INDUSTRIAL AND ENGINEERING CHEMISTRY
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tail than is necessary to prove its soundness. New procedures should be clearly described, that readers can easily duplicate the work. Loose directions should be avoided, unless the author knows that no possible harm can result from the most liberal interpretation that can be made of such expressions as “to the faintly acid solution,” “wash the precipitate,” “ignite,” etc. If new or uncommon reagents are needed, the author should state their probable cost, where they can be purchased if rare, or how they can be prepared, if not on the market. The author should distinguish carefully between precision and accuracy. Briefly but somewhat roughly stated, accuracy is a measure of degree of correctness; precision is a measure of reproducibility. The precision of a result does not necessarily have anything to do with its accuracy; it serves merely as a measure of the duplicability of the procedure in the hands of a given operator. No claim for accuracy should be made unless the author believes that he has satisfactorily established the correct result. The author should be frank and define the limitations of the method. Tests dealing with the effects of foreign compounds should be made on mixtures in
Vol. 7, No. 1
which the ratios of the compounds sought to the foreign compounds are varied and simulate conditions that are likely to be encountered in practice. If the author has made no such tests, he should state that he has no knowledge of the effectsof foreign substances. It is desirable that possible applications of methods should be stated. A summary or prefatory abstract should acquaint the reader with the main points of the article. This should give concisely where possible the substances determined, nature of material to which determination is applicable, interfering substances, range of concentration to which method is applicable, whether or not a sensible constant error is involved-that is, the accuracy of the method-and its precision. Either the summary or the prefatory abstract is so often used by abstractors that the author may well spend considerable time in their preparation, in order to be certain that proper emphasis is given to the main features of the contribution. Our “Suggestions to Authors” is available to those unfamiliar with the form of manuscript and illustrations preferred by INDUSTRIAL AND ENGINEERING CHEMISTRY.
Microdetermination of Lead Electrolytic-Colorimetric Method MERLERANDALL AND MARIAN N. SARQUIS, Chemical Laboratory, University of California, Berkeley, Calif.
I
N A study of the solubility of lead sulfate in aqueous
solutions of acetic acid it was necessary to determine accurately small amounts, 2 to 15 mg., of lead. From 95 to 99 per cent of the lead in solution was precipitated electrolytically as lead peroxide and the remainder was determined colorimetrically c io in the form of lead sulfide. The solution to be analyzed was measured into an e 1e c t r o 1y t i c beaker (9 cm. high, 3.8 cm. diameter) and 4 ml. of nitric acid (sp.gr. 1.42), 8 drops of 3 M sulfuric acid, and enough water to make the volume 35 ml. were added. The anode w a s a s m a l l cylinder (4.5 cm. high, 1 cm. diameter) of fine platinum gauze, 40 mesh per inch (16 mesh per om.) with a piece of platinum wire attached, and the cathode a spiral of platinum wire wound on a thin glass rod. The anode was secured by means of a gold-plated clamp, instead of the usual holder in which the end of a screw engages the electrode used for the cathode. It was f o u n d t h a t t h e a c t i o n of t h e screw against the electrodes caused a loss in weight of as much as
0.05 mg.
0
2 -0.10
P
3c
g -0.20 -0.30
0 -0.40
The solution was electrolyzed for 12 to 18 hours at approximately 10 volts with a current of 0.05 ampere. Enough water was added so that, without breaking the circuit, the cathode could be lifted from the center of the anode and laced to one side of it. The current was short-circuited througfi the cathode and simultaneously the anode was taken out of the solution and r i n s e d well, first with distilled water, and second with absolute alcohol. The rinse 1i q u i d s were collected in a c a s s e r o 1e placed below the anode. The anode with its deposit of lead peroxide was then placed for 2 hours in an oven maintained at 180” C., after which it was allowed to c o m e t o equilibrium w i t h r e s p e c t to the temperature and moisture content of the balance case, and carefully weighed on a microchemical balance with a sensitivity of 0.005 mg. Tares were used, so that the rider alone was sufficient to weigh the deposit. The theoretical factor wa s u s e d t o calculate the amount of lead precipitated.
,,,1 1
f/e;tro/; alone flectroiytic and colorimetric No acetic ac/d added
?.503.003.50 4.00 4.50 9.00 9.50 10.00 /2.5( Milligrams of lead in sample (not to scale)
FIGURE 1. ERRORIN MICRODETERMINATION OF LEAD
The procedure followed for the c o l o r i m e t r i c d e t e r m i n a t i o n was & comb i n a t i o n of the methods employed b y Francis, H a r v e y , a n d Buchan (2)
January 15, 1935
ANALYTICAL EDITION
and Allport and Skrimshire (1). The reagents and materials were tested to prove the absence of lead and sulfide. The sodium sulfide solution was made as suggested by Francis, Harvey, and Buchan (2). The solution remaining in the electrolytic beaker was poured into the casserole containing the liquids used in washing the anode, and the beaker and cathode were well rinsed. The solution in the casserole was then evaporated to a volume of 10 ml. over a steam bath. Any particles of lead peroxide which might have been present dissolved during the process. In the meanwhile, 2 ml. of 10 per cent potassium cyanide solution, 5 ml. of 6 N ammonium hydroxide, and 2 grams of ammonium acetate were $aced in each of two exactly similar tall 50-ml. Nessler cylinders. he solution in the casserole was neutralized with 6 N ammonium hydroxide and transferred to one of the cylinders. Water to the 50-ml. graduation and 3 drops of sodium sulfide solution were added. Finally, the cylinder was shaken repeatedly until the ammonium acetate was dissolved and the solution was uniform. The other cylinder was treated similarly, except that a known amount of a lead nitrate solution, containing 0.01 mg. of lead per ml., was added instead of the solution from the casserole, After a little ractice, it was easy to determine within 0.05 mg. the number or milligrams of lead present in the unknown solution by noticing the color intensity, and this quantity of lead was added to the standard cylinder for best comparison. The solutions were compared in a Leitz colorimeter. The colorimetric value of the lead left in solution after electrolysis was in error from 5 to 20 per cent, but since this value was a small addition to the value determined electrolytically, the result was a considerable lowering of the total percentage error, which without the colorimetric correction was from 2 to 10 per cent. The method was tested by analyzing measured volumes of a solution of lead nitrate containing 0.5 mg. of lead per ml., prepared by dissolving a weighed amount of pure test lead in nitric acid and diluting with water to the proper volume. Since the method was developed for determining the lead
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content of solutions containing a small amount of acetic acid (left after several hours’ boiling) a drop of glacial acetic acid was added to the known test solutions. A few of the solutions did not contain acetic acid (indicated in Figure 1) and it is readily seen that the presence of acetic acid in such small quantities did not affect the electrolytic precipitation of lead peroxide. With samples containing from 2.5 to 15 mg. of lead the deposit of lead peroxide adhered strongly t o the anode and did not drop off even under vigorous shaking. The results of the various test analyses are summarized in Table I, and in Figure 1 the absolute error in milligrams of the individual analyses is indicated. The vertical sections of the chart show the total amount of lead in the various test solutions. The accuracy in the most unfavorable cases was about 1.5 per cent, and in the majority of cases it lay well below 1 per cent.
TABLE I. MICRODETERMINATION OF LEADBY ELECTROLYTIC-COLORIMETRIC METHOD LEAD TAKEN
Me
.
15.00 12.50 10.00 9.00 5.00 4.50 4.00 3.50 3.00 2.50
AVERAQE LEAD No. OF FOUND EXPERI- ElectroColoriMENTS lytically metrically 12 2 12 4 1 3 21 2 7 3
haximum
-ERRORMinimum
AN
Average
Me.
Me.
Me.
Mg.
%
14.836 12.210 9.821 8.753 4.884 4.324 3.882 3.353 2.844 2.321
0.13 0.26 0.13 0.24 0.08 0.15 0.12 0.12 0.14 0.15
0.149 0.078 0.090 0.034 0.036 0.043 0.071 0.028 0.043 0.054
0.003 0.017 0.006 0.021
0.29 0.37 0.34 0.24 0.70 0.68 0.72 0.77 0.83 1.2
o:i)i7 0.002 0.026 0.002 0.009
LITERATURE CITED (1) Allport and Skrimshire, Analyst, 57,440 (1932). (2) Francis, Harvey, and Buchan,Ibid., 54,725 (1929). RmcmrvsD November 8, 1934.
Determination of Selenium Quantitative Determination on Animal Matter and Clinical Test in Urine H. C. DUDLEY AND H. G. BYERS, Bureau of Chemistry and Soils, Washington, D. C.
R
OBINSON, Dudley, Williams, and Byers (1) report
procedures for the determination of selenium and arsenic in a variety of materials. These methods were developed in response to the necessities of an investigation of the selenium content of minerals, shales, soils, vegetation, and animal tissues and products. Selenium has been found in the tissues and in the blood, feces, and urine of all animals which have ingested seleniferous food, and also in the milk of selenized cows, whether the cows have been fed seleniferous vegetation or have been given inorganic compounds of selenium. Selenium has also been demonstrated in eggs from selenized hens. The development of an accurate method of determination of selenium in these materials is essential for satisfactory work and a clinical test as an aid to diagnosis of selenium poisoning is urgently required. The procedures detailed in this paper are believed to be an advance upon those previously reported, QUANTITATIVE DETERMINATION IN ANIMALTISSUES AND
PRODUCTS 1. For blood, eggs, flesh, hair, bones, or hoofs, the quantity of material required is from 50 to 100 grams. The material in a suitable state of subdivision is placed in a beaker (400 to 600 cc. capacity), covered with 150 to 200 cc. of
concentrated nitric acid (sp. gr. 1.42), and allowed to stand a t room temperature for from 2 to 3 hours, during which period it is stirred vigorously at intervals. Fifty cubic centimeters of hydrogen peroxide (30 per cent by weight) are added and the mixture is allowed to stand overnight. If frothing occurs on addition of the hydrogen peroxide, foaming over is prevented by vigorous stirring of the foam. The foaming is T;articularly intense with blood, liver, and spleen. After stan ing overnight, the mixture is warmed slowly on the steam bath until frothing ceases, after which 50 cc. more of hydrogen peroxide are added, together with 20 cc. of concentrated sulfuric acid. The mixture is then taken t o essentially complete dryness on the steam bath or hot plate. The cooled black paste is treated with 100 cc. of hydrobromic acid (45 per cent HBr) to which has been added sufficient bromine to make it deep yellow in color. The material is then transferred to a distilling flask and 50 to 76 cc. of distillate are collected. Further procedure is as directed by Robinson et al.
2. For milk, the procedure is essentially as for other animal material, except that a quantity of from 500 to 1000 cc. is advised. The final evaporation should be carried out on a hot plate as soon as the fatty material has separated out as a clear yellow layer on the surface of the mixture. This fatty layer, which also appears with egg yolk and other fatty tissues, makes necessary the use of a hot plate for the complete evaporation. 3. The procedure for urine is similar to that for milk, ex-
