364 JOHNSON AND POLHILL: THE USE OF SODIUM HEXAMETAPHOSPHATE [VOl. 80

The Use of Sodium Hexametaphosphate in the Determination of Traces of Lead in Food

BY E. I. JOHNSON AND R. D. &4. POLHILL

The alkaline-earth phosphates, including magnesium phosphate, can be kept in solution, under conditions suitable for the dithizone extraction of lead, by the addition of sodium hexametaphosphate. This facilitates the use of a simple dithizone purification step in the determination of traces of lead in a wide range of foods.

IN the examination of foods and some other materials for traces of lead, it is sometimes necessary1 s 2 to separate lead from quantities of calcium and magnesium phosphates greater than can be maintained in alkaline solution by any reasonable amount of citrate. Separation as the sulphide is a tedious and lengthy process. Separation with diethylammonium dithio- carbamate from acid solution has been u ~ e d , ~ ? ~ but the subsequent evaporation of the chloroform solution and wet oxidation of the residue shows no great gain in ease of manipula- tion over the sulphide process. Clifford, in his report on lead,5 mentions that, although the dithizone extraction affords a convenient means of lead isolation, the presence of magnesium phosphate will vitiate the extraction because of occlusion of lead on the magnesium ammonium phosphate, which will be precipitated at any pH high enough for normal efficient lead extraction.

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May, 19551 IN THE DETERMIEATION OF TRACES OF LEAD IN FOOD 366

The procedure described below uses sodium hexametaphosphate to prevent or delay the precipitation of magnesium ammonium phosphate, and thus extends the applicability of dithizone extraction of lead from alkaline solution. The use of sodium hexametaphosphate for this purpose is new so far as we are aware. The rest of the procedure described below is taken from recent well-known work4j6 and adapted to give a method which we regard as suitable for routine use.

Either wet or dry ashing may be used for destroying the organic matter in the sample, but the use of an ashing aid containing calcium or magnesium is not advised for two reasons. The introduction of relatively large amounts of calcium or magnesium will tend to defeat the object of the sodium hexametaphosphate; and we have not found it easy to obtain solutions of the nitrates of these metals free from traces of lead. The use of the aqueous dithizone solution is taken from the recent report of the Society’s Lead Panel* and has been found to be a convenient and flexible device for introducing purified dithizone into a system without altering the volume of the chloroform phase. We have used it both in the preliminary extraction for the isolation of lead and, in conjunction with Snyder’s mono-colour technique,6 for the preparation of the final solution for colour measurement.

Most of the reagents used in this work are identical with those described in the recent report of the Society’s Lead Panel or have been derived from them by dilution or mixing. Solution A is taken from Snyder’s paper.6 So far as we know no special effort has yet been made to produce sodium hexametaphosphate as a “lead-free” reagent. The B.P.C. monograph allows it to contain up to 25p.p.m. of lead. A reagent grade as purchased was found to contain 0.2p.p.m. of lead.

The lead content of a 10 per cent. solution of sodium hexametaphosphate can be reduced to a very low level by adjusting it to pH 9 with ammonium hydroxide and extracting with dithizone in chloroform. The last of the dithizone can be. removed with chloroform after making the solution slightly acid. The solution should finally be re-adjusted to pH 9.5, as it is most stable in that state. Prepared in this way the solution has been found to be virtually free from lead and to retain its property of preventing precipitation of alkaline- earth phosphates under the conditions of use for at least 1 month.

METHOD REAGENTS-

preparation. All reagents must be substantially “lead free,” either by purchase or by special

Sul+huric acid, cowentrated. Nitric acid, concentrated. Hydrochloric acid, 5 N. Ammonium hydroxide, sp.gr. 0.880. Ammonium hydroxide, 5 N. Ammmium citrate solution-A 25 per cent. solution in water. Potassium cyanide solution-A 10 per cent. solution in water. Hydroxylamine hydrochloride-A 20 per cent. solution in water. Nitric acid, 1 in 100-One volume of the concentrated acid diluted to 100 volumes with

water. Solution A-Mix 340ml of ammonium hydroxide, sp.gr. 0.880, 75 ml of 2 per cent.

sodium sulphite, Na,SO,, solution, 30ml of 10 per cent. potassium cyanide solution and 605 ml of water.

Chloroform-Shake 250 ml of chloroform with 25 ml of water containing 1 ml of 10 per cent. potassium cyanide solution and about 20 drops of 5 N ammonium hydroxide, separate and reject the aqueous layer, wash the chloroform with water and filter it.

Dithixone, stock solution-0.1 per cent. of dithizone in chloroform. Filter and store in a refrigerator.

Dithizone, working solution-Shake 6ml of the stock dithizone solution with 9 m l of water and 1 ml of 5 N ammonium hydroxide. Separate and reject the lower layer. Centrifuge the aqueous layer until clear.

Standard lead solution, 1 ml = 10 pg of lead-(a) Dissolve 1.60 g of lead nitrate in water, add 10 ml of concentrated nitric acid and dilute to 1 litre. (b ) Dilute 1 volume of (a) to 100 volumes with water. Prepare dilution (b) freshly as required.

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366 JOHNSON AND POLHILL: THE USE OF SODIUM HEXAMETAPHOSPHATE [VOl. 80 PROCEDURE-

Destruction of organic matter-Destroy the organic matter by heating 5 g of the sample in a silica flask with 3 to 4 ml of sulphuric acid and small amounts of nitric acid until oxidation is complete, as shown by the liquid being colourless when cold. Add 5 ml of water and boil down again to fuming. Alternatively, ash the 5-g sample in a silica basin at a temperature not exceeding 500” C. The choice of method may depend on the character of the sample. Those which easily yield good bulky ashes should be dry-ashed; the others are best treated by wet combustion.

Extraction of lead-To the ash in the silica basin or the liquid in the flask, add 5 ml of water and 10 ml of 5 N hydrochloric acid, and boil gently for 5 minutes. If any insoluble matter remains, remove it by filtration, wash the filter with a few millilitres of hot water and add the washings to the main filtrate. Cool the solution and add 2 ml of 25 per cent. ammonium citrate solution and 10 ml of 10 per cent. sodium hexametaphosphate solution. Add a few drops of thymol blue indicator solution and sufficient ammonium hydroxide, sp.gr. 0.880, to give the blue - green colour indicating pH 9.0 to 9-5. Cool, add 1 ml of 10 per cent. potassium cyanide solution and, if much iron is present, add 1 ml of hydroxylamine hydrochloride solution. Transfer to a 100-ml separating funnel containing 10 ml of chloro- form, washing in with a few millilitres of water. The volume of the aqueous phase at this stage should be approximately 50ml. Add 10 drops of the dithizone working solution, shake vigorously for 1 minute and allow to separate. If the lower layer is red, add sufficient dithizone working solution until, after shaking, a purple, blue or green colour is obtained. Run off the chloroform layer into a small beaker and wash through with 1 or 2 ml of chloro- form. Add to the liquid in the separating funnel 3 ml of chloroform and 3 drops of dithizone solution. Shake vigorously for 30 seconds, allow the chloroform layer to separate and add it to the main chloroform extract. This last chloroform extract should be green. If it is not, further extractions with chlclroform and dithizone must be made until the green colour of the final extract indicates that all the lead has been extracted. Reject the aqueous layer, Return the chloroform extract to the separating funnel, washing it in with a little chloroform. Add 10ml of 1 in 100 nitric acid and shake vigorously for 1 minute. Allow to separate, and reject the chloroform layer as completely as possible.

Determination of lead-To the aqueous layer left in the separating funnel add 30 ml of solution A, exactly 10ml of chloroform and 10 drops of the dithizone working solution; shake vigorously for 1 minute and allow to settle. Run off a little of the chloroform layer. Insert a plug of cotton-wool into the dry stem of the funnel and, after rejecting the first runnings, fill a l-cm spectrophotometer cell with the chloroform solution. Measure the optical density against chloroform at 520 mp.

Blank or control-Prepare a reagent blank solution under the same conditions as the test, omitting only the sample, and determine the optical density.

Standard graph-Measure 0, 1.0, 2.0, 3.0 and 4.0ml of standard lead solution into separating funnels containing 10ml of 1 in 100 nitric acid and proceed as described under “determination of lead.”

Precautions against contamination-The method is a sensitive one and all the precautions usual in this type of work must be observed. These have been described elsewhere.4

RESULTS The method as described has been in use in this laboratory for several months and has

been used on a wide variety of foods. Recoveries of added lead are complete and consistent, and we have no doubts regarding its precision in the hands of experienced workers. Samples of syrup and cocoa have been examined both by this method and the method given in the Society’s recent Report on Lead.4

The results are given in Table I. TABLE I

DETERMINATION OF LEAD IN COCOA AND SYRUP

Lead by S.A.C. report Lead by present Sample method, method,

p.p.m. p.p.m. Cocoa . . . . . . . . 0.69, 0.84, 0.74 0.72, 0.88, 0.92 Syrup .. . . . . . . 0.12, 0.11, 0-25 0.14, 0.20, 0.22

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May, 19561 IN THE DETERMINATION OF TRACES OF LEAD IN FOOD

DISCUSSION The principle of using sodium hexametaphosphate in the way we have used it is capable

of extension to some existing methods for lead,’** which at present rely on prompt action in making extractions with dithizone to prevent serious occlusion of lead on the gradually precipitating phosphates.

In common with many other methods for lead, the present one is liable to interference from bismuth. We have tried to keep the method as simple as possible, but its structure clearly affords the opportunity of eliminating bismuth by extracting the chloroform solution with an acid bufferg instead of the 1 in 100 nitric acid. The wavelength at the point of maximum absorption gives useful information as to whether interference is serious5

Preliminary experiments on the subject of bismuth interference have given some indication that bismuth is not so readily extracted with dithizone from solutions containing sodium hexametaphosphate as is lead and that this part of the method may reduce potential bismuth interference without eliminating it completely. We have not so far followed up this observation.

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The authors wish to express their thanks to the Government Chemist for permission to publish this paper.

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3. 4. 5. 6. 7. 8. 9.

REFERENCES “Official Methods of Analysis, ” Seventh Edition, The Association of Official Agricultural Chemists,

Washington, D.C., 1950, p. 403. Analytical Methods Committee, ‘l The Determination of Lead in Food-colouring Materials,”

Analyst, 1935, 60, 541. Hart, H. V., Ibid., 1951, 76, 692. Analytical Methods Committee, “The Determination of Lead in Foodstuffs,” Ibid., 1954, 79, 397. Clifford, P. A., J. Ass. 08. Agric. Chem., 1943, 26, 26. Snyder, L. J., Anal. Chem., 1947, 19, 684. Allport, N. L., and Skrimshire, G. H., Anadyst, 1932, 57, 440. Monier-Williams, G. W., “Trace Elements in Foods,” Chapman & Hall Ltd., London, 1949. Bambach, K., and Burkey, R. E., Ind. Eng. Chem., Anal. Ed., 1942, 14, 904.

DEPARTMENT OF THE GOVERNMENT CHEMIST GOVERNMENT LABORATORY

CLEMENT’S INN PASSAGE STRAND, LONDON, W.C.2 November 12tk, 1954

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