Polarographic determination of magnesium in calcium carbonate

Talattta, 1965. Vat. 12. pp. 1009 to 1015. Pergamon Press Ltd. Printed in Northern Iteland

POLAROGRAPHIC DETERMINATION OF MAGNESIUM IN CALCIUM CARBONATE MERWN L. RICHARDSON John and E. Sturge Ltd., Lifford Lane, Kings Norton, Birmingham 30, England (Received 30 June 1965. Accepted 4 August 1965) Summary-Magnesium is determined in calcium carbonate by polarographically reducing the magnesium complex of the dyestti Solochrome Violet R.S. at pH > 11 using piperidine as buffer. The bulk of the calcium is first removed by means of a precipitation with oxalate; iron, when present at a level above one-tenth of the magnesium content, must be removed by a chloroform extraction of its cupferrate. The method has a coefficient of variation of less than 6% and it is suitable for magnesium levels up to at least 1%. A comparison of the proposed method is made with flame photometry. INTRODUCTION THE

polarographic technique described by Reynolds and his coworkers1s2 is here modified for the determination of magnesium in calcium carbonate in the range O-2,500 ppm of magnesium. The proposed procedure consists of polarographically reducing the magnesium complex of the dyestuff Solochrome Violet R.S. (C.I. Mordant Violet 5; 5-sulpho-2This complex is produced in solutions containing hydroxybenzene-2-naphthol). piperidine as buffer and the final solution is adjusted to pH > 11. In order to avoid interference from calcium, which forms a similar but much weaker complex with the dyestuff, it is necessary first to remove the bulk of the calcium, e.g., by precipitation with oxalate. Interference from iron (distortion of the polarographic waves) can also be serious when the level of iron is more than one-tenth of the magnesium content. This interference can be removed by extraction with chloroform of the iron complex with cupferron. EXPERIMENTAL Removal of calcium Table I shows the effect of various precipitants investigated for the removal of calcium. TABLE I

Effect of precipitant

Precipitant Sulphuric acid, aqueous and ethanolic Sulphite (by the procedure of Gehrke et al*) Fluoride Oxalate

Residual soluble calcium sutliciently high to cause interference subsequent magnesium determination. Residual soluble calcium too high (see immediately above).

with

Magnesium fluoride coprecipitated with the calcium fluoride. Satisfactory; no soluble calcium could be found at centrifuge tube stage (see Method) when examined by an Evans Electroselenium Ltd. filter flame photometer.*

* This procedure was independently

and simultaneously 1009

investigated by Pyburn and Reynolds.*

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M. L. RICHARDSON

Polarographic procedure From the findings of Reynolds and his coworkers1p8 and from our own observations using the K.1000 Cathode Ray Polarograph (Southern Analytical Ltd.) it was found that the quantities of reagents specified under Method gave the most reliable and satisfactory working conditions. Piperidine. The volume of piperidinium chloride buffer solution should be maintained between @9 and 1.1 ml. A smaller quantity gave no or very poorly shaped polarographic waves; a larger quantity gave waves with large slopes which were difficult or impossible to synchronise. Sodium hydroxide. Although the determination can be performed in the absence of sodium hydroxide, the waves exhibit poor s and are hence di&icult to interpret. The volume of 1M sodium hydroxide solution should !zakgreater than 0.5 ml but should not exceed 1.1 ml. A larger quantity gives a calibration relationship with a large negative intercept and also a poorer shaped polarogram. Solochrome Violet R.S. A smaller quantity of the dyestuff solution than 20 ml produced “tailing” of the calibration curves; a larger quantity tended to give a large preceding dyestuff wave which makes the base of the wave difficult to interpret and to synchronise. Using 15 ml of Solochrome Violet R.S. solution it is possible to determine magnesium down to at least 0.20 @ml of the final solution for polarography. For lower amounts of magnesium the quantity of dyestuff solution would need to be decreased.

4

,,a 3.

2

\ \ ,

FIG. l.-Effect

of interference of iron on magnesium polarogram: --.----a-lOOOppmofiron, _.-.500 ppm of iron, ------2OOppmofiron, . . . . . . . . . . . 100 ppm of iron, “blank”.

Interferences The effects on the final polarogram of various elements taken through the whole of the method (see p. 1013) are shown in Table II. These effects do not necessarily apply if the oxalate step is omitted. Barium, calcium and strontium. Experiments indicated that reliable results could not be obtained (either high or wholly non-reproducible) if ratios greater than calcium/magnesium 10/l strontium/magnesium 25/l barium/magnesium 40/I

Magnesium in calcium carbonate

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TABLB II Element * Aluminium(HI)

up to 5,000

Antimony(V) Arsenic(IIr) Barium(B) Beryllium(I1) Bismuth(II1) Cadmium(E) Calcium(Il) Chromium(III)

l,ooO Loo0 Loo0 1,000 5,000 see elsewhere up to 500

Cobalt(B)

up to 200

cOpper

5,000 5,000 up to 1,000

Fluoride Iron(II1)

_

Amount of element, Pp&

Ahnninium wave precedes magnesium wave. Above 2000ppm base of wave is difficult to interpret. However, ahuninium content of “Pharmaceutical grade” calcium carbonate is invariably below 500 ppm and interference effects are insigniiicant. No interference No interference

-

see elsewhere

Lead(I1) Manganese(E)

5@0 1,000 5,000

Molybdenum(V1) Nickel(B)

1,000 up to 500

Orthophosphate Strontium(H) Thallium(I) Tin(E) Vanadium(B) Zinc(B)

500 see elsewhere 5,000 5,000 1,000 5,000

* Added to 1 ml of 10% calcium carbonate t Calculated to solid calcium carbonate.

No interference No interference No interference

Chromium waved.&cec$sa~~esi!nwJg Above 200 ppm base of wave. Because chromium level is usually under 10 ppm, chromium interference is of no consequence. Cobalt wave just precedes magnesium wave. Slight interference effects observed with 100 ppm of cobalt and more serious interference effects at 200 ppm and above. Because cobalt level is usually less than 20 ppm, interference effects are of little consequence. No interference No interference Serious interference effects found with iron (see Fig. 1) its removal discussed elsewhere. “Pharmaceutical grade” calcium carbonate can contain up to 200 ppm of iron. Interference effects, however, not encountered with analytical reagent grade or luminescent grade calcium carbonate because iron content is below 10 ppm. No interference No interference Wave appears between dye wave and magnesium complex wave, which makes magnesium wave difficult to interpret. No interference Serious interference effects (wave from nickel complex coalesces with that from magnesium comnlex) found with 100 oom of nickel. but these are iot ‘likely to be of ‘consequence because “Pharmaceutical grade” calcium carbonate does not contain more than 10 ppm of nickel. No interference

No No No No

interference interference interference interference

solution containing

100 pg of magnesium.

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M. L.

RMXARDSON

were obtained in the tinal solution for polarography. If these ratios are exceeded it is essential to remove the elements by the oxalate procedure. Iron. To try and eliminate the interference from iron, various complexing agents were added before the addition of ammonium oxalate to a “solution” of calcium carbonate which contained 1,000 ppm of magnesium and 500 ppm of iron. The effects of these reagents are shown in Table III. TABLE III

Effect

Complexing agent Acetylacetone (0.1 ml) 10 % Sodium acetylsalicylate (1 ml) 10 % Sodium L-ascorbate (1 ml) 10% Ammonium thiocyanate (1 ml) O.OlM Cyclohexanediaminetetra-acetic

acid

(1 ml) 10% Cupferron (1 ml) 1% Sodium diethyldithiocarbamate (1 ml) 0025M 4,7-Diphenyl-l,lO-phenanthroline (50% ethanolic; 1 ml) 1% Dithizone (ethanolic; 1 ml) 001M Disodium ethylenediaminetetraacetate (1 ml) 10 % Hydroxylamine hydrochloride (1 ml) 10 % S-Hydroxyquinoline (50 % ethanolic; 1 ml) O-25% 1, 10-Phenanthroline (1 ml) 1% Phenyl-2-pyridylketoxime (ethanolic; 1 ml) 10 % Sodium rubeanate (1 ml) 10% Sodium tannate (1 ml) 10 % Sodium thioglycollate (1 ml) 50% Triethanolamine (in 1M sodium hydroxide ; 1 ml)

Wave form badly distorted No effect on iron interference Wave form badly distorted Wave form distorted Magnesium wave removed No effect on iron interference Wave form badly distorted No effect on iron interference* Wave form badly distorted Magnesium wave removed Wave form badly distorted Wave form badly distorted Magnesium wave enhanced* No effect on iron interference Wave form distorted Wave form badly distorted Wave form badly distorted Iron interference wave removed, but magnesium waves found not to be reproducible

* 1JO-Phenanthroline and 4,7-diphenyl-l,lO-phenanthroline agents because these tended to distort the wave forms.

used without the addition of reducing

Increasing the pH during the calcium-removal stage by addition of piperidine did not remove the iron interference. Cupferron was chosen as the most suitable complexing agent with which to try and remove the iron by solvent extraction because it was the only complexing agent which did not have a serious effect on the polarogram of the magnesium-dyestuff complex. This procedure also made the base-line flatter and “cleaned up” any irregularities from the polarogram. Details of the procedure are given under Method. By using a comparison technique the effect of the iron interference could also be overcome. A series of calibration relationships was prepared, containing an equivalent of 0,50,100, 150,200 and 250 ppm of added iron, respectively. The iron is then determined by l,lO-phenanthroline* and the result read from the appropriate relationship. The extraction procedure is preferred for eliminating interference from iron. Results Some results obtained by cathode ray polarography are shown in Table IV. Some of the samnles shown in Table IV were also examined bv the Unicam S.P. 900 flame soectrophotometer. 0.25 %-w/v Solutions of the samples in hydrochloricacid were compared with 0.25 % w/v “spec-pure” calcium carbonate in hydrochloric acid which contained known amounts of added magnesium. Measurements were carried out using the 285+2-rnp magnesium line; the spectra were scanned from 283.5 to 289.5 mp.

Magnesium in calcium carbonate

1013

TABLE N

Sample

Mg found, ppm (average)

Replicate determinations

Standard deviation, PPm

5 100 1200 930 450 990 1500 1540 4520 4070

;::

: 6 6 6 12 12 12 12 12

5873

51 21 58 33 79 320 131

Coefficient of variation, % 14.6 5.5 8.3 4.7 5.5 4.7 5.8 2.3 5-l 7.0 3.2

* Iron content less than 10 ppm. t Iron content about 150 ppm; comparative method used for eliminating iron interference effects. $ Iron content about 150 ppm; cupferrate extraction used for removal of iron. TABLE V

Sample

Mg found, ppm (average)

D E F G H I

105 74 1100 880 260 980 1500 4150

Replicate determinations 4 5 6 7 7 6 5 9

Standard deviation, PPm 67 74 2 38 37 58 150

Coefficient of variation, % 64 41 7.7 5.0 14.6 ;:; 4.0

Method Reagents All reagents should be of recognised analytical grade. 1M Hydrochloric acid 1M Sodium hydroxide Ammonium oxalate solution (saturated) 4 % w/v Cupferron solution (freshlyprepared) 0.05 % wlv Solochrome Violet R.S. (C.I. Mordant Violet 5) solution Chlo&fo;fn Piperidinium chloride bu$er. Dilute 20 ml of piperidine to 100 ml with water, then adjust the pH to 13.5 with 1M hvdrochloric acid. Store in a tiehtlv stormered bottle. Magnesium sto>k solution. Prepare from mageskm &nings so that 1 ml = 1000 ,ug of Mg. Iron stock solution. Prepare from spectroscopically pure iron so that 1 ml = 1000 pg of Fe. Calcium carbonate (“spec-pure”, Johnson Matthey & Co. Ltd). Preparation of sample solution For saqoles containing less than 2Oppm of iron. Transfer a 1.00-g sample to a 30-ml beaker, dissolve in the minimum quantity of 1M hydrochloric acid (ca. 22 ml) and evaporate almost to dryness. Transfer to a lo-ml volumetric flask and dilute to the mark with water. Transfer a l-ml aliquot to a lo-ml graduated centrifuge tube and proceed as described under Calcium removal and magnesium complex formation stages. For samples containing more than 20 ppm of iron. Proceed as described immediately above, but add 1 ml of 4% cupferron solution before diluting to 10 ml in the volumetric flask. Transfer the contents of the volumetric flask, without washing, to a dry 50-ml separatory funnel, add 10 ml of chloroform, shake for 30 set, allow the phases to separate and discard the chloroform phase. Repeat the extraction with chloroform until the organic extract is colourless. Transfer a l-ml aliquot to a 10-ml

1014

M. L.

~CEL4FCDSON

centrifuge tube and proceed as described under Calcium removal and magnesium complex formation stages. Calcium removal and magnesium complex formation stages To the centrifuge tube containing the l-ml aliquot, add 4 ml of ammonium oxalate solution and dilute to the lO-ml mark with water; insert a glass rod and mix thoroughly. Place in a boiling water bath for 3 min, then allow to cool. Centrifuge. Transfer 5 ml to a 50-ml volumetric flask. Add 1 ml of piperidinium chloride buffer and mix; add 1 ml of 1M sodium hydroxide solution and mix; add 20 ml (15 ml when the magnesium content is less than 500 ppm) of Solochrome Violet R.S. solution and dilute to 50 ml with water. Allow to stand 25-30 mm. (If the solution is allowed to stand for longer than 30 min, low results are obtained; if the solution is not allowed to stand for the stated period, erratic results are often obtained.) Transfer about 5 ml of this solution to a polarographic cell and deoxygenate with nitrogen for 5 min. Record the height of the magnesium polarographic wave, using the following conditions: mercury pool reference electrode start potential of -0.65 V. Read off the results from a calibration graph. Preparation of calibration graph when magnesium content less than 500ppm and 20 ppm of iron present Transfer l-ml aliauots of loo/, w/v calcium carbonate Ksnec-nure”) solution to a series of six lO-ml graduated cent;ifuge tubes’ and add 0, 10,20, 30, 40 and 50 ;g of’magnesium (as magnesium stock solution), respectively; add 4 ml of ammonium oxalate solution and proceed as described under Calcium removal and magnesium complex formation stages. Plot a calibration graph relating magnesium added and wave height. Preparation of calibration graph when magnesium content in 500-2500 ppm range and iron content reater than 20 ppm Transfer 5-ml aliquots of 20% w/v calcium carbonate (“spec-pure”) solution to a series of six lo-ml volumetric flasks and add 250 rg of iron (as iron stock solution) to each. Add 0, 500, 1000, 1500,200O and 2500 pg of magnesium (as magnesium stock solution), respectively, and 1 ml of 4% cupferron solution, then proceed as described under For samples containing more than 20 ppm of iron. Plot a calibration graph relating magnesium added to wave height. Note. If the magnesium content exceeds 2500 ppm, the initial quantity of sample should be reduced accordingly; the ammonium oxalate should also be reduced in proportion and a calibration graph drawn up using the appropriate quantities. CONCLUSIONS

The proposed method offers a fairly quick, but reliable procedure for the determination of magnesium in calcium carbonate and it $as been used successfully in our laboratories for some months. The chief interfering elements include aluminium, chromium, cobalt, iron and nickel, of which iron is particularly serious. Interference from iron can, however, be easily overcome by a simple solvent-extraction stage. The method has also been applied successfully to strontium and barium salts. Although the procedure is more lengthy than flame spectrophotometry, it is, in general, more accurate as judged by the coefficients of variations shown in Tables IV and V. Acknowledgments-l wish to thank the directors of John and E. Sturge Limited for permission to publish this paper. Thanks are also due to Dr. G. F. Reynolds, Loughborough College of Advanced Technology, for many helpful discussions, to Mrs. V. C. Mason and Mr. D. E. Woolley for carrying out much of the experimental work, to Mr. P. E. Luton for carrying out determinations on a number of samples, and to Dr. A. G. Catchpole and Mr. W. Foster, Wolverhampton and South Staffordshire College of Technology, for help with the flame spectrophotometric analyses. Zusammenfassuaa-Maanesium in Calciumcarbonat wird bestimmt durch polarographischg Reduktion des Magnesiumkomplexes mit dem Farbstoff Solochromviolett R.S. hei DH > 11 mit Pineridin als Puffer. Die Hauptmenge Calcium wird &erst durch Ox’aatf&llung

Magnesium in calcium carbonate

1015

entfernt; bei Anwesenheit von Eisen in einer Menge, mehr als ein Zehntel des Magnesiums mu8 es durch Extraktion seines Kupferronkomplexes mit Chloroform entfemt werden. Die Methode hat einen Variationskoetlixienten von weniger als 6% und ist f3r Magnesiumgehalte herauf bis wenigstens 1% gee&net. Die vorgeschlagene Methode wird mit der Flammenphotometrie verglichen. R&m&On dose le ma&sium dam le carbonate de calcium par r6duction polarographiqie du complexe magntsien du colorant Solochrome Violet R.S. a DH > 11 en utilisant la nineridine comme tampon. La majeure part6 du-calcium est d’abord &&n&s au moyen dune precipitation a l’oxalate; on doit &niner le fer par une extraction chloroformique de son cupferrate lorsqu’il est present en quantit6 plus que le dixieme de la teneur en magnesium. La methode pr6sente un coefficient de variation inf6rieur a 6% et elle convient pour des teneurs en magnesium allant jusqu’it 1% au moins. On compare la methode proposQ a la photometric de flamme. REFERENCES 1 S. M. Palmer and G. F. Reynolds, private communication. * C. M. Pyburn and G. F. Reynolds, Z. unalyt. Chem., in press. a G. W. Gehrke, H. E. Affsprung and V. C. Lee, A Direct Disodium Dihydrogen Ethylenediaminetetra-acetate F&ration Procedure for Magnesium in Limestone, Agr. Res. Bull. (University of Missouri), 1954, No. 569. ‘ M. L. Richardson, S.C.I. Monograph No. 18, The Analysis of Calcareous Materials, p. 172: Review of the Determination of Iron in Calcium Carbonate.