnesium the resulting A tungsten light source placed near the titration flask improves precipitate is a gelatinous m-hite 11-ith no red-brown color. the end point, of the direct calcium titration principally because the red complex of the indicator and calcium is greatly intensified. LITERATURE CITED T h e monoethanolamine-hydrochloric acid buffer ( 2 4 ) has a . h o c . Offic. d g r . Chemists, “Official Methods of Analysis.” buffering capacity equal to the ammonia-ammonium chloride 7th ed., 1950. buffer n-hich is commonly recommended for the titration of . 24, 1186 (1952). Banewica, J. J., Kenner, C. T., A s a ~CHmr. calcium and magnesium with (ethylenedinitrilo) tetraacetate. Beta, J. D., Noll, C. A , , J . Am. Water Works Assoc. 42, 49 Sharp end points are obtainable and the strong ammonia odor (1950). Biedermann, W.,Schwarxenbach, G., Chimia (Prague) 2, 56 is completely eliminated. The buffer has excellent keeping (1948). qualities. Diehl, H., Goeta, C. A , , Hach, C. C., J . Am. Water Works Assoc. Biedermann and Schwarzenbach (4)pointed out that magne42, 40 (1950). sium must be added to obtain a sharp end point when relatively Elliot, W.E., J . Biol. Chem. 197, 641 (1952). pure samples of calcium are titrated using Eriochrome Black T as Fales, F. W., Ibid., 204, 577 (1953). the indicator. Hence, magnesium is usually added t o the Gehrke, C. IT., ilffsprung, H., Lee, Y., SAL. CHEW 26, 1944 (ethylenedinitrilo) tetraacetate solution prior to the standardiza(1954) tion. Magnesium in the chelating solution prevents a sharp Groggins, P. H., “Vnit Processes in Organic Synthesis,” 2nd ed., p. 136, XIcGraw-Hill, Kew York and London, 1938. end point when calcium is titrated using 2-hydroxy-l-(2-hydroxyHoffman, W.h l . , Shapiro, H., J . Assoc. Ofic. Agr. Chemists 37, 4-sulfo-l-naphthylazo)-3-naphthoic acid as the indicator. Addi966 (1954). tion of romplexed magneeium to the buffer eliminates the need Schmidt, Julius, Maier, W., Ber. 64B, 767 (1931). of two standard (ethylenedinitrilo) tetraacetate solutions and Schwarzenbach, G., dckermann, H., Hela. Chim. Acta 31, 1029 assures an adequate amount of magnesium t o give a sharp end (1948). point, even when small amounts of pure calcium are titrated. Schwarzenbach, G., Biedermann, W.,Bangerter, F., Ibid., 29, determined gravimetrically on the boiler scale samples by the AOAC methods for soils and are given in the table as calcium oxide and magnesium oxide present. The maximum deviation from the average for the determinations of both calcium oxide and magnesium oxide in limestones and boiler scales was i O . O O % , . Corrections were made on the determinations of calcium and magnesium in Kational Bureau of Standards limestone l a in order to account for the titration of strontium 1%-ith(ethylenedinitrilo) tetraacetate.
I
811 (1946).
INTERFERENCES
Thies, H., Kallinich, G., Biochem. Z . 324, 485 (1953).
Ions which interfere in the usual titration of calcium and magnesium using Eriochrome Black T as the indicator also interfere
R E C E I V Efor D review > f a y 1 3 , 1955. Accepted RIarch 29, 1956
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