Trace Inorganics In Water - ACS Publications

17 A Thioacetamide-Precipitation Procedure for Determining Trace Elements in Water

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0073.ch017

E D W A R D C. M A L L O R Y , JR. U . S. Geological Survey, Denver, Colo.

When minor elements in water residues are determined spectrographically, major constituents sometimes interfere by producing unpredictable matrix effects, and by diluting the minor elements below limits of detection. Precipitation of minor elements in waters can be used to remove major elements and to concentrate minor ones before spectrographic analysis. In the procedure described here, thioacetamide is used as the precipitant for both the acid and the ammoniacal sulfide groups. Palladium is added to the samples as an internal standard. Tin is added to the acid sulfide group as a spectrographic buffer and coprecipitant; indium is used for the same purposes with the ammoniacal sulfide group. Quantitative precipitations are obtained for aluminum, antimony, arsenic, beryllium, bismuth, cadmium, chromium, copper, iron, lead, lanthanum, titanium, zinc, and zirconium.

T ^ e t e r m i n i n g trace elements i n natural waters involves a fairly large number of elements whose concentrations rarely exceed 1 milligram per liter, even i n highly mineralized waters. Most of these are subgroup elements, although several important elements, such as the rare alkalies, rare alkaline earths, and such elements as boron, aluminum, gallium, antimony, and lead are also of more than occasional interest and should be considered when trace element evaluations are made. Emission spectrographic methods are the most practical means of determining more than a few trace elements. The spectrographic detection limits for most elements are adequate, and as a means of .determining the greatest num ber of elements i n a single sample, the emission spectrographic method can scarcely be equalled. 281

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


282

T R A C E

I N O R G A N I C S

I N

W A T E R

The detection limits for the trace elements can be lowered and the spectrographic analysis simplified if the trace elements are separated from a comparatively large volume of water sample and from the major solutes present in the sample. One means of accomplishing such separa tions involves precipitation of the trace elements with a reagent or re agents which form very slightly soluble compounds with as many trace elements as possible. Such techniques have been described (4, 5, 7). Of particular interest from the standpoint of water analysis are the methods of Heggen and Strock (8), Mitchell and Scott ( J J ) , and Silvey and Brennan (15). In these methods, precipitation by organic précipitants at a controlled p H results in the quantitative or near-quantitative recovery of up to 17 minor elements. Although claims have been made that man ganese and beryllium are completely precipitated, our experience has shown that the recovery of these two elements is often far from complete. Furthermore, the organic reagents themselves, 8-hydroxyquinoline, thionalide, and tannic acid, contain traces of some of the metals sought and the ultimate detection limit for these metals is limited by the impurities in the reagents. The ideal precipitant, of course, does not contain as impurities any of the metals being sought. Such a reagent must also be capable of precipitating all or most of the important minor elements i n water analysis. M a n y heavy metals are quite insoluble as sulfides or hydroxides i n hydrogen sulfide solutions of properly adjusted p H . However, hydrogen sulfide has several disadvantages, such as being toxic and having an unpleasant odor. Thioacetamide has been used as a substitute for hydrogen sulfide. It is readily obtainable, and the commercial product tested was free of all heavy-metal sulfides except a trace of silver. Thioacetamide is very soluble in alcohol, benzene, or water. The neutral water solution is stable for long periods of time. A slight cloudiness may form in long-stored water solutions but this may be removed by filtration. A solution of thioacetamide can be added directly to solutions so there is no loss of precipitate in hydrogen sulfide delivery tubes. Thioacetamide hydrolyzes readily i n hot acid or alkaline solutions to give hydrogen sulfide. The equations for the hydrolysis are as follows: In acid: C H C S N H + 2 H 0 = C H C O O N H + H S 3

2

2

3

4

2

In base: C H C S N H + 30H" = C H C O O - + N H 4- S " 3

2

3

3

2

The rate of hydrolysis can be controlled by controlling the tempera ture of the solution so the rate of precipitation can be regulated. The precipitates obtained with thioacetamide are coarsely crystalline, easily filtered, easily washed, and not so likely to contain foreign ions. Since thioacetamide has so many advantages over hydrogen sulfide, an attempt



17.

M A L L O R Y ,

J R .

Thioacetamide-? recipitation

283

to evaluate the reagent as a precipitant to recover trace elements from water samples seemed worthwhile. Considerable basic work has been done concerning the reactions of thioacetamide, particularly with heavy metals, and the conditions which affect these reactions. Flaschka (6) discusses optimum conditions and some difficulties of precipitating antimony, arsenic, bismuth, cadmium, copper, lead, mercury, molybdenum, and tin with thioacetamide. Swift and Anson (16) discuss the analytical chemistry of thioacetamide. They also give preliminary results (17) for the mechanisms of sulfide precipi tation by thioacetamide for silver, mercury ( I I I ) , copper (II), tin ( I V ) , and molybdenum ( I V ) . Bowersox, Smith, and Swift used thioacetamide to precipitate nickel (2) and zinc (3) and studied reactions concerning these precipitations. Swift and Butler (18) have studied the reactions during the precipitation of lead with thioacetamide. M a n y others have directed their efforts to the separation of one or a few metals. References 1, 9,12, and 14 are only a few examples. For the present purpose, condi tions must be adjusted so as to ensure essentially complete precipitation of as many elements as possible but still maintain maximum simplicity of the operational procedure. For example, the sulfides of certain metals are quite insoluble in strongly acid solution but soluble in neutral or alkaline solution. For other elements of the sulfide group the reverse is true. W i t h the general insolubility of the sulfides for a guide, the use of thioacetamide as a precipitant for separation of the heavy metals was evaluated. Lundell and Hoffman (JO) and Waggoner (20) are good references for sulfide insolubility. Admittedly, sulfide precipitations offer no means of separating the important rare alkalies and alkaline earths. Nevertheless, an improved technique for the recovery of a significant number of the heavy metals would certainly be worthwhile. Accordingly, optimum conditions were established for the quantitative precipitation of the greatest number of trace elements. Initial experiments indicated the necessity for using two separate aliquots for the precipitations. A l l attempts first to precipitate those metal sulfides insoluble i n acid, filter them off, and then neutralize the filtrate to make the ammoniacal precipitation met with failure. Incomplete pre cipitation of aluminum, beryllium, cobalt, iron, molybdenum, manganese, nickel, and zinc always resulted with this procedure. W h e n one aliquot was used for the acid sulfide precipitate and another aliquot was used for the ammoniacal sulfide precipitate, much better results were obtained for aluminum, beryllium, iron, and zinc. Apparently the additional opera tions involved in reducing the volume of the sample and washings, and the readjustment of the p H , result in conditions which are far from ideal for the subsequent precipitation. Only by carrying out precipitations on


284

T R A C E INORGANICS I N W A T E R

two separate aliquots could acceptable recovery of a significant number of the elements of interest be achieved. W h i l e this defeats one of the goals of the investigation, the success i n accomplishing the isolation of a number of trace elements makes the method worthy of consideration as a technique for the analysis of water samples.


Preparation of Stock Solutions Four separate stock solutions are necessary to provide stable mixtures of a l l 19 elements investigated. The components of each of these are tabulated i n Tables I and II. Compounds used i n stock solutions must be of known high purity or be examined to demonstrate the absence of harmful impurities. A l l dilutions must be made with demineralized water. Table I. Stock Sol No. Element 1

2

As Bi Cd Cu Pb Mo Sb

Stock Solutions Nos. 1 and 2 ° Weight Required (mg./liter)

Compound Used

66.02 55.74 57.11 62.59 53.86 75.02

AsjjOa Bi 0 CdO CuO PbO Mo0 2

3

3

KSbOC H O 1/2H 0 4

4

e

•

137.13

Solvent HC1 (warm only) .... HC1 HC1 HC1 HC1 HC1

10 10 10 10 15 15

ml. ml. ml. ml. ml. ml.

H 0 2

2

a

50 mg./liter of each element.

Stock solutions Nos. 1 and 2 contain those elements whose sulfides are precipitated from strongly acid solution. High-purity oxides of a l l these elements except antimony are readily soluble i n hydrochloric acid. Antimony must be prepared as a separate stock solution because of its tendency to hydrolyze and precipitate as a basic chloride i n dilute hydro chloric acid solution. This stock solution is stable for at least 30 days. Stock solutions Nos. 3 and 4 contain those elements whose sulfides or hydroxides are precipitated from neutral or slightly ammoniacal solu tion. Either the high-purity oxide or sulfate of each element is used, except for chromium, where a dichromate salt is satisfactory, and titanium, where the most convenient compound is ammonium titanyl oxalate. The titanium stock solution is stable for at least 30 days. Logarithmic standards are made as follows: I. A . Dilute 100 m l . of stock solution N o . 1 to 1 liter with water. This gives a standard containing 5000 jLtg./liter of each element. B. Dilute 200 m l . of the 5000 /xg./liter standard with 229 m l . of water and 1 ml. of hydrochloric acid. This gives a standard of 2325 /ng./liter. C . Dilute 200 m l . of the 2325 /xg./liter standard with 229 ml. of water and 1 ml. of hydrochloric acid to make a standard of 1081 /jtg./liter.


17.

M A L L O R Y ,

Table II. Stock


Stock Solutions Nos. 3 and 4

A1 (S0 )

Al

Weight Required (mg./liter)

Compound Used

Sol No. Element 3

285

Thioacetamide-Precipitation

J R .

2

4

3.175 ml. of 10% specpure sol. 138.76 141.46 68.10 71.48

3

Be Cr Co Fe

BeO K Cr 0y Co 0 Fe 0

La Mn

La 0 Mn 0

Ni Ag Zn Zr

NiO Ag 0 ZnO Zr(S0 ) · 4H 0

Ti

(NH ) TiO(C 0 ) 2H 0

2

2

3

4

2

3

β

Solvent H 0 2

7 ml. H S 0 H 0 12 ml. H S 0 5 ml. H C l f o r solvent, then fume in 5 ml. H S0 3 ml. H S 0 5ml. HClfor solvent, then fume in 5 ml. H S0 12 ml. H S 0 5 ml. H S 0 5 ml. H S 0 5 ml. H S 0 2

2

56.84 69.41

3

3

4

2

63.63 53.71 62.23 194.81

2

4

4

2

2

2

2

4

2

306.93

2

4

2

4

2

4

2

4

4

2

4

2

4

2

4

2

4

HoO

2

' 50 mg./liter of each element.

D. Continue the dilutions as above so that standards of 503, 233, 108, 50, 23, 11, 5, 2.3, and 1.1 jug./liter are obtained. Each standard solution contains 46.5% of the elements found in the next higher standard. II. Standards made from stock solutions Nos. 2 and 4 are diluted the same as I except that 230 ml. of water are used. N o acid is necessary. III. Standards made from stock solution N o . 3 are diluted the same as I except that 1 m l . of sulfuric acid is used instead of the 1 m l . of hydro chloric acid. Reagents Ammonium chloropalladite (0.1 mg. P d / m l . ) . Dissolve 133.6 mg. of ammonium chloropalladite i n 450 ml. of demineralized water acidified with 5 m l . of concentrated H S 0 . Dilute to 500 ml. Dilute this solution 1:1 before using as an internal standard. Indium sulfate solution (1 mg. In/ml.). Dissolve 1.209 grams of l n 0 i n 40 m l . of concentrated H S 0 , heating until solution is complete. Dilute to 1 liter with demineralized water. Stannous chloride solution (1 mg. Sn/ml.). Dissolve 0.3902 grams of S n C l * 2 H 0 i n 4 m l . of concentrated H C 1 and dilute to 200 m l . with demineralized water. Prepare fresh before use. 2

2

3

4

2

2

4

2


286

T R A C E

I N O R G A N I C S

I N

Ammonium hydroxide solution ( I N ) . Use ammonia gas and de mineralized water to make 42V N H O H . Dilute 25 m l . of 42V N H O H to 100 m l . Hydrochloric acid (122V). Distill reagent grade concentrated HC1, collecting only the middle one-third portion of distillate. Hydrochloric acid (32V). Dilute 25 m l . of the redistilled 122V HC1 to 100 m l . with demineralized water. Ammonium chloride solution ( 2 % , wt./vol.). Dissolve 2 grams in 100 m l . of demineralized water. Ammonium sulfite solution (0.2%, wt./vol.). Dissolve 0.5 gram in 250 m l . of demineralized water. Thioacetamide solution ( 2 % , wt./vol.). Dissolve 2 grams i n 100 ml. of demineralized water. Stir or expose the solution mixture to ultra sonic agitation until dissolved. A n y small amount of insoluble white residue may be ignored. Wash solution I (acid precipitation). Dissolve 80 mg. of thioaceta mide in 1 liter of demineralized water. Adjust the p H to 0.75 with 32V HC1. Wash solution II (Ammoniacal precipitation). Dissolve 0.5 gram each of ammonium chloride and ammonium sulfite, and 2 grams of thioacetamide in 1 liter of demineralized water. Adjust the p H to 8.0 with 12V ammonium hydroxide. 4


W A T E R

Spectrographic

4

Equipment

A spectrograph with a 3-meter concave grating having 15,000 lines per inch and a reciprocal linear dispersion of 2.7 Angstroms per mm. is used to record the spectra on Eastman III-O spectroscopic plates. A 220volt d.c. arc of 6 amperes between graphite electrodes (about 5 mm. gap) is used for sample excitation. Each sample charge of 24 mg. is burned to completion in the arc. Plate development is by mechanical agitation in a commercially available plate processor unit at controlled temperature. Spectral line transmissions are measured in the usual way on a scanning, non-recording densitometer. A l l calculations are made by the system of Trandafir (19) in which transmissions of standards and samples are plotted directly, eliminating the need for plate calibration and calculation of intensity ratios. Procedure The sample to be analyzed should first be filtered through a micro pore membrane filter of 0.45 micron average pore diameter. This effec tively removes suspended matter and insures subsequent determination of only that material considered to be in true solution. The suspended material may then be examined by x-ray analysis. If the thioacetamide method were used to analyze the unfiltered sample, some of the sus pended sediment might not dissolve and would remain to contaminate the sulfide precipitate. The amount of sample taken for analysis must provide adequate amounts of the trace elements being determined. A 100-ml, aliquot is a convenient volume in which to make the précipita-



17.

M A L L O R Y ,

J R .

Thioacetamide-Precipitation

287

tions. Larger volumes, up to 1 liter or more, of dilute waters can be evaporated to 100 m l . Highly mineralized waters cannot be concentrated to such an extent, hence correspondingly smaller volumes must be taken. For brines, a sample of less than 100 ml. may be necessary and such samples must be diluted to 100 m l . with demineralized water. The specific conductance of a water can be used as an aid in deter mining the size of the aliquot needed. A 1-liter aliquot is generally suffi cient for waters with a conductivity less than 2,000 micromhos. Samples with a greater conductivity require smaller aliquots. However, if a water is high in some elements and low in others, both a large and a small aliquot may be necessary. The samples are conveniently evaporated in the 400-ml. beakers in which the subsequent precipitations are made to minimize both the risk of contamination and the possible loss of trace elements from the solution. Aliquots ( 100 ml. ) of the standard solutions are tested by the rest of the procedure. Acid Precipitation. Evaporate a measured portion of the sample to about 100 ml. or, if less than 100-ml. aliquots are used, adjust the final volume to about 100 ml. A d d 3 m l . of the ammonium chloropalladite solution and 30 ml. of stannous chloride solution. Adjust the p H to 0.75 with 3 N HC1 and add 20 m l . of the thioacetamide solution. Cover the beaker with a watch glass, heat the sample to just below the boiling point, and allow it to stand overnight at room temperature. Filter the solution through a fine filter paper (Whatman No. 42 or equivalent). Wash the precipitate several times, using the acid-thioacetamide wash solution ( N o . I ) . Discard the filtrate and washings. Place the filter paper and precipitate in an uncovered porcelain crucible and ash at 450°C. Thoroughly grind and mix the cool, dry precipitate with an equal weight of spectrographic-grade powdered graph ite in an agate mortar. The mixture is then ready for spectrographic analysis. W e i g h a 24-mg. portion of the sample-graphite mixture and tamp gently into the cavity of a graphite electrode (Ultra Carbon 1590, Met-Bay 1120, and similar to National Carbon L 3942, except for modi fication to 0.015-inch wall thickness). Burn to completion in a 220-volt d.c. arc of about 6 amperes. Ammoniacal Precipitation. Evaporate a measured portion of the sample to about 100 m l . or, if less than 100-ml. aliquots are used, adjust the final volume to about 100 m l . A d d successively 3 ml. of ammonium chloropalladite solution, 30 m l . of indium sulfate solution, and 25 ml. of ammonium chloride solution. Adjust the p H of the solution to 7.2 with I N ammonium hydroxide and add successively 20 m l . of ammonium sulfite solution and 20 ml. of thio acetamide solution. Adjust the p H of the sample to 8.0 with I N am monium hydroxide and allow the sample to stand overnight at room temperature. Filter the solution through a fine filter paper (Whatman No. 42 or equivalent) and wash the precipitate with several portions of wash solu tion II, the ammoniacal thioacetamide solution containing small amounts of ammonium chloride and ammonium sulfite. Ash the precipitate at 450°C., cool, and mix with an equal weight of powdered graphite. M i x the ashed precipitate and powdered graphite thoroughly by grinding


288

T R A C E

I N O R G A N I C S

I N

W A T E R

together in an agate mortar. A portion of the precipitate-graphite mix ture weighing 2Λ mg. is analyzed spectrographically the same as the precipitates obtained from the acid precipitations.


Discussion Palladium has been recommended as an internal standard for spec trographic analysis of precipitated heavy metals such as are involved i n this procedure (15). Palladium sulfide precipitates readily and com pletely under both acid and ammoniacal conditions, and, therefore, it seemed satisfactory for the purpose. The choice of an appropriate spectroscopic buffer and collector for the precipitated trace elements is somewhat more difficult. Adding stan nous chloride to the acid precipitation and indium sulfate to the am moniacal precipitation appeared satisfactory. Although tin is a fairly common element and its determination in water is occasionally of interest, it is of sufficiently infrequent occurrence in water so it can be used with little disadvantage. Indium is only rarely detected in natural water and, since its ammoniacal sulfide is readily precipitated under the conditions of this procedure, it serves adequately as both a collector for the pre cipitate and as a spectrographic buffer. According to Lundell and Hoffman (JO), the optimum conditions for the complete precipitation of the sulfides which are insoluble i n strong acid solution require adjustment of the solution to 0.2N with hydrochloric acid. Hydrochloric acid is preferable to sulfuric acid primarily because of the difficulty of completely precipitating lead from strong sulfuric acid solution. The p H of the acidified aliquot is therefore adjusted to 0.75 with 32V hydrochloric acid. T o determine the optimum p H for the precipitation of the metal sulfides insoluble in ammoniacal solution, several precipitations were made under identical conditions except that p H values of 7.0, 7.5, 8.0, 8.5, and 9.0 were used. The filtrates from each precipitation were ex amined to measure the completeness of precipitation. Maximum recovery of most elements occurred in the solution adjusted to a p H of 8.0. Zir conium precipitated completely at all p H values. Adding ammonium chloride decreases the solubility of aluminum hydroxide and prevents the precipitation of magnesium hydroxide. Scott (13) states that the precipitation of nickel, cobalt, manganese, and zinc sulfides may be incomplete because of the formation of polysulfides in the presence of air or other oxidizing agents. A small amount of am monium sulfite is therefore added to promote their precipitation. In spite of this precaution, cobalt, nickel, and manganese were never quantita tively recovered and the procedure is not considered entirely satisfactory for these elements.



17.

M A L L O R Y ,

J R .

289

Thioacetamide-Prectpitation

The optimum conditions for forming the acid and ammoniacal sulfide precipitates were also studied. Whether a moderate or a large excess of thioacetamide is added appears to have little or no effect on the com pleteness of precipitation. Precipitation of the acid-insoluble sulfides occurs slowly at room temperature; however, precipitation may be has tened by heating the solution to just below boiling. To ensure complete precipitation and an easily filtered precipitate, the solution should be filtered only after the residue has settled for several hours or overnight. A n examination of the filtrate showed complete removal of antimony, arsenic, bismuth, cadmium, copper, and lead; only molybdenum was not completely removed. O n the other hand, the metallic sulfides and hydroxides which pre cipitate from the ammoniacal solution form readily at a p H of 8.0 and no heating is necessary. The precipitates are allowed to stand overnight before filtering. Attempts to recover the precipitates on micropore membrane filters were unsuccessful both because of the loss of precipitate between the edges of the filter disc and the filter holder and because of the difficulty Table III.

Elements Completely or Nearly Completely Precipitated Over the Concentration Range Shown

Element Antimony Arsenic Bismuth Cadmium Copper Lead

Acid Precipitations Concentration Analytical Line Range, pg./liter (A.) 2598.1 2780.2 3067.7 2989.0 3261.1 2824.4 2873.3

5.00-500 10.80-500 0.23-10.8 10.80-232 5.00-232 5.00-500 5.00-232

Concentration in Filtrate, /liter None None None None None None None

Ammoniacal Precipitations Element

Analytical Line (A.)

Concentration Range, ^g./liter

Concentration in Filtrate, μ^./liter

Beryllium

3131.1

Chromium Iron Titanium Zinc

3021.6 2723.6 3242.0 3345.0

Zirconium

3392.0

0.23-5.0 10.80 0.50-23 0.50-232 1.10-50 2.30-50 108.00 232.00 500.00 0.23-23

None

Trace Inorganics In Water - ACS Publications

Recommend Documents