727
V O L U M E 2 2 , N O . 5, M A Y 1 9 5 0 man (6) used a sample of Fisher's technical grade that contained nearly 0.03 microgram per gram. One sample of C.P. arsenic trioxide contained a substance that interfered with the iodidecatalyzed reduction of cerate by arsenite. With the authors' still, boiling for 7 minutes after addition of the phosphorous acid gave recoveries of 94 to 100% when arsenite was used in the trap. Using arsenite in the distillation trap Greenman ( 5 ) reports recoveries averaging 95% for iodide in diiodotyrosine compared with low variable amounts without the use of arsenite. Since this manuscript was prepared another article describing a suitable still for this purpose has come to the attention of the authors ( 4 ) .
LITERATURE CITED
(1) (2) (3) (4) (5)
Alford, W. C., J . I d . H y g . Tozkol., 29, 3 9 G 9 (1947). Barker, 5. B., J . B i d . Chem., 173, 715-24 (1948). Chaney, A. L., IND. ENG.CHEM.,ANAL.E D . , 12, 179-81 (1940). Connor, A. C., et al., Surgery. 25, 510-17 (1949). Greenman, University of Pittsburgh, personal communication.
1948. ( 6 ) Martinek, M. J., and Marti, W. C., IND.ENQ.CHEM.,ANAL.EO., 3, 408 (1931). (7) Talbot, N. B., Butler, A. M., Salzman, A. H., and Rodriguez. P. M J . Biol. Chem., 153, 479-88 (1944). (8) Taurog, A., and Chaikoff, I . L., Zbid., 163, 313-22 (1946). RECEIVED June 8. 1949.
Polarographic Data on Zinc in Small Concentrations Deviations from the Ilkovic Equation LUDWIG S . CH.4MPA AND ABRAHAM WALLACH Division of Industrial Hygiene, Montana S t a t e Bonrd of Health, Helena, .Wont.
the development of a method for the determination D URING of small amounts of zinc in various concentrations of very
dilute sulfuric acid by polarographic means, a lack of proportionality between wave height and concentration was noted. A closer study verified these initial findings and the results that were obtained under varying conditions of drop time and concentration are reported below. APPARATUS
The determinations were made with a Sargent Model XI1 polarograph. Lithium chloride (0.1 N ) was w e d as the supporting electrolyte, because maxima effects-reported with other alkali chlorides-were absent. The galvanometer calibration factor, according to the manufacturer, was 0.0051 microampere per millimeter. The ratios of the sensitivity settings (Zto 5, 5 to 10, 10 t o 20) were checked in triplicate, using various concentrations of cadmium (with zinc there was a noticeable decrease in diffusion current with an increase in applied e.m.f. after 1.0 volt), in order to make sure that conversion from each of these four sensitivities was permissible on the instrument. To ensure more accurate measurements of the wave heights, the concentration of cadmium in each case was such that the wave height for the less sensitive reading was from 46 to 60 mm. and that for the corresponding more sensitive reading was 112 to 120 mm. The maximum deviation during each set of runs varied from less than 0.5% for the JO, 20 comparison to less than 1% for the 5, 10 and 2, 5 comparisons. The ratio of average wave heights for the 10, 20 comparison was exactly 2.00; for the 5, 10 sensitivity comparison was 1.99 (instead of 2.00); and for the 2, 5 comparison was 2.53 (instead of 2.50). EXPERIMENTAL
Kolthoff and Lingane (3)have stated that with a drop time between 3 and 6 seconds there is strict linear proportionality between the diffusion current and the concentration of metal ions analyzed. However, they mention that other investigators (1, 6) have claimed that at very small concentrations the diffusion current is greater than corresponds to strict linear proportionality with the concentration. Because the authors were dealing with small amounts of zinc, a series of experiments was run with dropping times varying from 2.77 to 5.90 seconds to determine whether or not there was strict proportionality between current and concentration. The first series of experiments was made with five different concentrations of zinc, varying from 0.10 to 0.005 mg. per ml. (1.53 X loe3to 0.77 X 10-4 M ) . A primary solution was prepared by dissolving zinc sulfate heptahydrate, and standardized by the potassium ferrocyanide method mentioned by Treadwell and Hall (7). From this solution a standard was prepared containing 0.1 mg. of zinc per ml. in 0.1 N lithium chloride. The other concentrations were obtained by taking aliquot portions (25 or 50 ml.) of the 0.1 N standard or weaker concentrations alread made, and diluting them in proper volumetric flasks with 0.1 N Ethium chloride.
The sensitivity used for 0.10 mg. per ml. was 20; for 0.05 mg. per ml., 10; for 0.02 mg. per ml., 5 ; and for 0.01 and 0.005 mg.oper ml., 2. The temperature of the water bath was kept a t 25 * 0.2" C. In measuring the wave heights for zinc, it waa found that reproducible results were best obtained by drawing the limiting current line parallel to the residual current whenever its angle with the horizontal was less than that formed by the residual current. (There is a decrease in diffusion current with an increase in e.m.f. above 1.0 volt.) Corrections for residual current were made, as indicated by Kolthoff and Lingane ( d ) , in measuring diffusion current. The polarogram obtained at the 3.Gsecond drop time were run in triplicate on each of two separate portions of solution. With the other drop times, determinations were mostly made in quadruplicate on the same portion of solution. Where the deviation on one of these four results was greater by four times the average deviation of the remaining results, this single result wm discarded. The data are shown in Table I. DISCUSSION
Ion Concentration. As can be seen from Table I, the ratio of diffusipn current to concentration is not constant within the range of zinc concentrations tested, but increases progressively with decreasing concentrations a t drop times less than 3.7 seconds, being more marked with lower drop times. The ratio of diffusion current to concentration also increases a t higher drop times (4.18 and 4.90 seconds). Xhether it is progressive or not was difficult to ascertain, because an experimental variation in measurement of the polaroprams of even 0.5 mm. a t these drop times can cause a relative difference of 1%. For example, if one were to use the 43.8 figure (drop time 4.18, concentration 0.005) the average wave height would be 43.5 and the per cent deviation 3.3 instead of3.1. Effective Pressure. In view of these observations, the data were analyzed to see whether (with all other factors constant) the diffusion current was Broportional to the square root of the effective pressure on the dropping mercury. According to data indicated by Kolthoff and Lingane ( 4 ) the back pressure due to interfacial tension a t the surface of the mercury drop varies, with usual capillaries used, from 2 to 3 cm. of mercury. In making this correction on the height of the mercury column, the figure 2.5 cm. was used. Any error due to a deviation of 0.5 cm. would make little difference in the square root of the effective heights used in the data in Table 11. The data in Table I1 indicate that although with a concentration of 0.1 mg. of zinc per ml., the ratio of id/h1/2does not vary to any greater degree with varying pressures of mercury columns than the data obtained by Maas ( 6 ) , this is not so with lower concentrations of zinc. At a concentration of 0.005 mg. of zinc per ml., the deviation is about 14.2y0 with a difference in pressure of only 257,; and for a range of pressure that varies twofold, there is a deviation in the ratio of i d / h l / *of about 17.9%.
ANALYTICAL CHEMISTRY
728 ~-
Table I . Dfopping Tiroe See. 2.77
Height of H g Zn Column Concn. Cm. Mg./.lfl. 93.0
Wa\e Height Mm.
0.10
107.6 107.0 108.0 106.8 A v . 107.4
0.02
94.0 93.9 91.9 94.6 94.4
Av. 0.01
119.9 119.8 120.3 Av. 120 0
0.005
Av.
63 8 61 3 63 0 63 0 63.5
.4v. 103.2 0.05
104.7 103.2 103.4 104.6 103.6 102.8 Av. 103.7
85.3 85.2 85.7 85.0 84.5 Av 85.1 0.01 117.7 116.4 116.9 118.0 116.3 117.3 Av. 117.1
Height Correupondin$ to 20 Sensitivity .Urn.
Ratio, Height/ Conrn.
Determination of Zinc Drop-
Deviation
Tiiiir
51
Sei.
Height of Hg %n Column Concn. Cm. Mg./.Ml. 0.01
107.4
0.005
23.b
1180
9.9 3.65
12.0
1200
71.0
11.8 0.05
6.33
103.2
1270
18.3
0.01
1032
1037
0.5 4.18
21.3
1065
62.0
79.0
0.10
100.1 99.8 99.5 AY. 99.8
0.05
102.0 101.3 100.6 AY. 101.3
0.02
82.7 82.2 81.9 82.3 82.3
Av.
1060
6.0
,1098
10.0
( .
/c
Av.
91.6 91.8 92.0 91.5 91 7
91 7
917
..
92 1 92 0 93.4 AY. 92.5
46 3
926
1. o
945
3.1
5.49
96.4 96.0 95.9 AY. 96.1
9.61
961
4.8
Av.
47.4 47.0 48.2 47.3 47.5
4.75
950
3.6
AY.
83.7 84.2 84.6 84.2
842
..
870
3.3
868
3.1
0.10
84.2
~
0.005 11.7
1170
13.5
5.90 6.13
1226
46.0
43.8 (1) 43.2 43.4 43.5 AY. 43.4
0.10
18.9
99.8
50.7
20.6
998
1014
1030
..
Av.
72.0
720
..
Av.
59.5 60.0 60.0 59.8
15.0
750
4.1
0.01
73.0 73.0 71.6 AY. 72.5
7.25
725
0.7
0.005
36.3 36.1 36.4 36.2 AY. 36.2
3.62
724
0.6
1.5
3.2
or
- Khs.t.
Thus a straight line should be obtained if iz is plotted against happ. This was nearly so with the six points from the data on the concentration of 0.1 mg. of zinc per ml., but definitely not so with
4.34
72.6 71.6 71.7 72.0
0.02
-
= KLpp.
10.6
Deviation
3.1
I n order to check any error by having assumed an incorrect back pressure due to interfacial tension, the applied height of mercury was plotted against the diffusion current squared. If id 0: d h a p p , ha.t. where h..t. is the back pressure due to interfacial tension, and hspp. is the hydrostatic pressure on the mercury drops, then i z a hwp. - h8.t.
iz
Ratio, Height/ Concn.
Av.
0.005 51.8
Height Corresponding t o 20 Sensitivity Ym .
54.8 55.2 54.8 54.Y 54.9
0.10
0.005
3.25
Wave Height .Vm.
106.4 105.0 106.0 105.5 Av. 105.7
1074
0.02
61.2 61.3 61.4 61.3 61.7 61.0 AY. 61.3
piny
data obtained on the 0.02, 0.01, and 0.005 mg. of zinc per ml. runs. Acid Concentration. Because the samples contained small amounts of sulfuric acid, a series of experiments was run to determine the effect on diffusion current by varying the concentration of acid. The authors did not have equipment for measuring the p H accurately and easily, so that the acid concentrations are expressed in normality. All runs were made with a dropping time of 3.0 seconds. Solutions containing no acid and 0.001, 0.002, 0.0025, and 0.005 N acids all having 0.10, 0.05, and 0.010 mg. of zinc per ml., respectively, were run in duplicate. The results showed that the diffusion current for each particular concentration of zinc, irrespective of acid concentration, did not vary more than 1% of the average of results obtained. Thus, in
V O L U M E 2 2 , N O . 5, M A Y 1 9 5 0
229
current to the square root of effective pressure on the merConcentration of Zinc, Mg./MI. cury drop increases with a de0.02 0.01 O.lO_ h hl!l h __crease in dropping time. Suli d id/h’!a i d i d / h l l ’ Corrected Corrected id id/h’!’ Applied furic acid concentration has no M m . Mm. Mm. Cm. Cm. Cm. appreciable effect on diffusion 63.5 6.69 90.5 9.51 107.4 11.31 94.4 9.94 120.0 12.63 93.0 61.5 6.69 84.5 9.20 103.2 11.27 85.1 9.25 117.1 12.72 87.0 current up to a strength of 54.9 6.30 79.0 76.5 8.74 99.8 11.43 82.3 9.42 105.7 12.21 0.005 N . The use of lithium 47.5 5.74 71.0 68.5 8.28 91.7 11.09 75.4 9.10 96.1 11.63 43.4 5.63 ... 62.0 59.5 7.71 84.2 10.94 89.6 9.03 chloride as a supporting 36.2 5.49 43.5 6.60 72.0 10.90 59.8 9.05 72.5 l1:OO 46.0 electrolyte obviates the need for a suppressor of maxima usually obtained with other alkali chlorides when running zinc. such weak concentrations of sulfuric acid, the ratio of diffusion current to the concentration of zinc-as in the experiments run in LITERATURE CITED neutral solution-is also not constant but increases progressively. (1) Hsmsmoto, E., Collecfion Czechoaloo. Chem. Commun., 5, 427 Table 11. Relation between Diffusion Current and Pressure on Dropping Mercury
SYNOPSIS
A se,ries of experiments was run on polarographic analysis of zinc in concentrations from 0.005 to 0.10 mg. per ml. in various concentrations of sulfuric acid. At such low concentrations of zinc, the ratio of diffusion current to concentration increases progressively with decreasing concentration in either neutral or acid solutions up to 0.005 N , especially with smaller dropping times. Within this range of no acid to 0.005 Ai sulfuric acid, and especially a t the lower concentrations of zinc, the ratio of the diffusion
(1 933). (2) Kolthoff. I. M., and Lingane, J. J., “Polarography,” p. 57, New York, Interacience Publishers, 1946, revised reprint. (3) Zbid., p. 60. (4) Ibid.. PP. 67, 6 s . (5) p. 71* (6) Thanheiser, G . , and Maassen, G., Mitt. Koiser-Wilhelm-Insl. Eisenfwach. Dilsreldorf9 19, 27 (1937). (7) Treadwell and Hall, “Analytical Chemistry,” Vol. 11. 5th Print ing, p. 667, New York, John Wiley & Sons, 1947.
RECEIVED February 28, 1949.
Direct Determination of Tin in Pig Tin SILVE KALLMANN Ledoux & Co., 155 S i x t h Ave., New York, N. Y HE evaluation of pig tin involves the determination of imTpurities, such as lead, copper, antimony, arsenic, bismuth, cadmium, zinc, iron, nickel, cobalt, and sulfur; tin is calculated “by difference.” This indirect determination of tin provides a n accurate tin figure and reveals the nature and percentage of contaminating elements. Unfortunately, it is very timeconsuming and involved. Frequently, the producer and buyer of pig tin are not so much interested in the impurities as in whether the material meets certain specifications based on its tin content. In such cases a rapid, accurate, and direct determination of tin is of great importance. Unfortunately, the usual iodometric determination of tin must be run on a small portion of the sample and is therefore scarcely more accurate than ~ 0 . 2 5 % . A method for the determination of tin in Bolivian tin concentrates ( 1 ) to a large extent obviates the difficulties of the usual iodometric determination of tin. Based on several years of experience with the method in its original form, and on the advice of several chemists, certain changes were worked out. PROCEDURE
Weigh 5.0000-gram portions of pig tin in the form of sawings or drillings and also 5.0000-gram portions of standard C.P. tin into 750-ml. Erlenmeyer flasks. If an accuracy better than 0.05% is desired, use 10.0000-gram portions. Add 100 ml. of concentrated hydrochloric acid and cover with a glaea cover to avoid loss by spraying and to prevent oxidation of stannous chloride; 5 ams of tin usually dissolve in 2 to 3 hours in the cold acid. gentle warming on a late with a surface temperature not higher than 60” C. speeis up solution of the sample. Disregard any small residue consisting of undissolved copper, bismuth, or antimony. If the residual metallic sponge looks larger than a few milligrams and is suspected to hold back tin (particularly with lower grades of tin), add a few milligrams of potassium chlorate. Avoid any excess. The potassium chlorate will oxidize a small quantity of stannous chloride t o stannic chloride, which in turn will dissolve any copper, antimony, or lead.
Add about 10 grams of sodium chloride and dilute to about 300 If potassium chlorate was used, precipitate any copper, antimony, or bismuth by warming gently with a few grams of iron wire or drillings low in carbon, keeping the flask covered continually with a small cover glass. Omit the treatment with iron if no potassium chlorate was used. Introduce into the Erlenmeyer flasks two nickel strips or foils weighing a t least 5 grams each. Close the flasks with rubber stoppers containing a bentcglass tube extending on the outside to the bottom of the flasks. Boil the solution gently for about 75 minutes, or until the volume has been reduced to about 200 ml., then seal the end of the glass tube with a hot solution of sodium bicarbonate in a 250-ml. beaker. Remove the flasks from the hot plate and cool in running water to below 15’ C. ml. with hot water.
TITRATION
When dealing with 5.0000-gram portions of pig tin, weigh accurately 2.9400 grams of potassium iodate into small beakers, and add about 0.5 gram of sodium bicarbonate and about 100 ml. of water (SO’ to 70” C.) which has previously been boiled. Stir gently with a glass rod until the salts have dissolved but prevent any losses; 2.9400 grams of potmsium iodate theoretically oxidize 4.8918 grams or 97.84% of the tin present in a 5gram sample. Therefore less potassium iodate should be added for pig tins lower than 98%. Prepare a dilute solution of potassium iodate by dissolving 3.oooO grams of potassium iodate in a 1000-ml. volumetric flask in warm water. Cool, fill to the mark with cold water, and mix. Remove the rubber stopper from the Erlenmeyer flask containing the reduced tin solution and add immediately and
Table I.
a
Accuracy of Potassium Iodate RIethod Tin Calculated by DiBerence, 70
Tin by PotasPiiirn Iodate Methoda, % 99.83 99.26 99.64 99.92 98.42 97.12 Average of three.
99.83
99.23 99.64 99.90 98.48
!)7,09
