January 15,1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
27
Electrolytic Determination of Lead as Lead Dioxide' W. T. Schrenk and Philip H. Delano MIssoURI SCHOOL O F
MINESAND METALLURGY, ROLLA,M O .
The electrolytic method for the determination of lead has been studied and the best results were obtained when the following conditions were observed: The sample should be electrolyzed for l 1 / 2 to 2 hours with a current of 3 amperes and placed on the board a t a temperature of 90' C. The anodes should be sand blasted and clean and the amount of lead present between 5 and 150 mg. The solution should contain from 20 to 30 per cent free nitric acid and about cc. of sulfuric acid. Interfering elements should be absent. The effects of interfering impurities were investigated and silver, bismuth, manganese, tin, arsenic, antimony, mercury, chromate, and phosphate were found to cause
poor results. If these elements are present in any great amount, a separation should be made before electrolyzing. Potassium, sodium, ammonia, calcium, magnesium, cadmium, copper, zinc, and iron were found to have no effect unless the solution contains large amounts of the salt. The precision of the method was determined. It was found that the average variation obtained was less than 0.2 mg. provided the lead content of the sample is between 5 and 150 mg. Samples of higher lead content can be r u n but the error is usually somewhat larger. The error is practically always negative. The theoretical factor should always be used with this method in order to avoid low results.
..........,... HE electrolytic method for lead has been adopted as a control method especially for ores in which the lead content is less than 7 per cent. The method is rapid and accurate under certain conditions but variations in conditions produce erratic and irregular results which may check each other but will not check the true lead content. Because of this fact a thorough study of the conditions affecting the electrolytic deposition of lead dioxide was undertaken. The following factors may affect the determination of lead: (1) apparatus, (2) temperature, (3) time of electrolysis, (4)concentration of acid, (5) current, (6) amount of lead present, (7) added reagents, and (8) type of material analyzed.
T
Apparatus and Solutions
A slight modification of the apparatus first used by Guess ( I ) , and further described by Holmes and Morgan (2), was used in this work. A diagram of the apparatus is given in Figure 1. The anodes should be made of 0.005-inch (0.0127-cm.) sand blasted sheet platinum. Sleeves of rubber tubing placed over the split terminals furnish contact pressure to hold the anodes and cathodes in place. The cathodes are firmly held in place by an additional bakelite sleeve placed over the end of the terminal. The cathodes are stiffer and last longer if crimped as shown. The terminals should be of rolled hard drawn aluminum in order to resist the acid fumes to which they are exposed. The base may be either cast or machined but the terminals should be welded to it to insure against poor contact. The wiring should be so arranged that any number of cells may be removed from the board without disturbing the remaining cells. The wiring in the diagram has this advantage. Lead nitrate was prepared by dissolving pure lead in nitric acid. The solution was filtered through a quartz filter, and precipitated with concentrated nitric acid. The recrystallization was repeated several times and the salt dried. Solutions of known lead content were prepared from this purified lead nitrate and checked gravimetrically by the sulfate method. lReceived Septembei 5, 1930. Presented before the Division of Rubber Chemistry a t the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September 8 t o 12, 1930.
Preliminary Procedure I n the preliminary experiments the conditions were selected which had been recommended by other workers and had been used in this laboratory. All factors were held constant save one which was varied and the effect of this variable was studied. The procedure a t first used was to take a suitable weight of the sample and digest with 20 cc. of nitric acid, When brown fumes ceased to be evolved, 15 cc. of a saturated solution of ammonium nitrate was added and the solution boiled for approximately 1 minute. The sample was then diluted with cold water and electrolyzed for 11/2 to 2 hours. If a pure lead solution were used instead of a
ANALYTICAL EDITION
28
standard sample, the procedure was approximately the same. Effect of Time and Temperature
The usual value for current with the type of anodes used in these experiments is 3 amperes (a current density of about 7 amperes per square decimeter), giving good results in ll/z to 2 hours. It was found that complete deposition in the cold could only be secured by electrolysis over a period of 8 or more hours, the slowness of deposition being due to the plating of lead on the cathode. Electrolysis of a solution at an initial temperature of 90" to 95"C. yielded a quantitative deposition of lead in a period of 11/z to 2 hours. The temperature during this time dropped to about 50" C. The lead is not completely deposited from the solution if the high temperature is maintained during the electrolysis. Table I-Effect of Time a n d Temperature Conditions of experiment: 140 cc. s o h ; 30 cc. "0s; 3 amperes Gram PbOataken 0.0288 Average of 8 determinations (cold) l'/z hours.. 0 0254 -~ 0.0003 Maximum deviation from averaae Error 0.0034 PbO*taken 0.0250 Average of 6 determinations (cold) 8 hours.. 0.0249 Maximum deviation from average.. 0.0002 Error 0.0001 0.0250 PbOptaken. ....................................... Average of 22 deferminations (hot) 1i/z hours. 0.02501 Maximum deviation from average.. 0.0002 0.00001 Error
........................................ ........ .................. ............................................ ........................................ .......... ................ ...........................................
Vol. 3, No. 1
by giving a firmer and more coherent deposit and increasing the amount of lead dioxide which could be quantitatively determined. A concentration of sulfuric acid in greater amounts than 2.5 cc. produces incomplete deposition. The concentration of sulfate ion produced by the addition of sodium sulfate, potassium sulfate, or calcium sulfate produced the same effect as did sulfuric acid. When running a sulfide ore there is always a little sulfate present, either from the original ore or formed by the action of the nitric acid during the solution of the sample. This sulfate probably explains the excellent results usually obtained in the electrolytic analysis of sulfide ores. I n these experiments, noted in Table 11,it was demonstrated that ammonium nitrate is not essential and in larger amounts tends to produce scaling. Also it has been found that satisfactory results are obtained when the electrodes are dried rapidly on a hot plate or in an oven a t a temperature between 150" and 200" C. The anodes are removed from the hot plate as soon as dry. Interfering Elements
The effect of certain salts on the deposition of lead dioxide was studied. The salts were those which might be obtained in the solution of a lead ore. Calcium nitrate, cadmium nitrate, zinc nitrate, potassium nitrate, sodium nitrate, ......... cupric nitrate, ferric nitrate, and perchloric acid were without ................ ........................................... effect when sulfuric acid was present. Certain elements, however, do interfere with the deterThese results, given in Table I, indicate that the tem- mination. An investigation of the effects of silver, bismuth, perature is one of the most important factors in the elec- arsenic, antimony, mercury, manganese, tin, chromate, and trolysis of a lead solution. Results in the cold are poor phosphate shows them to be harmful to the determination. unless a long period of electrolysis is used, while results Some of these impurities deposit with the lead while others with a hot solution are very good in l l / g to 2 hours. The prevent deposition or cause scaling of the lead dioxide deposit. experiments also brought out the fact that checking results I n any case in which these elements are present in an amount could be obtained under practically all conditions, even large enough to cause trouble, a separation must be perthough all of the lead was not deposited from the solution. formed. Traces of these impurities may be present withThis emphasizes the fact that checking results do not mean out the production of any appreciable interference. The quantitative results. results of this investigation are given in Table 111. A small amount of chromate does not harm the determinaTable 11-Effect of Ammonium Nitrate, Nitric Acid, a n d tion so that i t wouId be possible to precipitate lead from Sulfuric Acid solution as lead chromate, redissolve, and deposit electroConditions of experiment: 140 cc. sola.; 90' C.; 2 hours; 3 amperes PbOi PbOn lytically as lead dioxide. TAKEN FOUND NH4NOa HNOa Ha04 REMARES Gram 0.0025 0.0025 0.0025 0.0250 0.0250 0.0250 0.1156 0.1156 0.0188 0.0469 0.1407 0.0188 0.0469 0.1407 0.0188 0.0469 0.1407 , 0,2887 0.2887 0.2887 0.2887 0.1156 0.1156 0.1156 0.1156 0.1156 0.1156 0.1156
Gram 0.0009 0.0016 0.0014 0.0096 0.0238 0.0250 0.1125 0.1148 0.0182 0.0454 0.1351 0.0175 0.0452 0.1384 0.0187 0.0470 0.1410 0,2864 0,2806 0.2876 0.2878 0.1155 0.1156 0.1154 0.1155 0.1156 0.1080
Cc.
....
Cc.
.... .... .... .... .... .... .... ....
..... .... ...... ..
j.
.... ....
15 15 15
... .
8'd& 8 drops 8 drops
15
....
.. .. 15 .,.. 15
15
15
..
15 15
CG.
.... ....
30 30 30 30
8'd&s 8 drops 0.5cc. 1.5 cc. 1.5 cc. 2.5 cc. 2.5 cc. 5 cc. 30 cc.
Table 111-Effect of Interfering Elements Conditions of experiment: 30 cc. "0s; 1.5-2 hours; 3 amperes; 0.25 cc. HaSO4; 80" C.; 140 cc. soh. IMPURITYWEIGHTOR PbOz PbOz PRESENT IMPURITY TAKEN FOUND REMARKS Gram Gram Gram 0.0100 0.0064 0.0050 As 0,0416 0.0100 0.0500 Deposit with PbOi loosens 0.0108 0.0100 0.0300 MU deposit 0 0504 0.0200 0.0500 Scaling with large amounts of Ma Deposit with PbOz may 0,0500 0.0524 0.0050 Bi loosen deposit 0.0550 0.0100 0.0500 Scaling 0.0500 0.0500 0.0552 0.0500 0.2500 SU 0.0580 0.5000 0.0500 0.0520 0.0500 0.0100 Sb 0.1000 0,0500 0.0484 Deposits with PbOi 0.0506 0.0500 0.0100 Ag 0.0510 0.0500 0,1000 Hg on cathode holds Pb 0.0426 0.0500 0.0100 Hg 0.0422 0.0500 0.1000 Occluded 0.1157 0.1156 0.0500 CrOc 0.1168 0.1156 0.2500 0.1186 1.0000 0.1156 0.0500 Very low 1 cc. Hap04 N o deposit 0.0500 5 cc. 0.1147 0.1156 0.0500 NazHPOi 0.1100 0.1156 0.2500 0.0980 0,5000 0.1156 %
Slight scaling Scaling increased
Very low results No deposit
Effect of Acid Concentrations
The series of experiments to determine the effect of acid concentrations brought out the fact that complete deposition is best secured when from 20 to 30 per cent free nitric acid is present. A series of experiments was run to determine the effect of sulfuric acid and the sulfate ion. From 0.25 to 0.5 cc. of sulfuric acid aided the deposition materially
Amount of Lead
The amount of lead which may be quantitatively deposited was determined and the data presented in Table I V and Figures 2 and 3. These indicate that the method gives
January 15, 1931
INDUXTRIAL AND ENGINEERING CHEMISTRY
excellent results when the lead content of the sample is between 5 and 150 mg. Within this range the average error of the method is less than 0.2 mg. of lead. When the sample contains less than 5 mg. of lead, low results are obtained. Should the lead content of the solution be more than 150 mg., as much as 1 mg. of lead may remain in solution. 1 rcrd d r p r r t r d
-
Arid m
I
I .
P*
I
until brown fumes cease to be evolved. The sides of the beaker are washed and 0.25 cc. sulfuric acid added. The solution is boiled again for 1 to 2 minutes, diluted to 140 CC., and heated to a temperature of 90" C. It is then electrolyzed for 2 hours a t 3 amperes. The anodes are removed from the board, washed in water, alcohol, dried rapidly in an oven or on a hot plate, cooled, and weighed. The weight of lead dioxide deposited is equal to the per cent of lead when a factor weight of the sample has been used. When interfering elements are present, it is necessaryto perform a separation before electrolyzing. I n applying this method to the analysis of lead concentrates it is usually best to take a one-fourth factor weight sample. A one-half factor weight sample may be run but the error is usually greater. Care must be taken to assure a homogeneous sample. I n some instances it, is necessary to grind the sample to pass a 20Qmesh screen before satisfactory results can be obtained. This is illustrated by the data given in Table V.
Con,."+
Figure 2
Table V-Analysis
Other workers have recommended factors less than theoretical. According to this the method should give high results when the theoretical factor is used, but in these experiments high results were never obtained unless some interfering element, such as bismuth or tin, were present. The factor lead taken over lead dioxide deposited, is always 0.8662 if the proper conditions are observed. Hydrogen sulfide was used to test for the complete deposition of lead and-only traces were found after electrolysis. Table IV-Deposition of Various Amounts of PbOa Conditions of experiment: 140 cc. soln: 0.25 cc. Hasob; 2 hours; 3 PbOn TAKEN Gram
29
amperes: 90' C.: 3; cc. HNOs PbOi N O . OF FOUND DBTNS. Gram
26 6 1 1 5 1 4 2 2 22 18 5 3 5 4 5
0 0044 0.0049 0.0070 0.0100 0.02501 0.11555 0.1727 0.2295 0.2870 0.3451 0.4024
A v ER A G E DEVIATION Gram
.....
%
of Lead Concentrates SAMPLE SAMPLE 150-MESH 2OO-MESH
% 53.88 53.68 53.60 53.70 53.88 54.08
5
Run
by-A. W.
% 53 .92 53.92 54 04 53. 88O 53.84' 53.84' Av. 53.91 ;h 0.05
Holmes, Bureau of Mines.
Literature Cited (1) Guess, Bull. Am. Insl. Mining Eng., 1239 (1905). (2) Holmes and Morgan, IND. ENQ. CHEM.,Anal. Ed., 1, 210 (1929).
.....
0.0013 0.0008 0.0007 0 0004 0.0001 0.0001 0.0001 0.0000 0.00001 0.00005 0,0005 0.0014 0.0012 0.0013 0.0017
Procedure
The procedure as finally adopted and recommended for use when there is no interference from other elements is as fol-
PbO, cont.nt d f
ORIGINALSAMPLE ~OO-MESH
so/".,;b"
Figure 3
lows: A sample, the amount depending upon the per cent of lead, is weighed into an ordinary 150 cc. electrolytic beaker, 30 to 35 cc. of nitric acid are added, and the sample is boiled
Determination of Hydrogen Sulfide in Refinery Still Gases' A. R. Scharnagel and A. W. Trusty LOUISIANA OIL REFINING CORPORATION, SHREVEPORT, LA.
HE iodine titration method has always been a standard method for the determination of hydrogen sulfide, but with refinery still gases this method is inaccurate because of the reaction of the gaseous unsaturated hydrocarbons with the iodine. The following method has been used in our laboratory: 100 cc. of 0.1 N acid lead nitrate is placed in a Fisher gas washing bottle or any convenient scrubber. The gas to be tested is passed through the bottle and metered with a gas meter, or a known volume of water is displaced. A second wash bottle with 100 cc. of 0.1 N lead nitrate is placed directly after the first wash bottle. With the slightest discoloration of the second bottle, the rate of flow of the gas is reduced to insure all of the hydrogen sulfide being caught in the first bottle. The second bottle is useful also when testing a gas in which the quantity of hydrogen sulfide is unknown, as the second bottle will be an indicator when the first bottle becomes spent. The first bottle is Gltered, made up to volume, and aliquot portions titrated with ammonium molybdate solution. With convenient factors on the solutions, the method is rapid and accurate. 1
Received N6vembec 3, 1930.
