306 Arcalyst, May, 1968, Vol. 93, +$. 306-310 The Determination of Magnesium in Silicate and Carbonate Rocks by the Titan Yellow Spectrophotometric Method BY W. H. EVANS (Ministry of Technology, Laboratory of the Government Chemist, Cornwall House, Stamford Street, London, S.E.l) A method is proposed involving an initial separation of ammonia group elements by means of a succinate precipitation, followed by a spectrophoto- metric determination of magnesium with Titan yellow reagent; calcium interference is minimal. Satisfactory agreement with the pyrophosphate gravimetric method is obtained for a wide range of magnesium values in silicate and carbonate rocks. THE existing pyrophosphate gravimetric method of determining magnesium in silicate and carbonate rocks is time consuming and tedious; the need exists for a more rapid method also applicable over a wide range of values of magnesium.The possibilities are complexo- metric titration, involving the difference between two titrations and giving rise to large errors at the lower end of the scale ; atomic-absorption photometry that requires expensive equipment for precise results; and spectrophotometric methods. The majority of methods proposed for the latter are based on the deposition of a dyestuff on magnesium hydroxide to form a strongly coloured lake in alkali-metal hydroxide solution; they are subject to many interferences, in particular from ammonia group elements. Of these reagents the triazole dye, Titan yellow, has been used most often. In its recent applications to the determination of magnesium in silicates ,l *2 s3 masking reagents to overcome interference from ammonia group elements are used, and buffering to high concentrations is recommended to overcome inter- ference from aluminium and calcium.These additions tend to affect the stability of the lake and impair the reproducibility of results. A procedure is described wherein the ammonia group elements are removed by a double succinate precipitation, and in the spectrophoto metric method described calcium interference is largely nullified. EXPERIMENTAL SPECTROPHOTOMETRIC DETERMINATION- Of many colloids examined as stabilisers for the Titan yellow lake, poly(viny1 alcohol) , or sodium polyacrylate, in conjunction with glycerol has proved superior.* The spectrophoto- metric procedure described by Bradfield: in which a 0-001 per cent.solution of Titan yellow, 0.01 per cent. poly(viny1 alcohol) and 10 per cent. glycerol final concentration are used, provided a suitable basis and, with modification, gave excellent reproducibility. It was found desirable to keep the alkali strength as low as possible as poly(viny1 alcohol) reacts with alkali to give a complex absorbing at 490 mp. A final volume of 50 ml was used to decrease interference from other elements. Under these conditions, and measuring at 540mp, a smooth calibration graph could be drawn between 5 and 60pg of magnesium that was substantially linear between 10 and 50 pg. Several authors396 have examined the variation in the composition of the Titan yellow dyestuff in detail recently, while subsequent to their work King, Pruden and Janess recom- mended a procedure for the preparation of a refined Titan yellow reagent.A commercial 0 SAC; Crown Copyright Reserved.EVANS 307 product, Titan Yellow C.I. 19546 (Clayton yellow) reagent for magnesium, supplied by Hopkin and Williams Ltd., was used for this investigation and proved satisfactory at a 0*001 per cent. final concentration. INTERFERENCES- The effect of low level concentrations of interfering elements on the spectrophotometric measurement, referred to above, is shown by the results summarised in Table I; at higher levels interference increases rapidly. TABLE I EFFECT OF LOW LEVEL CONCENTRATIONS OF INTERFERING ELEMENTS ON MAGNESIUM DETERMINATIONS Magnesium concentration , p.p.m. 0.3 0.6 0.9 0.3 0.6 0.9 0.3 0.6 0.9 0-6 0.6 0.8 0-6 Error at varying concentrations of interfering elements, per cent.A r \ Interfering element 0.1 p.p.m. 0.2 p.p.m. 0-4 p.p.m. 1.0 p.p.m. 2.0 p.p.m. (as oxide) + 1.2 Nil Nil + 1.2 + 2.0 + 0.4 Nil + 1.2 Nil Nil Nil Nil Nil Nil - 1.0 - 2.2 + 1.3 - 0.4 - 0.4 + 1.2 + 2.0 + 0.4 - 0.7 Nil Nil Nil - 1.2 - 2.0 - 2.6 - 2.6 - 4.6 - 1.3 + 6.4 + 4.6 + 2.1 - 1.3 + 1.2 - 0.4 - 2.7 - - Fe - - Fe - Fe - - Al - - Al - - Al - - Mn - - Mn - - Mn - - Ti + 0.7 Nil Ba - - 1.4 Sr -0.7 - 1.2 P(as PsO,) + 1 mg of CaO The spectrophotometric measurement is sensitive to ammonium salts, hence an am- monium hydroxide precipitation of ammonia group elements followed by oxalate removal of calcium cannot be considered without subsequent removal of all ammonium salts.Willard and Tang7 precipitated aluminium quantitatively by means of a succinate precipitation under carefully controlled conditions. These involved the gradual hydrolysis of urea to ammonia to give a final pH of 4-4; calcium, magnesium and the greater part of the manganese did not interfere. It has now been found that 99 per cent. separation of both calcium and magnesium is possible by a single rapid precipitation of ammonia group elements by using sodium succinate and a final pH of 6.0; this separation becomes quantitative, for both synthetic mixtures and rock solutions, with a second precipitation. The quantity of succinate has little effect on the result of the spectrophotometric determination, except for a slight blank not exceeding 3 pg of magnesium per gram of succinate, and has the added advantage of controlling the initial pH of the solution used for spectrophotometric measurement.The levels of iron and titanium in the resulting solution are negligible. Fluoride interferes seriously in the separation of aluminium, giving rise to levels in excess of the limits imposed by the results shown in Table I. All but the last traces of fluoride could be removed after wet decomposition of samples, by evaporating to fumes once with a small volume of concen- trated sulphuric acid before applying the succinate precipitation to the final perchloric acid solution. If this condition was fulfilled, aluminium levels (as alumina) in all samples did not exceed 0.16 p.p.m. Manganese, as manganese oxide, enhanced the magnesium figure for concentrations greater than 0.2 p.p.m.; the incompletely separated manganese after a succinate precipitation is at a level not exceeding this concentration. Similarly phosphate, in the presence of calcium, and barium and strontium, are seldom present in silicate and carbonate rocks at levels sufficient to cause interference in the spectrophotometric deter- mination (Table I); the effect of the remaining commonly occurring trace elements could be disregarded. The effect of different concentrations of calcium varies with the amounts of magnesium present (Fig. 1); a level of calcium equivalent to 110 p.p.m. of calcium oxide could not be exceeded by this method. Attempts to diminish this effect with altered alkali conditions or by using differential solubilities or complex formation of the calcium and magnesium salts as carbonates, borates, tungstates, hydroxy acids or dibasic acids were unsuccessful.Inter- ference could be lessened by increased glycerol concentration but this was impractical because308 EVANS: DETERMINATION OF MAGNESIUM IN SILICATE AND CARBONATE [Ana(ySt, VOl. 93 of the difficulty in handling solutions of high viscosity. Sucrose, at a 0.3 to 0.4 per cent. concentration, however, was effective and had no detrimental effect on the spectrophotometric determination. Fig. 1 indicates this to be so for the concentrations of calcium most likely to be encountered in rock analysis. Calcium oxide concentration, p.p.m. (log scale) Fig. 1 Effect of different concentrations of calcium (as oxide) on known magnesium content: 0, with sucrose; x, without sucrose METHOD REAGENTS- Reagents should be of analytical-reagent grade.HydroJuoric acid, 40 per cent. vlv. Perchloric acid, 60 per cent. v/v. Sodium succinate solution, 0-05 per cent. Glycerol. Sucrose. Sodizlm hydroxide solution, 8 per cent. w/v. Titan yellow solution, 0.2 per cent. w/v-Dissolve, with boiling, 50 mg of poly(viny1 alcohol) in 20 or 30 ml of distilled water, add 100 mg of a suitable sample of Titan yellow and dilute to 50 ml. Titan yellow reagent-Dissolve, with boiling, 100 mg of poly(viny1 alcohol) in 20 or 30 ml of distilled water. Decant the solution into a 200-ml flask, add 5 ml of the 0.2 per cent. Titan yellow solution, 4 g of sucrose and 100 ml of glycerol. Magnesium standard soldion-Dissolve 0.603 g of clean magnesium ribbon in 10 ml of perchloric acid and dilute to 1 litre.This solution contains the equivalent of 1 mg of mag- nesium oxide per ml. Dilute this solution to give a working solution containing 5 pg of magnesium oxide per ml, and adjust to pH 6 with 0.05 per cent. sodium succinate solution. A Hilger Uvispek spectrophotometer or equivalent instrument is suitable for the measurement of optical densities. PROCEDURE- Decompose 1 g of silicate or carbonate rock with 20 ml of hydrofluoric acid by digestion overnight in a platinum or PTFE basin. Add 5 ml of perchloric acid and evaporate to dryness on an air-bath. Repeat the evaporation, until no further fumes are evolved, with three further separate 5-ml portions of perchloric acid, to the first of which are added 2ml of sulphuric acid (1 + 1).Finally dissolve the residue in 5 to 10 ml of perchloric acid and dilute to 200 ml. This solution retains its activity for some months. Dilute to 200 ml.day, 19681 ROCKS BY THE TITAN YELLOW SPECTROPHOTOMETRIC METHOD 309 Dilute 20 ml of this solution (equivalent to 100 mg of rock) to 40 ml and adjust to pH 2 with the aid of a pH meter by adding 8 per cent. sodium hydroxide solution. Add 2 g of sodium succinate and a Whatman accelerator, boil for 2 to 3 minutes and allow to settle. Filter the solution through a Whatman No. 40 filter-paper into a 200-ml graduated flask, washing the precipitate with a 0.05 per cent. sodium succinate solution. Re-dissolve the precipitate with 1 ml of perchloric acid, re-precipitate as above, and make the solution up to 200 ml.Withdraw a suitable aliquot (1 to 20 ml) of the solution containing 10 to 50 pg of magnesium, and dilute to 30 ml in a 50-ml flask (pre-washed with acid). Add by pipette, successively, 10ml of the Titan yellow reagent and 5ml of 8 per cent. sodium hydroxide solution, shaking the solution after each addition; dilute to 50 ml. After 1 hour measure the optical density in 2-cm cells at 540 mp. Prepare a series of standards containing from 0 to 50 pg of magnesium at the same time, and plot a curve of the optical density, corrected for the blank value, against magnesium present. A blank determination for the entire procedure should be carried out concurrently; this should not exceed the equivalent of 0-2 pg of mag- nesium per mg of rock.RESULTS TABLE I1 RECOVERY OF MAGNESIUM ADDED TO ROCKS OF LOW MAGNESIUM CONTENT Magnesium as mg of magnesium oxide present 0.16 0.19 0.17 0.38 0-25 0.25 Magnesium as mg of magnesium oxide added 0.20 0.20 1.00 1.00 5.00 15.0 Magnesium as mg of magnesium oxide recovered 0.37 1.20 1.38 5-28, 6-22 15-26, 15.23 0.38, 0.39 Recoveries obtained for the entire procedure for synthetic mixtures containing magnesium at the 0.50, 1.00, 5.0 and 15.0-mg levels, together with iron, aluminium, titanium and calcium, average 99.9 per cent., with a relative standard deviation of 0.016. Recoveries of magnesium added to rock material low in magnesium content are shown in Table 11. In this case the mean value of recovery is 100.3 per cent., with a relative standard deviation of 0.015.TABLE I11 COMPARISON OF MAGNESIUM VALUES OF SOME SILICATE AND CARBONATE ROCKS Magnesium oxide, Magnesium oxide, per cent. (Titan yellow) 0.39, 0.38 1.89, 1-85, 1-86 6-74, 6.67 per cent. (gravimetric) Granite G.I . . .. .. 0.38, 0.39. 0-37 0.41,* 0.35T Tonalite T.l . . .. .. 1-84, 1.86, 1.86 1.89 Diabase W.l . . .. .. 6.67, 6-62, 6-63 6.62,* 6.52t Granite . . .. * . .. 0.15 0-17 Olivine-basalt . . .. .. 4.70 4.74 Olivine-basalt . . .. .. 10.2 10.2 Tremolite-schist . . .. .. 19.1 19.1 Limestone . . .. .. 0.23 0.20 Limestone . . .. .. 4-73 4.70 Limestone . . .. .. 7.06 7-08 Dolomite. . .. .. .. 14.7 14.8 Porphyritic granophyre . . 0.78 0.80 All values recorded are averages of duplicate determinations. * Preferred value.* T Preferred value.s This method has been applied to some eighty silicate and carbonate rocks, and the values within the range 0-2 to 20.0 per cent.of magnesium oxide compared with results obtained by the gravimetric pyrophosphate procedure. The maximum differences from the gravimetric values were -0.20 and +0.27 per cent. The correlation coefficient was calculated as 0.998,310 EVANS with a relative standard deviation of 0.014. Comparative figures for a few rocks are shown in Table 111. The solution obtained after succinate precipitation of ammonia group elements is also a suitable medium for the complexometric determination of calcium. This paper is published by permission of the Government Chemist and the Director, Institute of Geological Sciences. 1. 2. 3. 4. 6. 6. 7. 8. 9. REFERENCES Shapiro, L., Chemist Analyst, 1969, 48, 73. Meyrowitz, R., Amer. Miner, 1964, 49, 769. King, H. G. C., and Pruden, G., Analyst. 1967, 92, 83. Bradfield, J., Analytica Chim. Ada, 1962, 27, 262. Hall, R. J., Gray, G. A., and Flynn, L. R., Analyst, 1966, 91, 102. King, H. G. C., Pruden, G., and Janes, N. F., Ibid., 1967, 92, 696. Willard, H. H., and Tang, N. K., Ind. Engng Chem. Anatyt. Edn, 1937,9, 367. Fleischer, M., and Stevens, R. E., Geodim. Cosmodim. Ada, 1962, 26, 626. Ingamells, C. O., and Suhr, N. H., Ibid., 1963, 27, 897. Received June 20th, 1967