of organic compounds which undergo quantitative iodoform reaction. An obvious disadvantage is the expected interference by other iodine-substituted organic compounds, which must be eliminated from the assay mixture, because most of the iodine compounds are affected by light. Another disadvantage is its dependence on sunlight. The photooxidation takes place in daylight also, but requires many
days for completion. White light from a mercury vapor lamp may be used as a substitute, although trials with such a lamp have not been carried out by the author. LITERATURE CITED
(1) Ellist c . ,
A. Heyrotll, F. F., “The Chemical Action of Ultraviolet Rays,” p. 395, Reinhold, S e w York, 1941.
( 2 ) Emschwiller, G., Bull.
SOC. chim. 6 , 551-60, 561-70 (1939). ( 3 ) Emschwiller, G., Compt. rend. 207, 1201-3 (1938). (4) Messinger, J., Ber. 21, 3366-72 (1888). (5) hlitchell, J., Kolthoff, I. M., Proskauer, E. S., Fpsberger, A., “Organic Analysis, Vol. I, p. 269, Interscience, New York, 1953.
RECEIVEDfor review August 23, 1957. Accepted January 6 , 1958.
Dithizone Method for Determination of Lead in Monazite R. A. POWELL‘ and C. U. S. Geological Survey,
A. KINSER Washington
25, D. C.
b In the determination of lead in monazite-to b e used as the basis for geologic age measurements-it was necessary to eliminate interferences due to the presence of phosphates o f thorium and the rare earth metals. The method involves attacking the monazite samples with hot, concentrated sulfuric acid, then taking them up with dilute nitric acid. Lead i s extracted as the dithizonate and determined spectrophotometrically at 520 mp. Rapid determinations were made with good reproducibility on a series of monazite samples.
I
on the determination of lead in monazite led the authors to believe that thorium-rare earth phosphates were a source of interference. Sandell (8) reported that dithizone extraction fails in the presence of much calcium (or magnesium) and phosphorus because the phosphates of these metals are only slightly soluble in ammoniacal citrate solution and carry down lead strongly. Thorium and rare earth phosphates behave similarly. To minimize or eliminate the interferences in the analysis of monazites and other related materials, it became necessary to study various methods of sample attack, Iead separation, and reagent concentrations. The method developed as a result of this investigation is based on several established spectrophotometric methods (1-3, 6, 8) for the determination of lead, but has been altered a t several points that mere found to be critical when working with samples containing thorium and rare earth phosphates. iJ7ith this procedure lead determinations can be made rapidly on a routine basis. Present address, Union Carbide Ore Co., Sterling Forest Research Lab., Tuxedo, N. Y. SVESTIGATIONS
The method has given excellent reproducibility in the laboratory, and its accuracy was proved by check analysis with two other laboratories using mass spectrographic techniques. PRELIMINARY STUDY
The established method, upon d i i c h the following experimental work is based, consists of dissolution of the sample; addition of citrate to prevent precipitation of metal hydroxides, cyanide to complex other metals, sulfurous acid to reduce iron, and ammonium hydroxide to adjust the pH to 9.2; extraction of lead into a chloroform solution of dithizone; stripping of the lead into a dilute nitric acid solution; reextraction of the lead into a standard dithizone solution; and measurement of the absorbance of the extract a t 520 mp against water as the reference solution. Sample Treatment. Several methods of sample treatment were investigated in a n effort t o select the one best suited t o effect solution prior t o the lead determination. Sintering the sample with sodium peroxide (7, 9) seemed promising, but its use was discontinued because of erratic lead analyses obtained after this initial treatment. Later work indicated that the erratic results might have been caused by the difficulty of removing all of the excess peroxide before the lead extraction. Fusion with sodium carbonate, potassium bisulfate, or a 3 to 1 mixture of sodium carbonate and sodium borate failed to give complete decomposition of monazites. Samples seemed to be completely decomposed when the sodium carbonate-sodium borate mixture ratio was changed to 1 to 3; however, the lead results were variable, When so-
dium borate alone was used as the flux, clear melts mere obtained which were exceedingly difficult to dissolve. The treatment with sulfuric acid was the most successful method (4) investigated for the attack of monazite samples. By this means decomposition of samples mas completed within 1 hour, the fusion in platinum vessels, a possible source of error, was eliminated. After the initial treatment with sulfuric acid and the subsequent addition of dilute nitric acid, a trace of residue was usually apparent. However, it was generally insignificant and could be ignored. Of the nine monazite samples analyzed, only one (labeled impure) gave an appreciable residue. Spectrographic analysis showed the major constituent to be silicon, although it also disclosed the presence of lead. This residue was easily decomposed by hydrofluoric and nitric acids. Sample and Reagent Concentration. Cloudiness or precipitation, attributable t o thorium and rare earth phosphates, was observed occasionally in the sample solutions after adjustment of the p H to approximately 9.2. This phenomenon sometimes appeared immediately, while a t other times it was delayed until after the extraction of lead had been completed. As this precipitation seemed to be associated mainly with larger samples, experiments were undertaken to establish the relationship between sample size, time, and amount of precipitation and lead recovery. For the sake of convenience and sample conservation, a synthetic monazite solution was prepared from cerium and thorium nitrates and disodium phosphate. The solution contained the equivalent of 5 mg. of a typical monazite per ml. in dilute nitric acid (I t o I) Preliminary experiments with the esVOL. 30, NO. 6, JUNE 1958
1139
tablished dithizone method that had been used successfully on other types of samples showed that, when applied to 1-ml. aliquots of the synthetic monazite solution to which had been added known amounts of lead, recoveries were consistently 3.591, low. A twofold increase in the sodium citrate made complete recoveries again possible. This change was incorporated in the newly developed method described below. A series of solutions was prepared containing increasing amounts of the synthetic monazite solution, a constant amount of lead, and redistilled water to make all volumes the same. Sodium citrate, potassium cyanide, sulfurous acid, and ammonium hydroxide were added to each, bringing the pH of the solutions to 9.2. The solutions were then allowed to stand undisturbed for 1 hour. The solutions that contained 5 to 10 mg. of monazite were only slightly cloudy, those containing 15 to 25 mg. were slightly precipitated, and the ones that contained 30 to 50 mg. appeared to be completely precipitated within a few minutes after p H adjustment. Another series was prepared exactly as previously described, except that these were extracted immediately after the addition of reagents. Complete recovery of lead was obtained from the solutions that contained 5 to 25 mg. of monazite; however, the recoveries ranged downward from 89 to 45% on those containing 30 to 50 mg. Extractions were made on a similar series that had been allowed t o stand for varying periods of time following the p H adjustment. After standing for 15 minutes, only the solutions containing 5 to 20 mg. of monazite gave complete lead recoveries. Those that contained 25 to 50 nig. gave downward from 74 to less than 10% recoveries. After standing for 1 hour, only the solutions containing 5 to 10 mg. gave complete recoveries. Additional tests have shown that complete recoveries of lead can be obtained on samples as large as 50 mg., if proportional increases of all reagents and final volumes are made and the lead is extracted immediately. APPARATUS A N D REAGENTS
Only a borosilicate type of glassware should be used. It should be cleaned routinely with analytical reagent grade dilute nitric acid (1 to 1) and rinsed with distilled water. Transfer pipets should be used exclusively to eliminate the possibility of lead contamination from the enamel markings of measuring pipets (6). A spectrophotometer with adjustable wave lengths is necessary, and matched Corex cells with a 1-cm. light path. Only analytical reagent grade chemicals should be used, and these should be tested to assure the lowest possible
1140
ANALYTICAL CHEMISTRY
lead blank by extracting with dithizone solution as follows. The absorbance of the dithizone solution before extraction is normally greater than that following the extraction because of its solubility in alkaline solutions. Therefore, when the absorbance of the dithizone solution following extraction in the presence of a reagent was less than its initial absorbance, the lead content of the reagent was deemed to be negligible. K h e n tested in this manner, sulfuric acid, sulfurous acid, and chloroforni were found to require no further purification, Nitric, hydrochloric, and hydrofluoric acids and ammonium hydroxide were found to contain more lead than could be tolerated. If it is necessary to purify any of the foregoing reagents further, these procedures are recommended : Sulfuric acid. Distill from borosilicate glass apparatus. Sulfurous acid. Dissolve pure, bottled sulfur dioxide in chilled, redistilled water. Chloroform. Purify by the procedure described by Bambach and Burkey ( 2 ) . Xitric acid. Distill fiom borosilicate glass apparatus. Hydrochloric acid. Distill dilute (1 to 1) acid from borosilicate glass apparatus. Hydrofluoric acid. Distill from a platinum still and collect in a little chilled, redistilled water in a polyethylene bottle. Ammonium hydroxide. Distill from borosilicate glass apparatus and collect in chilled, redistilled water in a polyethylene bottle; 463 ml. of water gassed to 1 liter yields 15N ammonium hydroxide. Distilled water should be redistilled from borosilicate glass apparatus. Sodium Citrate. Dissolve 500 grams of sodium citrate in distilled water and add ammonium hydroxide to a p H of 9.2. Dilute the soliition to 1 liter with distilled water and shake with successive small portions of dithizone solution (50 mg. of purified dithizone per liter of chloroform) until the final portion retains its green color. Shake with small portions of chloroform to remove dithizone retained by the solution. Potassium Cyanide Solutions. Although the potassium cyanide required no further purification, lead can be removed if necessary by the procedure of Bambach m d Burkey ( 2 ) . Dissolve 100 grams of the salt in redistilled mater and dilute to 1 liter with redistilled water. Also prepare an ammoniacal solution by adding 7.5 ml. of 15.V distilled ammonium hydroxide and 325 ml. of redistilled water to 100 ml. of the potassium cyanide solution. m-Cresol Purple. Dissolve 100 mg. of m-cresol purple in 5.2 ml. of 0.05N sodium hydroxide solution, dilute to 100 ml. with redistilled water, and filter through a medium porosity filter paper. Dithizone. Dithizone solutions deteriorate quickly when exposed to heat or direct sunlight (8). It is recommended that ruby glass bottles be used for storage and that the solutions, when not in use, be kept in a cool dark place, preferably under refrigeration. If these precautions are taken and purified dithizone is used, the solution can be used for at least 1 month with little
deterioration. Unpurified dithizone was found t o be unsatisfactory even for use in the purification of reagents. T o purify dithizone, dissolve 0.5 gram in 50 ml. of chloroform and filter the solution through a course, frittedglass crucible. Extract the dithizone from the chloroform by shaking the solution with four successive 50- to 75-ml. portions of distilled ammonium hydroxide. Discard the chloroform phase. Pass the combined aqueous extracts through a small plug of cotton and collect the solution in a separatory funnel. Make the solution slightly acid by adding distilled hydrochloric acid (1 to l ) , which precipitates the dithizone, and extract with two or three 15- to 20-ml. portions of chloroform. Shake the combined chloroform extracts twice with an equal volume of redistilled water. Deliver the chloroform solution into a small glass dish, preferably of silica, and evaporate the chloroform at 50" C. Dry finally in a desiccator. Prepare a concentrated dithizone solution by dissolving 18 mg. of purified dithizone in 1 liter of chloroform. Prepare a standard solution containing 8 nig. of purified material in 1 liter of chloroform. Lead Solutions. Prepare a stock standard by dissolving 160.0 mg. of lead nitrate, previously dried a t 100' to 110" C., in 300 ml. of dilute (1 t o 2) distilled nitric acid in a 1-liter ~ o l u metric flask. Dilute to 1 liter with redistilled water and mix well. This solution contains 100 y of lead per ml. To prepare a working standard, transfer 5.00 ml. of the stock solution to a 500nil. volumetric flask, dilute to the mark with redistilled water, and mix well. This solution contains 1.00 y of lead per ml.; it should not be kept more than 1 or 2 days. PROCEDURE
Sample Solution. Weigh a sample of monazite (usually about 50 mg.) containing approximately 150 y of lead, transfer to a 100-m1. roundbottomed flask, add 3 ml. of sulfuric acid, and heat for 1 hour a t 150" to 200' C. Place a short-stemmed funnel in the neck of the flask to reduce loss of acid during the heating. Allow the solution t o cool, add 30 ml. of dilute nitric acid (1 to a), and boil the solution to remove oxides of nitrogen. Add 20 ml. of hot, redistilled water and digest on the steam bath for 15 minutes. If no appreciable residue remains, transfer the solution quantitatively to a 100-nil. volumetric flask. Dilute almost to 100 ml. with hot, redistilled water and cool to room temperature. Dilute to 100 ml. and mix well. If appreciable residue is present following the sulfuric and nitric acid treatments and the addition of water, filter the hot solution through a medium porosity filter paper into a 100 ml. volumetric flask and wash the paper with hot dilute nitric acid (I t o 99). Place the paper in a small platinum crucible which has been cleaned in boiling dilute nitric acid (1 to 1). Saturate the
paper and residue m-ith sulfuric acid and ignite a t 500” C. until the paper is completely ashed. Allow the crucible to cool, treat the residue with hydrofluoric acid and nitric acid, and evaporate to dryness. Repeat the process of adding nitric acid and evaporating to dryness several times to remove hydrofluoric acid. Dissolve the residue in a small amount of hot dilute nitric acid (1 to 9) and add this solution to the main part of the sample solution in the volumetric flask. Dilute as directed above. Isolation of Lead. Transfer a 10.00-ml. aliquot of the sample solution t o a 125-ml. separatory funnel and add 5 ml. of 6y0 sulfurous acid, 5 ml. of sodium citrate solution, and a few drops of nz-cresol purple indicator solution. Add ammonium hydroxide dropwise until t h e solution turns purple and then add a n additional 4.5 ml. of ammonium hydroxide and 2.5 ml. of potassium cyanide solution.
Table I. Comparison of Methods for Determination of Lead in Monazite
Lead, 70 Sample No. Std 1 45 AIT 266 SQ 81 49 OT 22L 44 ?\IT 121 49 LIT 15
52 MT 8 GS/436/55 GS/451/’55
Mass spectrographic
Dithizone 0 0 0 0 0 0 0 0 0
358, 0 183, 0 184, 0 178>0 176. 0 124, 0 126 040, 0 084, 0
0 372 0 193
353 183 186 174 177 125,
0 173
040 086,
0 083-, 0- 086 - ~
0.109, 0.107 0.617, 0.621 0.524, 0.514
-
0.107 0.681 0.540
Add 10 ml. of concentrated dithizone solution to the sample solution, which is now approximately a p H 9.2, and shake for 1 minute. Drain the dithizone extract into a 60-ml. separatory funnel. Add 10 ml. more of the concentrated dithizone solution to the aqueous phase and shake again for 30 seconds. Add this dithizone extract t o the first. Add 10 ml. of chloroform to the aqueous phase and shake the solutions again for 5 seconds. Drain the chloroform layer into the funnel that contains the combined dithizone extracts and discard the aqueous phase. Add 25 ml. of dilute nitric acid (1 to 99) t o the combined dithizone extracts and shake for 30 seconds to strip the lead from the dithizone phase. Discard the dithizone phase. Add 25 ml. of chloroform to the aqueous phase and shake for 5 seconds. Discard the chloroform layer and add 5 ml. of ammoniacal potassium cyanide solution to the aqueous phase to bring the pH again to approximately 9.2. Add
exactly 15 ml. of the standard dithizone solution to the aqueous phase and shake for 2 minutes for the final extraction of lead. Allow 2 minutes for complete separation of phases. Insert a small roll of filter paper into the stem of the funnel. Draw off and discard the first 2 or 3 ml. of the dithizone extract. Deliver the remainder of this layer into a glass-stoppered test tube. The roll of filter paper absorbs suspended droplets of water in the dithizone extract. [Bismuth was below the limits of spectrographic detection in the monazite samples analyzed. If bismuth is present in significant amounts, separate it from the lead by stripping the lead from the dithizone extract with a p H 3.4 buffer solution ( 2 ) instead of the dilute nitric acid.] Absorbance Measurement. Transfer the dithizone extract from the test tube into a 1-em. Corev cell and measure the absorbance against redistilled n a t e r as the reference solution in a matched cell. Optimum wave length for the measurement is 520 n ~ F . A Beckman Model B spectrophotometer was used in the work reported here. Prepare a standard curve from data obtained by carrying reagent blanks and a series of lead standards containing 5 to 20 y of lead through the entire procedure, but omitting the preliminary heat treatment described in sample preparation. The curve follows Beer’s lam. It intersects the ordinate above the origin, indicating a blank of about 0.13 absorbance unit; this is due mostly to the absorbance of the unreacted dithizone, which is always present when the “mixed color” method is used. A water reference solution is used in preference to a blank extract of dithizone because of the vaporization potential of chloroform and the instability of dithizone. Calculation. The average slope of the standard curves was 0.0160 absorbance unit per me;. of lead. This figure was so highly reproducible t h a t frequent preparation of a standard curve was not necessary. Instead, only reagent blanks were run daily and the lead concentration \\-as computed by the following formula: Absorbance of unknown-absorbance of blank 0.0160 =
micrograms of lead
Absorbance readings can be converted to micrograms of lead directly from the standard curve or calculated by the above formula. RESULTS
Table I shows the lead analysis by this method of nine monazites from various localities of the United States. Their lead content ranged from 0.04 to 0.6%. Also included are lead determinations on six samples by the mass
spectrographic nique.
isotope-dilution
tech-
DISCUSSION AND CONCLUSIONS
Dithizone extracts should be made immediately after addition of reagents and adjustment of the p H t o about 9.2. The amount of sample in the aliquot taken for lead extraction must be less than 20 mg. t o prevent loss of lead by coprecipitation with thorium and rare earth phosphates. If the procedure is modified by increasing all solution volumes and reagents proportionally, samples as large as 50 mg. can be used. The method described for the initial sample attack was successfully applied to all pure monazite samples. Only one impure monazite gave an appreciable residue, and this was easily decomposed by hydrofluoric and nitric acids. The presence of lead in the residue indicates the importance of the further treatment of any appreciable residue present after the acid attack. Good reproducibility has been obtained consistently, and the accuracy has been verified by independent checks using mass spectrographic methods. ACKNOWLEDGMENT
The authors nish t o thank their colleagues a t the U.S.Geological Survey: Carmen 11.Cialella and Lorin R. Stieff for the mass spectrographic analyses included in Table I, and Joseph Haffty, Katherine V. Hazel, and Claude L. Waring for the spectrographic analyses referred to in the text. LITERATURE CITED
(1) Bambach, K., IXD.EXG. CHEJI., i l s a ~ED. . 1i, 400-3 (1939). ( 2 ) Bambach. IC.. Burkev. R. E.. Ibid.. 14, 904-7 (1942). (3) Clifford, P. A., Wichmann, H. J., J . Assoc. Ojic. Agr. Chemists 19, 130-56 (1936). (4) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I.,
.,
“
I
“Applied Inorganic Analysis,” p.
i.5) \ - I
(6)
550, Riley, New York, 1953.
Hubbard. D. >I.. IXD.ENG.CHEM.. ANAL. E D . 9,493-5 (1937). Maltby, J. G., Analyst 79, 786-7 11454) ,- - - ,. Rafter, T. A., Ibid., 75, 486-92 (1950). Sandell, E. B., “Colorimetric Deterination of Traces of Metals.” 2nd ed., Interscience, New York; 1950. Seelye, T. T., Rafter, T. Nature 165, I
(7) (8) (9)
317 (1950).
RECEIVEDfor review June 13, 1957. Accepted February 13, 1958. Part of a program conducted by the U. S. Geological Survey on behalf of the Division of Raw Materials, U. S. Atomic Energy Commission.
VOL. 30, NO. 6, JUNE 1958
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