determination of mercury in soil

8/2/2019 Determination of Mercury in Soil

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706 P I C K A R D G M A R T I N - M E R C U R Y I N S O I L

DETERMINATION OF MERCURY IN SOILBy J. A. PICKARD and J. T. MARTIN

An analy tical procedure for th e determin ation of mercury in soil is described. Afterth e digestion of organic matter , the mercury is removed from the soil by distillation inhydrochloric acid gas, extracted from the distillate with dithizone and measured spectro-photometrically.

IntroductionAn earlier report1 described a method for the determination of mercury in plant material.In work on the uptake of mercury by plants2? a method was required for the determination of

mercury in soil. Stock and his co -w or ker ~~ -~eated powdered minerals a t 800' for several hours,and the volatilised mercury was collected and electrolytically deposited on copper, from whichit was distilled to condense as droplets whose sizes were measured. Substances such as coalwhich gave interfering distillates were wet-digested to bring the mercury into solution. Arableand forest soils showed 0.01-0.3 .p.m. and compost from gardens contained 0.8 p.p.m. of Hg.Stock' also used the electrolytic deposition procedure to examine soils and other materials ;soils showed 0.03-0.3 p.p.m., coal 0,02 p.p.m., street dust 0.9 p.p.m. and soot up t o 28 p.p.m. Hg.Polley & Millers destroyed the organic matter in minerals with hydrogen peroxide and deter-mined mercury by the dithizone reaction. Popea & Jem5neanug digested soils with potassiumnitrate-sulphuric acid, removed interfering metals with potassium thiocyanate and ethylene-diaminetetra-acetate (EDTA), and determined mercury with dithizone. We have found tha tinterference by metals such as copper in soil is severe, but may be overcome by separation ofthe mercury from the soil by distillation in a current of hydrochloric acid gas. The mercury isthen extracted from the distillate with dithizone and measured spectrophotometrically.ExperimentalPreparation of sample and des t rwt ion of organic matter

Thesample is air-dried, sieved from stones if present, and ground to pass a 2-mm. sieve. Organicmatter is destroyed by digestion with sulphuric acid-nitric acid, with the addition of selenium.Distillation and extraction of mercury

After digestion, the mercury is distilled from the mixture with hydrochloric acid gas ofpurity 99.8% supplied from a cylinder of liquefied hydrogen chloride. The purification ofreagents (see below) and the incorporation of EDTA treatment in the extraction process reduceblank values to a low level. The mono-colour method discussed earlier1 is used for the deter-mination of the mercury.Preparation of reagents

A soil sample of up to 40 g. is used, depending upon the amount of mercury present.

Reagents should be of analytical grade wherever possible.Ammonium hydroxide solution (5yo).-Dilute aq. ammonia sp. gr. 0.880 ( I vol.) with water (19 vol.).Buffer solution.-Prepare a N-sodium acetate solution. Test with dithizone for freedom from interferenceTo 50 ml. addnd p uri fy if necessary as described below for hydroxylamine hydrochloride solution.13 ml. of N-hydrochloric acid and dilute to 250 ml.Carbon tetrachloride.-Fractionate throug h a Widmer column.Dilhizone stock solution.-Dissolve 50--60 mg. of dithizone in 20 ml. of carbon tetrac hlori de an d filter. Ex trac twith 10 0 ml. of 5% ammonium solution and wash the alkaline layer twice with 5-ml. portions of carbontetrachloride. Make th e alkaline layer just acid with hydrochloric acid, add 5 ml. of hydroxylaminehydrochloride solution (see below) an d re-ex tract with 1 0 0 ml. of carbon tetrachloride. Wash th e carbontetrach loride solution twice with 15-ml. portions of water and store in the refrigerator.Dithizone dilute solution.-Dilute th e stock solution ten times with carbon tetrachloride.EDTA solu ti0n. -4~~ disodium ethylenediaminetetra-acetate dihydrate.Hydroxylamine hydrochloride solution (2 0%w/v) ,-Extract by shaking with small quan tities of diluted dithizone

solution until th e extr act is colourless afte r removal of excess dithizone with 5% ammonium hydroxidesolution. Ex trac t an y remaining dithizone with carbon tetrachloride unti l two successive washings areJ. Sci. Fd Agric., 1963, Vol. 14, October


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PICKARD G MARTIN-MERCURY I N SOIL 707colourless.to remove any entrained carbon tetrachloride.Filter the hydroxylamine hydrochloride solution through a wetted Whatman No. 54 paper

Selenium powder.-Selected for low mercury con ten t.Sodium metubisulphite solution (2 0yo w/v) .-Treat as for hydroxylamine hydrochloride solution. The extrac-tion is slow and a mechanical shaker is helpful.Sodium hypochlorite solution.-Titrate th e app rox . 15% commercial product, and dilute to 5% availablechlorine. Store in refrigerator. (A suitable ' low in mercury ' product is available on the market.lO)Sodium thiosulphute solution.-1.5 % (w/v), freshly prepared.ApparatusDigestion apparatus.-A 500-ml. two-necked, round-bottomed digestion flask carrying an air-

condenser (30 x 3.5cm.) in the central neck (B.34oint) and a 50-ml. cylindrical separatingfunnel in the side neck (B.19 joint). The air condenser supports a double-surface watercondenser (20 cm.). A 500-ml. 250-W heating mantle, controlled by an auto-transformer,is used for heating purposes. Thedistillation flask B (used for the digestion) carries an air-leak C and air-condenser D(ISx 3.5 cm.). The double-surface condenser E (Quickfit& Quartz CatalogueNo. C 11/23)leads to a 500-ml. receiver F. The U-tubes G, containing sufficient distilled water to closethe bends, are connected to a water pump.

Distillation apparatus (Fig. I) -The gas-scrubber A contains sulphuric acid, sp. gr. 1-84.

ProcedureTo a representative sample (up to 40 g.) of soil in the 500-ml. reaction flask, add 0.1 g.

of selenium powder. Add a mixture of 50 ml. of sulphuric acid (sp. gr. 1.84) nd 5 ml. of nitricacid (sp. gr. 1.42) mix well and digest slowly at low heat for about 2 h. Carefully add morenitric acid, sp. gr. 1.50, ml. a t a time, if there is any indication of charring. Maintain simmeringgently for 30 min. Allow the digest to cool thoroughly and wash down the condenser slowlywith 40 ml. of water, swirling the flask.

Transfer the reaction flask to the distillation apparatus and add 50 ml. of sulphuric acid(sp. gr. 1.84). Assemble the apparatus and ensure that all the joints are gas-tight. With theair-leak partially open, draw a slow steady stream of air through the apparatus by means ofthe water pump, and apply gentle heat to the reaction flask. When the reaction mixture isboiling steadily and two-thirds of the water has distilled over, with the air-leak still partiallyopen allow a steady stream of hydrochloric acid gas (about 25-30 ml./min.) to pass throughthe reaction flask. Maintain sufficient suction to ensure a steady flow through the absorptiontubes, and adjust the air-leak as necessary. Continue the distillation with hydrochloric acidgas for 1k-2 h. Wash the condenserand U-tubes with water and add the washings to the distillate. The volume of distillate andwashings should be about 125 ml.

Filter if necessary through a Whatman No. 541 ilter paper into a 600-ml. graduated beakerand just neutralise to litmus with ammonia solution (sp. gr. 0.880). Make just acid to litmus

Allow the apparatus to cool and remove the distillate.

FIG.I-Distillation apparatusFor A-G see text

J. Sci. Fd Agric., 1963, Vol. 14, October


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708 P I C K A R D & M A R T I N - M E R C U R Y I N SO ILby the addition, drop by drop, of concentrated hydrochloric acid. Add 10 ml. of sodiummetabisulphite solution, mix well, add 25 ml. of buffer solution and 5 ml. of EDTA solutionand dilute to 250-300 ml. The pH of the solution should be 5 ; test with indicator paper andadjust if necessary.

Transfer to a 400-ml. separating funnel and extract with 10-, - and 5-ml. portions ofdilute dithizone solution. If mercury is suspected in the third extract (indicated by a changeof colour of the dithizone solution), a fourth extraction with 5 ml. of dithizone solution shouldbe made. Transfer the dithizone ext racts to a 100-ml. separating funnel containing 25 ml. ofo-IN-hydrochloric acid. Add 5 ml. of hydroxylamine hydrochloride solution, shake well andafter separation transfer the carbon tetrachloride layer to a second Ioo-ml. separating funnelcontaining 50 ml. of o.IN-hydrochloric acid. Wash the aqueous layer with 5 ml. of carbontetrachloride and add the washings to the second separating funnel.

Add 2 ml. of sodium thiosulphate solution and shake for I min. Run off the carbon tet ra-chloride layer and discard. Wash the acid layer with two 3-ml. portions of carbon tetrachlorideand discard the washings. Add 3 ml. of sodium hypochlorite solution, shake vigorously forI min., blow off any chlorine and shake again. Extract twice with 3-ml. portions of carbontetrachloride and reject the extracts.

Add 7 ml. of carbontetrachloride and I ml. of dilute dithizone solution and extract with vigorous shaking. Ifnecessary add dithizone solution a few drops a t a time until an excess of dithizone is present.Repeat the extraction using 4ml. of carbon tetrachloride and a few drops of dithizone solution.Combine the dithizone extracts in a Ioo-ml. separating funnel and extract the excess ofdithizone by shaking twice with 15-ml. portions of 5% ammonium hydroxide solution. Runth e carbon tetrachloride solution containing the mercury-dithizone complex through 1-2 g.of granulated anhydrous sodium sulphate into a zo-ml. graduated amber glass flask and adjus tthe volume to 20 ml. with carbon tetrachloride. Measure the optical density (2-cm. cell) a t490 mP-

Add 20 ml. of buffer solution, 2 ml. of EDTA solution and mix.

Calibration with mercuric chlorideKnown amounts of mercury from o to 25 pg., as mercuric chloride in aqueous solution,were added to 50-ml. portions of o-IN-hydrochloric acid. Hydroxylamine hydrochloride

solution (5 ml.), buffer solution (20 ml.) and EDTA solution (2 ml.) were added, the mercurywas extracted with dithizone and the analysis continued as described. The optical densitiesrecorded in a 2-cm. cell and corrected for the reagent blank (0.005) were as follows:

Mercury, pg. 5 I0 I5 2 0 25Optical density 0.170 0.338 0.512 0.681 0.860Notes(I ) The precautions necessary to avoid loss of mercury during the digestion and extraction

processes have already been discussed.l The mixture of sulphuric and nitric acids must beevenly distributed throughout the sample before the digestion is commenced. During digestion,minimum heat from a controlled electric heating mantle is used, and charring must be avoided.A water-trap is not necessary. During distillation, the temperature of the mixture is maintainednear that of boiling sulphuric acid ; at lower temperatures the recovery is incomplete.

(2 ) The cylinder containing liquefied hydrogen chloride and instructions for safe handlingwere obtained from Badische Anilin & Soda-Fabrik A.-G., Ludwigshafen am Rhein. Thecylinder carried a reducing valve suitable for use with hydrochloric acid gas (obtained fromThe British Ermeto Corporation, Maidenhead). For safety, a three-way stop-tap was insertedbetween the reducing valve and gas-scrubber. Difficulty was met in finding suitable tubingto convey the gas from the cylinder to the distillation apparatus. Polyvinyl chloride tubingwas finally adopted ; fresh tubing gave high blank values, but the tubing became satisfactoryafter being in use. The gas flow-rate (about 25-30 ml./min.) was assessed visually from a know-ledge of the performance of the sintered disc of the gas-scrubber when passing measured amountsof nitrogen through sulphuric acid.



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PICKARD & MARTIN-MERCURY I N SOIL 709(3 ) The mercury-dithizone complex fades on exposure to sunlight ; this necessitates the(4) Standard solutions of mercuric chloride in water (jpg./ml.) lose strength when keptStandard solutions should therefore be'freshly prepared.

use of a flask of low-actinic amber glass for the final solution.due to adsorption on the glass surface.Recoveries of mercury from soil samples

Two soils that had not been treated with a mercurial compound showed on analysis 0.02and 0.06 p.p.m. Hg. Mercury in the form of a freshly-prepared solution of mercuric chloridein water (5 pg. of Hg/ml.) was added, in known amounts up to 25 pg., to 40-g. samples of thesoils before digestion. The added water on each occasion was adjusted to 5 ml. The recoveriesby the method, allowing for the optical density value (0.01-0.015) given by the reagents andapparatus, are shown in Table I.

Table IRecovevy of m e r cu r y f r o m soil

Original Hg, p.p.m.soil Added Found0.02 0.13 0.15, 0.14, 0.150.25 0.27, 0.27, 0.260.38 0 .3 8 , 0.38, 0.390.50 0'53, 0.52, 0'53

Original Hg, p.p.m.soil Added Found0.06 0.25 0.32 0.310.31 0.38

0.34 0'370.44 0.49, 0.510.56 0.620.59 0.63The recoveries of added mercury were good, indicating that no losses occurred in the

digestion, distillation and extraction processes. Whether the method yields all the naturally-occurring mercury in soil is difficult to ascertain. The soil particles were small (98% passeda I-mm. and 70% passed a 0.05-mm. sieve) and the digestion procedure was severe, but somemercury may have been protected by occlusion within the particles. The method, however,was designed primarily for work on the levels and persistence of mercury after addition to soil,and for this purpose it should be satisfactory.

Dept. of Agriculture & HorticultureLong Ashton Research StationUniversity of Bristol

Received 15 February, 1963 : amended manuscript 13 May, 1963

References1 Pickard, J . A,, & Martin, J . T., J . Sci.F d A g r ic . , Stock, A. , & Cucuel, F., N a t u r w i s s e n s c h a f t e n , 1934.

1960, 11, 374 22, 390Pickard, J . Martin, J . T., Annu. Long 7 Stock, A, , S u e n s k , h e m . T i d s k r , , 1938, 50 , 24 2A s h t o n R e s . S t a . , 1959, p. 933 Pickard, J. A,, & Martin, J . T., Annu. R e p , L o n g

H.? '5 Stock, A,, Cucuel, F., & Kohle, H., 2 . angem.

* Polley, Dorothy, & Miller, V. L., A n a l y t . C h e m . ,9 popes, F,, & JemBneanu, M., Stu d, Cercet&j Chi ', ,A s h t o n R e s . S t a . , 1960, p. 85Stock, A., Lux, H., Cucuel, F.,angew. Chem. , 1933, 46, 62

C h e m . , 1933. 46, 187 lo Anon., A n a l y s t , 1961, 86, 6 0 8

1955, 27, I162B u c u r e s t i , 1960, 8, 607


determination of mercury in soil

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