968 Analyst, November, 1969, Vol. 94, jbp. 968-975 The Determination of Trace Amounts of Calcium in Stainless Steels by Solvent Extraction Followed by Atomic-absorption Spectrophotome try BY J. B. HEADRIDGE AND J. RICHARDSON (Department of Chemistry, The University, Shefield. S3 7HF) A method is described for the atomic-absorption spectrophotometric determination of 2 to 60 pg g-l of calcium in stainIess steels after solvent extraction of most of the iron, chromium and nickel with acetylacetone and pyridine. For thirteen B.C.S. steels, the calcium contents varied from 4 to 51 pg g-l. The standard deviation in the errors from the means for the various steels (fifty-eight determinations) was 1.4 pg g-l. The results of applying the solvent-extraction procedure to twenty-five other elements of interest to the steelmaker are also reported.THE metallurgist is increasingly requiring information on the concentration of trace elements in stainless steel, as even minute traces of some elements can have detrimental effects on the desired properties of these alloys. In this paper a method is described for the determination of calcium in stainless steel at concentrations of 2 to 60 pg g-l. So that the interference of trace amounts of phosphate and silicate in the atomic- absorption spectrophotometric determination of calcium can readily be eliminated, a nitrous oxide - acetylene flame is used. With this flame the 1 per cent. absorption value for calcium is about 0.08 pg rnl-l for many commercial instruments in the presence of high concentrations of another ionisable metal.The standard deviation of the background “noise” is usually about one sixth of the 1 per cent. absorption value for cheaper atomic-absorption spectro- photometers. If the standard deviation of the sample “noise” is the same as the background “noise” (this is the minimum; it is usually appreciably greater), then the standard deviation in measured absorbance for a sample is twice the standard deviation in the background “noise,” or about one quarter of the 1 per cent. absorption value. Therefore, under the best conditions, the standard deviation in the error for a calcium determination would be about 0.02 pg ml-l with most British commercial instruments. For a 1 per cent. w/v solution of stainless steel this amounts to 2 pg g-l; in practice, the figure is likely to be nearer 5 pg g-1.However, a precision approaching 1 pg g-l was considered necessary for the determination of calcium in stainless steel and this could only be accomplished by using, in a direct method, a more concentrated solution of the steel, with the increased possibility of burner clogging, or by extracting the base elements from the steel into an organic solvent and concentrating the aqueous phase, which contained the calcium, before nebulisation. It was decided to adopt the latter method, for it has the additional advantage of allowing calibration graphs to be prepared from pure calcium solutions without the necessity of adding weighed amounts of Specpure base elements, which may contain calcium, to the standard solutions.This arises because the residual amounts of base elements, which are left in the aqueous phase after solvent extraction, have no interfering effect on the calcium signal. A literature survey was undertaken to obtain information on suitable solvent-extraction methods for iron, chromium and nickel. Acetylacetone, which forms a separate layer with aqueous solutions, is a suitable extractant for iron(II1) within the pH range of 1 to about 7.112 The neutral tris(acetylacetonato)iron(III) is extracted. With nickel,2 the maximum percen- tage extraction with 0.1 M acetylacetone in benzene occurs a t pH 10 but only to the extent of 25 per cent. The low percentage extraction of nickel could result because most of the nickel may be present in the aqueous phase as Ni(acac),-.Ni2+ + 3Hacac --+ Ni(acac),- + 3H+. Alternatively, the complex in the aqueous phase could be [Ni(acac),(H20),]0, and hydrogen bonding between water molecules on the complex and those in the bulk of the solution would keep the neutral complex in the aqueous phase. 0 SAC and the authors.HEADRIDGE AND RICHARDSON 969 To induce the nickel to extract more completely into the acetylacetone layer, it is necessary to add a neutral non-hydrogen bonding complexing agent that will oust the third acetylacetonate anion or the water molecules and give the neutral complex, [Ni(aca~)~L?]o. Pyridine was selected for this purpose. From a buffer solution of equal amounts of pyndme and its hydrochloride at pH 5-2, the extraction of nickel was now almost complete (see below).Since this work was started, Tanino and Kitahara have also reported that nickel can be extracted from aqueous solution with chloroform containing acetylacetone and ~yridine.~ Starvl states that chromium(II1) is only extracted from aqueous solutions into pure acetylacetone if the chromium(II1) solution is refluxed with acetylacetone. The dissociation of aquochromium( 111) complexes before complex formation is very slow at room temperature, hence the necessity for refluxing. However, when acetylacetone plus pyridine were used with chromium(II1) solutions that had been prepared by dissolving the metal in aqua regia and removing most of the acid by evaporation, it was found that 97 to 98 per cent. of the chromium was consistently extracted. This difference in behaviour of chromium( 111) is under further investigation, but we have verified that no extraction of chromium(II1) occurs at room temperature in the absence of pyridine.The solvent-extraction procedure for the removal of most of the iron, chromium and nickel from stainless steels is described below. Calcium is not extracted at pH 5.2 by acetylacetone and pyridine and is left in the aqueous phase for subsequent determination by at omic-absorpt ion spectrophot ome try. EXPERIMENTAL APPARATUS- a nitrous oxide - acetylene burner. Atomic-absorption spectrophotometry was carried out on a Unicam SP90, fitted with REAGENTS- Hydrochloric acid, sp.gr. 1-18. Nitric acid, sp.gr. 1.42. Hydrojuoric acid, 40 per cent. w/w. Potassium chloride. Calcium carbonate. These reagents were of analytical-reagent grade.Iron, chromium, nickel, manganese and titanium metals and molybdenum trioxide were of Specpure quality (Johnson, Matthey Ltd.). A cetylacetone-General-purpose reagent. Pyridine-General-purpose reagent. Re-distil both reagents before use. Freshly distilled water-Store in a polypropylene aspirator. Standard calcium solution A-Dissolve 0-624 g of dried calcium carbonate in the minimum amount of hydrochloric acid and dilute the solution to 1 litre in a graduated flask. Transfer the solution at once to a dry polythene bottle. Transfer the solution at once to a dry polythene bottle. 1 ml of solution B = 25 pg of calcium. 1 ml of solution A = 250 pg of calcium. Standard calcium solution B-Dilute 10 ml of solution A to 100 ml in a graduated flask.PRELIMINARY INVESTIGATIONS- It was hoped to develop a satisfactory method for the determination of calcium by dissolving 1 g of stainless steel in aqua regia, evaporating the solution to the first appearance of solid, dissolving this solid in water, diluting the solution to 50ml and extracting most of the iron(II1) into an organic phase by shaking this solution with 50 ml of acetylacetone in a separating funnel. On adding 10 ml of pyridine and re-shaking the solution, it was then expected that most of the chromium(II1) and nickel(I1) would also be extracted into the organic phase, and cakium ions remain in the aqueous phase. When acetylacetone reacts with metal ions to form complexes, hydrogen ions are released. M3+ + 3Hacac -+ M(acac), + 3H+.970 [Analyst, Vol.94 From 1 g of iron metal, 54 mmoles of hydrogen ions are produced. Another source of hydrogen ions in the solution is obviously the acid that remains in the solution when the steel solution has been evaporated to the first appearance of solid. It was established by titration with standard alkali that the 50 ml of steel solution, which were ready to be extracted with acetylacetone, always contained about 40 mmoles of hydrogen ions; 94 mmoles of hydrogen ions react with 7.6ml of pyridine to produce pyridinium ions and, as excess of pyridine in the aqueous layer is necessary for complexation of the nickel, it was decided to use 10 ml of pyridine in the solvent-extraction procedure. Therefore, the following tentative method was devised for preparing from stainless steel a solution suitable for nebulisation in the atomic-absorption spectrophotometer to determine the calcium content of the steel. HEADRIDGE AND RICHARDSON : DETERMINATION OF TRACE AMOUNTS TENTATIVE METHOD- Dissolve 1 g of steel in 20 ml of concentrated hydrochloric acid and 5 ml of concentrated nitric acid.Evaporate the resulting solution to the first appearance of appreciable solid material (about 7 ml). Take up the solid in water and dilute the solution to 50 ml. Transfer the solution to a separating funnel and shake the solution with 50 rnl of acetylacetone. Add 10ml of pyridine and re-shake the solution. Remove the lower aqueous layer containing the calcium ions and evaporate it to the first appearance of solid. Add 1 ml of potassium chloride solution (2.5 per cent.w/v in potassium ions) to suppress the ionisation of calcium atoms in the flame, take up the solid in water and dilute the solution to 25 ml in a graduated flask. Transfer the solution immediately to a dry polythene bottle. The usual procedure for a blank solution is to replace 1 g of stainless steel by 1 g of Specpure iron and to carry through the procedure exactly as for a steel. However, it cannot be assumed that this iron is completely free from calcium, which is a trace impurity in most materials. Therefore, it was decided to dispense with the use of 1 g of Specpure iron and replace it with 54 mmoles of hydrogen ions, added as hydrochloric acid. (The calcium content of this acid is negligible.) The tentative procedure with the blank was, therefore, as follows.TENTATIVE PROCEDURE WITH BLANK- Evaporate 20 ml of concentrated hydrochloric acid and 5 ml of concentrated nitric acid to 7 ml in a beaker. Add 10-8 ml of 5 M hydrochloric acid and dilute the solution to 50 ml. Transfer the solution to a separating funnel and proceed as for the steel solution. Conditions had now been devised to test the effectiveness of the solvent-extraction pro- cedure for iron, chromium and nickel. For convenience, the concentration of metal in the aqueous phase before extraction was taken as 0.2 per cent. w/v. The procedure was similar to that outlined above for steel except that 100 mg of Specpure metal were used. The con- centration of metal ions left in the aqueous phase after solvent extraction was determined by atomic-absorption spectrophotometry.Standard solutions for the calibration graphs were prepared by adding suitable aliquots of more concentrated aqueous standard solutions to the aqueous phase of a.n extracted blank solution. The results obtained by solvent extraction with an atomic-absorption spectrophotometric finish are shown in Table I. TABLE I DATA ON THE SOLVENT EXTRACTION OF IRON, CHROMIUM AND NICKEL Wavelength, Slit width, Lamp current, Percentage Inorganic species nm mm mA extracted Iron(II1) .. .. 248.3 0.10 15 98 Chromium(II1j . . .. 357-9 0.08 10 97 Nickel(I1) . . .. .. 232.0 0.10 15 95 The light path was 1 cm above the burner top, the acetylene flow-rate 3.0 1 minute-1 at 15 p.s.i. pressure and the nitrous oxide flow-rate 5.0 1 minute-l at 30 p.s.i.These results show that the iron, chromium and nickel are almost completely extracted with the tentative procedure. It was hoped that it would be possible to prepare calibration graphs for calcium in stainless steels by adding aliquots of standard calcium solutions to the extracted blank solution together with the potassium chloride solution. However, before this method was adopted,November, 19691 971 it was necessary to check that the low concentration of residual base elements had no effect on the calcium signal obtained in their absence, and that no calcium is removed into the organic phase in the extraction procedure. OF CALCIUM IN STAINLESS STEELS BY SOLVENT EXTRACTION Calcium concentration, pg ml” Fig. 1. Calcium calibration A calibration graph obtained by adding aliquots of standard calcium solution to the extracted blank solution is shown in Fig. 1.An identical calibration graph was obtained when 20-mg amounts of iron(II1) were added together with the aliquots of standard calcium solution. This proves that residual amounts of iron have no effect on the calcium signal, which is not unexpected, for the residual concentration of iron in the 25 ml of final solution is only 800pgml-1, but the concentrations of potassium ion and pyridinium chloride are 1000 pg ml-l and about 3.8 M, respectively. Similarly a calibration graph obtained by adding aliquots of standard calcium solution to blank solutions before extraction, followed by the solvent-extraction procedure and work- up, was identical with that shown in Fig.1. This proves that, as expected, no calcium is lost in the solvent-extraction step. In this instance the blank solution was prepared by using 1 g of Specpure iron per 50 ml of solution to be extracted rather than by adding 54 mmoles of hydrogen ions. Finally, it was necessary to investigate the possible interference effects of other con- stituents of stainless steels on the calcium signals. This was carried out by replacing x g of the 1 g of iron by x g of the other element, where lOOx per cent. is the maximum concentration of that element in stainless steels, adding aliquots of standard calcium solution, carrying through the solvent-extraction procedure and obtaining a calibration graph. The elements used with their respective concentrations are shown below.graph Element . . . . Chromium Nickel Molybdenum Manganese Titanium Percentage present . . 30 25 3 2 1 These calibration graphs were identical with that obtained for extracted solutions of 1 g of Specpure iron plus added amounts of calcium. These results showed that residual amounts of other elements likely to be present in the aqueous phase of a steel solution, after solvent extraction, will have no effect on the calcium signals obtained in their absence. However, it should be pointed out that the above remarks have only been substantiated for amounts of calcium not in excess of 6 pg ml-l in the solutions that are ready for spraying. This corresponds to a calcium concentration of 125 p g g-l in a stainless steel. For larger amounts of calcium there may be interference effects.The instrumental conditions for the atomic-absorption spectrophotometric determination of calcium are given later. MAIN STUDY A method was now available for the determination of calcium in stainless steels. How- ever, two slight modifications to the tentative procedure were considered desirable. When the tentative procedure was applied to stainless steels, the final solution usually contained972 [Analyst, Vol. 94 a small precipitate of hydrated silica. As this precipitate could possibly contain a trace of a calcium silicate mineral that had remained undissolved on dissolution of the steel, it was decided to bring all of the constituents into solution at an early stage in the analysis. This was achieved by dissolving the steel with aqua regia in a Teflon beaker, evaporating the solution to about 7 ml, adding 3 drops (0.15 ml) of hydrofluoric acid solution (40 per cent. w/w) and simmering the solution for 15 minutes, with the beaker partly covered with a Teflon cover.Secondly, the volume of pyridine used in the solvent extraction was increased from 10 to 15 ml. With 15 ml of pyridine the concentrations of pyridine and pyridinium ion in the aqueous phase are about equal and the buffering capacity of the solution is at a maximum. With 10 ml of pyridine the ratio of pyridine to pyridinium ion is about 1 : 3. The final method for the determination of calcium in stainless steels is, therefore, as follows. HEADRIDGE AND RICHARDSON : DETERMINATION OF TRACE AMOUNTS When this procedure was adopted the final solutions were always clear.METHOD Dissolve 1 g of steel in 20 ml of concentrated hydrochloric acid and 5 ml of concentrated nitric acid in a 150-ml Teflon beaker. Evaporate the resulting solution to the first appearance of appreciable amounts of solid material (about 7 ml). Add 3 drops (0.15 ml) of hydrofluoric acid (40 per cent. w/w) and simmer the solution for 15 minutes, with the beaker partly covered with a Teflon cover. Take up the solid in water and dilute the solution to 50 ml. Transfer the solution to a Pyrex separating funnel and shake the solution with 50 ml of acetylacetone. Add 15ml of pyridine and re-shake it. Remove the lower aqueous layer containing the calcium ions and evaporate it to the first appearance of solid. Add 1 ml of potassium chloride solution (2-5 per cent.w/v in potassium ions), take up the solid in water and dilute the solution to 25 ml in a Pyrex graduated flask. Transfer the solution immediately to a dry polythene bottle. Nebulise the solution in the Unicam SP90 atomic-absorption spectro- photometer by using the conditions given in Table 11, and determine the absorbance for this solution. TABLE I1 INSTRUMENTAL CONDITIONS FOR THE DETERMINATION OF CALCIUM Acetylene flow-rate a t 15 p.s.i., 1 minute-' . . . . .. 3.1 Nitrous oxide flow-rate a t 30 p.s.i., 1 minute-1 . . .. . . 5-0 Wavelength for use with the calcium lamp, nm . . . . 422.7 Slit width, mm . . .. . . .. .. . . .. 0-02 Lamp current, mA . . . . . . .. . . . . .. 12 Distance of centre of light path above burner, mm . . .. 10 Scale expansion .. .. . . .. .. .. .. x 3 Prepare the calibration graph for calcium in the following way. Evaporate in each of six 150-ml Teflon beakers, 20 ml of concentrated hydrochloric acid and 5 ml of concentrated nitric acid to 7ml. To each beaker add 3 drops (0.15ml) of hydrofluoric acid (40 per cent. w/w) and simmer the solutions for 15 minutes, with the beakers partly covered with Teflon covers. To each solution add 10.8 ml of 5 M hydrochloric acid and dilute to 50 ml with distilled water. Transfer the solutions to separating funnels and shake with 50 ml of acetyl- acetone. Add 15ml of pyridine and re-shake them. In all instances, remove the lower aqueous layers and evaporate them to volumes of about 10ml. To six 25-ml graduated flasks marked 0, 0.5, 1.0, 1.5, 2.0 and 2-5 pg ml-l of added calcium, add 0, 0.5, 1.0, 1.5, 2.0 and 2.5 ml, respectively, of standard calcium solution B.To each flask add 1 ml of potassium chloride solution (2.5 per cent. w/v in potassium ions). Then transfer the extracted and con- centrated solutions to each flask. Wash out the beakers with distilled water, add the washings to the appropriate flasks and make up to the marks. The flask marked 0 pg ml-l of added calcium is the blank solution for both the calibration graph and the steel solutions. It contains trace amounts of calcium from all of the reagents. Transfer the solutions immediately to dry polythene bottles. If a large Teflon beaker is available, a mixed acid stock solution, which has been extracted with acetylacetone and pyridine, can be prepared; 10-ml aliquots of this stock solution are then transferred by pipette into each 25-ml graduated flask.The solutions are now ready for nebulisation with the Unicam SP90 spectrophotometer. The following spraying procedure should be adopted. With a x3 scale expansion on the recorder, spray the 2.5 pgml-l calcium solution, then distilled water and finally theNovember, 19691 OF CALCIUM IN STAINLESS STEELS BY SOLVENT EXTRACTION 973 0 pg ml-l solution (the blank solution). Subtract the absorbance of the 0 pg ml-l solution from that of the 2-5 pg ml-l solution. The difference, which is the absorbance of the 2.5 pg ml-I solution corrected for the blank, should be about 0.08. If so, the instrument settings are correct for a satisfactory determination of calcium. Now spray the solutions in the following order: (i), distilled water; (ii), 2.5 pg ml-l calcium solution; (iii), distilled water; (iv), 2-0 pg ml-l calcium solution; (v), distilled water; (vi), 2.5 pg ml-l calcium solution; (vii), distilled water; (viii), 1.5 pg ml-l calcium solution; (ix), distilled water; (x), 2.5 pg ml-l calcium solution, and so on.Every second solution should be distilled water and every fourth solution the most concentrated standard solution. Continue in this way until all of the standard solutions, the blank solution and the steel solutions have been sprayed. The transmission of any solution containing calcium is determined by drawing the best lines through the “noise” on the recording for this solution and on the recordings for the distilled water immediately preceding and following that for the solution containing calcium, averaging the values for the distilled water and subtracting this average from the measured value for the solution containing calcium.This procedure corrects for slight drift in the base-line. The transmissions of every calcium-containing solution are converted into absorbances by using appropriate tables, and the absorbance of each solution is divided by the average absorbance for the two sprayings of the most concentrated standard solution that come immediately before and after it. This procedure corrects for any slight drift in flame tem- perature, rate of nebulisation, etc. Construct the calibration graph by plotting these absorbance ratios against concentration of the calcium solution. Read off the concentration of calcium in the steel solution from this graph. The graph does not pass through the origin because there is some calcium in the blank solution.It will be appreciated that the same stock solutions of acids, etc., must be used in the construction of the calibration graph and in preparing the steel solutions. A new calibration graph is necessary only when fresh stock solutions have to be used. RESULTS By using the above method the calcium contents of thirteen stainless steels were deter- mined. These results are shown in Table 111. The blank solution usually contained about 0.6 pg ml-I of calcium. The absorbance for a solution containing 2.5 pg ml-1 of calcium was always about 0.08, after correction for the blank.This gives a 1 per cent. absorption value of 0.12(5) pg ml-l. TABLE I11 RESULTS FOR THE DETERMINATION OF CALCIUM IX STAINLESS STEELS B.C.S. Steel 235/2 246 261 33 1 332 333 334 335 336 337 338 339 340 Calcium content, pg g-l, by the described method 4, 4, 5, 8 7, 7, 10, 11 49, 51, 52, 52 3*, 4*, 4, 4, 5, 6 4, 6, 6, 6*, 7*, 8 2*, 2, 3, 5, 5, 5* 21, 22, 23, 23 12, 13, 14, 16 7, 9, 10, 11 6, 6, 7, 8 12, 12, 14, 16 10, 11, 12, 14 6, 6, 8, 11 * These results were obtained after subtracting a standard addition of calcium equivalent to 25 pg g-’ of calcium in the steel. SOLVENT EXTRACTION OF OTHER ELEMENTS WITH ACETYLACETONE- The percentages of iron, chromium, nickel and twenty-six other elements of interest to the steelmaker, which are extracted under the final conditions used for the analysis of stainless steels, are given in Table IV, together with information on related extraction procedures.974 HEADRIDGE AND RICHARDSON : DETERMINATION OF TRACE AMOUNTS [Analyst, VOl.94 TABLE IV EXTENT OF EXTRACTION OF VARIOUS ELEMENTS FROM THE AQUEOUS PHASE Element Aluminium . . Antimony(V) . . Arsenic(V) . . Bismuth . . Boron . . .. Calcium . . Cerium (111) Chromium(II1j * Cobalt(I1) . . Copper(I1) . . Iron(II1) . . Lead . . .. Magnesium . . Manganese(I1) Molybdenum(V1) Nickel . . .. Niobium(V) . . Phosphorus(V) Selenium(V1) . . Sulphur(V1) . . Tantalum(V) . . Tellurium(V1) . . Thallium(1) . . Titanium(1V) . . Tungsten(V1) . . Vanadium(V) . . Zinc . . . . Zirconium . . Tin(1V) .. .. .. .. .. .. .. . . .. .. .. . . .. .. .... .. . . .. .. . . .. . . . . .. .. .. .. . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . .. .. . . .. . . .. . . .. . . . . . . . . Extraction system and percentage extraded I I1 1x1 96 96 96 0 0 0 0 0 0 96 100 100 2 2 2 0 0 0 90 90 91 98 98 99 96 96 96 97 98 98 97 98 99 81 81 82 0 . 0 0 70 70 70 53 85 87 96 98 99 93 93 93 0'. 0 0 1. 1 1 0 , 0 0 88 88 90 0 0 0 2 2 2 96 100 100 6 8 8 2 0 0 100 100 100 87 92 91 91 92 92 r A -% for calcium, magnesium, iron, chromium and nickel, 50-ml In all instances. exceDt aliquots of aqueous solutiin containing 50mg-or 0.5 mmoles of the element were used in the solvent-extraction procedure. With calcium, magnesium, iron, chromium and nickel, 250 pg, 250 pg, 1 g, 0.3 g and 0.2 g were used in 50 ml of solution, respectively.The 50-ml aliquots also contained 8.25 ml of concentrated hydrochloric acid and 2 drops (0.10 ml) of hydrofluoric acid solution (40 per cent. w/w). Separate aliquots were treated in the following way. Procedure I-Shake the solution with 50 ml of acetylacetone, then add 15 ml of pyridine and re-shake it. (This is the procedure used in the analysis of the stainless steels.) Procedure II-Carry out procedure I, remove the lower aqueous layer and re-shake it with 50ml of acetylactone. Procedure III-Carry out procedure I, remove the lower aqueous layer and re-shake it with 50 ml of chloroform - acetylacetone mixture (4 + 1 v/v). In all instances the extracted aqueous layers were evaporated to the first appearance of solid. This solid was dissolved in a little water and the solution diluted to 25 ml.Aliquots of these solutions were analysed for element content by well established atomic-absorption spectrophotometric, solution spectrophotometric, titrimetric and gravimetric methods. DISCUSSION The results for the determination of calcium in stainless steels are considered to be very satisfactory. The standard deviation in the errors from the mean for the various steels is 1.4 pg g-l. Less precise results for the determination of calcium in some of these stainless steels by two direct methods are available in a restricted report.4 In nearly all instances our resulls are in good agreement with these results. The results of the solvent-extraction studies (procedure I) indicate that a method similar to that used for calcium could probably be used for the determination of antimony, arsenic, magnesium, selenium, tellurium and thallium in steels. The 2 per cent. of thallium extracted can be attributed to aerial oxidation of a small amount of thallium(1) to thallium(III), whichNovember, 19691 OF CALCIUM IN STAINLESS STEELS BY SOLVENT EXTRACTION 975 is then extracted. This could be prevented by the presence of a suitable reducing agent. It will be noted in Table IV that re-shaking of the aqueous phase with more acetylacetone or with an acetylacetone - chloroform mixture results in more complete extraction with a few of the elements, but with most a second extraction makes little difference to the amount extracted by one shaking with acetylacetone and pyridine. We thank the BISRA/Inter-Group Laboratories of the British Steel Corporation for a grant towards this work, and Mr. P. H. Scholes of BISRA and Dr. M. S. Taylor of Firth-Brown Ltd. for discussions on the project. REFERENCES 1. 2. 3. 4. BISRA Report, MG/D/404/69. Starg, J., with the assistance of Irving, H., “The Solvent Extraction of Metal Chelates,” Pergamon Tanino, K., and Kitahara, S., Sci. Pap. Inst. Phys. Chew. Res., Tokyo, 1967, 61, 35. Press, Oxford, 1964, p. 56. - , op. cit., p. 53. Received March 18th. 1969 Accepted April 29th, 1969