Masking of iron with fluoride in the extractive atomic-absorption spectrometric determination of chromium in steel

541

SHORT C%W.iUNICA~ONB

~-Mi~~~rn quantities of silver and copper ions in aqueous solutions are collected on dithizone precipitates, which are then floated with the aid of small nitrogen bubbles. This separation technique has been sua~s&dly applied to the atomic-absorption spectrophotometric determination of down to a tenth ppm of silver and copper in high-purity lead and zinc metals.

i%lanm, Vol. 22, pp 541-543. Pergamon Press, 1975 Printed in Great Britain

MASKING OF IRON WITH J?LUORIDE IN THE EXTRACTIVE ATOMIC-ABSORPTION SPECTROMETRIC DETERMINATION OF CHROMIUM IN STEEL (Received 22 July 1974. Revised 31 October 1974. Accepted 14 vovember

The ~te~tion of chromium by means of atomicabsorption spectrometry has been the subject of extensive study particularly aimed at overcoming interference from iron when using an air-acetylene flame. The mechanism of this interference has been discussed by Roos and coworkers,‘sr and more recently by Ottaway and Pradhan3 who have suggested 8hydroxyquinoline to be. an effective releasing agent. Thomerson and Price? recommended a procedure using a nitrous oxide-acetylene flame. Iron causes an enhancement of the chromium absorption signal in the nitrous oxide flame and this increases slightly with increase in iron concentration; thus, iron has to be added to the calibration solutions. The present study was made in an attempt to develop a procedure that is completely free from iron interference, that uses an air-acetytene flame and that does not require the use of releasing agents. This can only be done by completely separating the chromium from the iron matrix. Bryan and Dear? determined chromium by flame photometry after extracting chromium(VI) into 4-methyl-2-pentanone; persulphate catalysed by silver(I) was used to oxidize chromium(II1) to chromium(VI). Feldman and Purdy6 applied this extraction procedure to the determination of chromium by atomi~~b~~tion spectrometry, permanganate being used as the oxidant; iron interference, however, was not studied. Blundy,7 in developing an extractive calorimetric procedure for the determination of chromium in 1958, made a study of the effectiveness of several oxidants for chromium(III) and concluded that of those studied only cerium(IV) gave complete oxidation and good reproducib~~ty. The atom-abso~tion method developed in this work is based on Blundy’s extract& procedure, and iron(iI1) is masked by means of fluoride.

dichromate solution and with a standard c~orniu~1~~ solution prepared by reducing potassium dichromate solution with sulphur dioxide and then boiling off the excess of sulphur dioxide. The absorbance values obtained after oxidation [in the case of chromium(III)], extraction into 4-methyl-2pentanone and spraying into the flame were identical. Iron(II1) was a&o found to be extracted into 4-methyl-2pentanone at the hy~~~o~c acid concentrations (i.e., l-3&4) required to extract cbromium(VI), and the atomicabsorption signal of chromium was greatly reduced as a result. A thousandfold ratio of iron(II1) to chromium in the aqueous phase reduced the absorbance signal obtained on spraying the organic extract bv about 50%. Several methods of ¶ting iron were tried. Extract& of iron with di-isourouvl ether from 7.75825M hvdrochloric‘ acid8 before ihe b;xidation of ~~o~u~~j was effective, but the high hydrochloric acid concentration adversely affected the subsequent oxidation by cerium(IV). Cupferron extraction of iron and precipitation of iron(II1) hydroxide were both ineffective, in that low recoveries of chromium were obtained. The final procedure adopted was to mask the iron with fluoride; this proved to be highly effective up to at least a thou~dfold ratio of iron to chromium. Pmushottam and co-workers’ had shown previously that guoride suppresses interference by iron when aqueous chromium samples are sprayed into the atomicabsorption flame. The following procedure is recommended for the determination of chromium in steel. If the addition of fluoride is omitted from this procedure, interference by iron becomes apparent at iron to chromium ratios above 5:l. No difference in results was observed when the fluoride was added after the oxidation step, but the results given here were determined with fluoride added before oxidation of chromium.

EXPERIMENTAL.

Atomic-ab~rption ~asuremen~ were made with a Hilger and Watts Atomsnek H 1170. using a chromium hollow-cathode lamp supplied by V.. A. Howe, Ltd. Complete oxidation of chromium(II1) was checked by comparing results obtained with a standard potassium

1974)

Reagents Stdphuric acid, 25% v/v. Sulphuric acid, 12.5% v/v. Hydrogen peroxide, 100~vol.

542

SHORT

COMMUNlCATIONS

Sodium jIucride solution, 4.6% w/v. Ammonium hexanitratocerate(lV), 1.1% solution in 1M

sulphuric acid. Hydrochloric acid SM. 4-Methyl-2-pentanone saturated with 1M hydrochloric

acid. Shake together vigorously equal volumes of 4-methyl2-pentanone and 1M hydrochloric acid, and discard the (lower) aqueous layer. Droplets of the acid should be allowed to separate completely or removed by passing the solvent through a small filter paper. Stock standard chromium(V1) solution, 1000 H/ml. Dissolve 2.8282 g of analytical-reagent grade potassium dichromate in water and dilute to 1 litre in a volumetric flask. Working

standard

chromium(VI)

solution,

20

&ml.

Dilute 10 ml of the stock chromium(V1) solution to 500 ml with water in a volumetric flask.

of water and continue as for the calibration curve procedure beginning at “add 1Oml of 8M hydrochloric acid”. In the case of certain steel samples the organic extracts are not particularly stable and the solvent should be sprayed within 5 min of extraction (Note 3). Notes

1. The final hydrochloric acid concentration must be l-3M for complete extraction of chromium. 2. For steel samples containing <0.1x, &l-0.3% and 0.305% of chromium take 2Oml, lo-ml and 5-ml aliquots of sample solution respectively. 3. The reason for this is not known but the relative instability of the extract appears to occur at high iron to chromium ratios. RESULTS

Dissolution of steel samples

Weigh 0500 g of steel sample into a 2%ml conical flask and add 50 ml of 25% sulphuric acid. Heat gently on a hot-plate until hydrogen evolution ceases. Remove the flask from the hot-plate, allow it to cool slightly and add lOO-vol hydrogen peroxide dropwise to oxidize any carbon or carbide residues. Boil the solution gently to destroy excess of hydrogen peroxide. Cool the solution and dilute to 100 ml with water in a volumetric flask. Preparation of calibration curve

Pipette suitable volumes of working standard chromium(V1) solution containing up to 400 pg of chromium into 10%ml separating funnels. Dilute to 60 ml with water, add 10 ml of 8M hydrochloric acid (Note 1) and 20 ml (by pipette) of 4-methyl-Zpentanone saturated with 1M hydrochloric acid and shake the mixture for 1 min. Allow the two phases to separate, discard the (lower) aqueous phase and make atomic-absorption measurements on the organic layer. The organic extracts are stable for at least 4 hr. The optimum burner height was found to be similar to that used for determining chromium in aqueous systems, i.e., observations are made just above the primary reaction zone, but the flame should be less fuel-rich. Ontimum instrument conditions for the Atomspek H 1170 were found to be as follows. Wavelength Lamp current Slit-width Burner Observation height above burner head Air flowmeter

Acetylene flowmeter

357.9 nm 10 mA 10 pm 13cm air-acetylene, 7.5 mm

single slot

12.5 (nebulixer) at 30 psig feed pressure * 12 (auxiliary air) at 30 psig feed pressure * 3 at 5 psig feed pressure

* Values for use with aqueous chromium solutions: 3.6 (auxiliary air) and 4.3 (acetylene). Sensitivities quoted by instrument manufacturers for the determination of chromium when aqueous solutions are sprayed are within the range 0.05-015 &ml. In the present work sensitivities of 0.12 and 0.07 &ml were obtained with aqueous and organic solutions respectively.

AND DISCUSSION

The calibration curve was nearly rectilinear and it was shown that up to a thousandfold ratio of iron to chromium could be tolerated without change in absorbance signal. Results obtained with a range of low alloy and mild steels are shown in Table 1. Table 1. Analyses of British Chemical samples

TyPe of steel

BCS No

Standardued chromium content, %

Low alloy

251/l

051

Low alloy

252/I

0.42

Mild Mdd Mdd

273 325 321

0.07, 0.22 0.106

Mdd

322

0 039

Standards

Chromum

steel

found_*

% 051, O-51, 0.51, @50, 053, 0 51 044, 041,044, O-43, 042, 0.42 0.070, O-073, 0070 0 23, 0.22, 0.22 0106,o 110.0100. 0100 Oao, eO40, 0039

* Each value given is for an individual dissolution of steel sample. The extractive procedure recommended is an alternative to methods in which aqueous sample solutions are sprayed into the flame. The procedure has two advantages over the other methods in that sensitivity is slightly increased and the chromium is separated from iron which interferes. Even the procedure of Ottaway and Pradhan, which possibly comes nearest to being interference-free, is only so at particular observation heights and with particular types of burners.’ The procedure recommended herein has of course, the disadvantage that the oxidation and extraction steps are additional to the steps carried out in the usual procedure.

Acknowledgement-One

of us (S.S.) is grateful to the University of Tab&, Iran, for study leave.

Chemistry Department University of Technology Loughborough Leics, U.K.

A. G. FOGG S. S~LEYMANLOO

D.

THORBURN

BURNS

Procedure

Pipette an aliquot of the solution containing the steel sample (see Note 2) into a 100~ml conical flask and dilute the solution to 20 ml with 12.5% sulphuric acid. Add 4 ml of sodium fluoride solution and 25 ml of ammonium hexanitratocerate(IV) solution and heat in a boiling waterbath for 25 min. Cool in an ice-bath to 10” or less. Transfer the.solution to a lOO-ml separating funnel with 20 ml

REFERENCES

1. J. T. H. Roos and W. J. Price, Spectrochim. Acta, 1971, 26B, 441. 2. J. T. H. Roos, ibid., 1972, 27B, 473. 3. J. M. Ottaway and N. K. Pradhan, Talanta, 1973, 20, 921.

SHORT

543

COMMUNICATIONS

4. D. R. Thomerson and W. J. Price, Analyst, 1971, 96, 321. 5. H. A. Bryan and J. A. Dean, Anal. Chem., 1957, 29, 1289. 6. F. J. Feldman and W. C. Purdy, Anal. Chim. Acta, 1965, 33, 273.

7. P. D. Bhmdy, Analyst, 1958, 83, 555. 8. K. Kodama, Methods of Quantitative Inorganic Analysis, p. 135. Interscience, New York, 1963. 9. A. Purushottam, P. P. Naidu and S. S. Lal, Talanta, 1973, 20, 631.

Summary-Chromium in steel is determined by oxidation to dichromate, extraction into methyl isobutyl ketone from l-3M hydrochloric acid, and atomic-absorption measurements on the extract. The interference of iron in the atomic absorption is eliminated by using fluoride to keep the iron(II1) in the aqueous phase in the extraction step.

Tahnta,

Vol. 22, pp. 543-544.

Pergamon

Press, 1975. Printed m Great Entam

DETERMINATION OF TIN AS STANNITE-KESTERITE CASSITERITE IN ORES

AND

(Receiwd 22 August 1974. Accepted 5 November 1974)

The Mines Branch has recently issued a zinc-tin-copperlead ore, MP-I, as a certified reference material with recommended values for nine elements. Certain difficulties associated with the volumetric methods used in the certi& cation of MP-1 for tin, however, became apparent in the interlaboratory programme.’ In view of the future issuing of a second reference material, KC-l, to be certified for tin, an investigation was undertaken to find the source of these problems. For this purpose, it was desirable to have a knowledge of the relative proportions of the various tin minerals such as stannite-kesterite and cassiterite in MP-1 and KC-l. Tin as stannite, CuzFeSnS,, has been determined in the presence of cassiterite, SnOz, by the selective decomposition of stannite with bromine in carbon tetrachloride2*3 or with potassium chlorate in concentrated hydrochloric acid.4 This present work describes the selective decomposition of stannite-kesterite with sodium nitrate in glacial acetic acid. This method is simple and rapid and avoids the use of liquid bromine. Its applicability to the analysis of two certified reference ores is described.

EXPERIMEmAL

Decomposition of stannite-kesterite

One g of sodium nitrate and 10 ml of glacial acetic acid are added to a ml-2 g sample of ore in a 600~ml beaker and mixed by gentle swirling. After a further addition of 60 ml of acetic acid, the uncovered beaker is heated to allow the acetic acid to evaporate. When the volume of the contents has decreased to approximately 5 ml or less the beaker wall is washed down with water and 5.0 ml of concentrated hydrochloric acid and more water, if necessary, are added to make the volume approximately 50 ml. The insoluble material, composed of unattacked minerals and elemental sulphur*, is filtered off on a Whatman No. 42 paper and thoroughly washed with hot water. The filtrate is reserved for the determination of the tin present as stannite-kesterite. The tin present as cassiterite is given by the difference between the total tin content of the ore and the tin present as stannite-kesterite or it may be determined directly in *The sulphur produced during decomposition of the sulphidic components remains in solution in acetic acid but precipitates in finely divided form on addition of water.

the insoluble material. If determined directly, great care must be taken to ensure a quantitative transfer of the high-density cassiterite from the reaction vessel to the filter paper. The paper containing the insoluble material is ashed in a zirconium crucible and the tin content determined as below. Iodometric determination of tin

For the determination of total tin or of tin in the insoluble material, i.e., tin present as cassiterite, solid samples must be fused with a 1:l mixture of sodium peroxide and sodium carbonate in a zirconium crucible, quenched in water and then acidified with concentrated hydrochloric acid.5 The tin is separated from interfering ions such as copper and molybdenum by precipitation as the hydrous oxide: with ammonia solution. After redissolution of the hydrous oxide in hydrochloric acid, the tin is reduced to the stannous state with iron metal in an air-free atmosphere6 and is titrated with a standardized solution of potassium iodate. The tin present as stannite-kesterite must be precipitated from the filtrate as the hydrous oxide for two reasons. First, the tin must be separated from the copper derived from stannite itself and, furthermore, boiling sodium nitrate-acetic acid mixture attacks other sulphide minerals containing elements that interfere in the iodometric determination of tin. Secondly, the presence of unreacted sodium nitrate in the filtrate would strongly interfere in the iodometric determination when the tin solution is acidified with hydrochloric acid. Tin-bearing ores studied

The reactivity of stannite-kesterite with sodium nitrate acetic acid mixture was tested with a stannite concentrate prepared from ore from Stannex Mines, British Columbia, Canada. The inertness of SnOz to treatment with sodium nitrate-acetic acid mixture was tested with a Bolivian cassiterite concentrate (NBS 137, 5664% Sn) and reagent grade stannic oxide (Fisher Scientific). The certified reference materials, MP-1 and KC-l, were treated with sodium nitrate-acetic acid mixture to determine the tin present as stannite-kesterite and as cassiterite. RESULTS

AND DISCUSSION

The results of the chemical determination of the tin minerals in the stannite concentrate, cassiterite and stannic