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 SYNOPSIS (1 933). (2) Kolthoff. I. M., and Lingane, J. J., “Polarography,” p. 57, New A se,ries of experiments was run on polarographic analysis of York, Interacience Publishers, 1946, revised reprint. zinc in concentrations from 0.005 to 0.10 mg. per ml. in various (3) Zbid., p. 60. (4) Ibid.. PP. 67, 6 s . concentrations of sulfuric acid. At such low concentrations of (5) p. 71* zinc, the ratio of diffusion current to concentration increases pro(6) Thanheiser, G . , and Maassen, G., Mitt. Koiser-Wilhelm-Insl. gressively with decreasing concentration in either neutral or acid Eisenfwach. Dilsreldorf9 19, 27 (1937). solutions up to 0.005 N , especially with smaller dropping times. (7) Treadwell and Hall, “Analytical Chemistry,” Vol. 11. 5th Print ing, p. 667, New York, John Wiley & Sons, 1947. 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 RECEIVED February 28, 1949.
Table 11. Relation between Diffusion Current and Pressure on Dropping Mercury
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 by PotasPiiirn Iodate Methoda, % 99.83 99.26 99.64 99.92 98.42 97.12 Average of three.
Tin Calculated by DiBerence, 70 99.83
99.23 99.64 99.90 98.48
!)7,09
ANALYTICAL CHEMISTRY
730
-~ Example 5 Grams oi Pig Tin Taken 6 Grams of C.P. Standard Tin Taken k ,9400 KIOI added. giams 2.9400 KIOa titrated. ml. 20.50 10.00 10.00 ml. KIOI = 0.0300 gram KIOI 0.0615 gram KIOI 20.50 ml. KIOI ~
Total
Calculation. Theoretically, because 1 mole of potassium iodate 3 moles of tin, 214.02 grams of potassium iodate = 356.1 grams oi tin.
2.9700 grama KIOx 2.9700 X 1.6658 X 100
5 . WOO Factor. = 1.6658 3.0015
Sn
Theoretically. 214.02 = 1.6639
Sn = 98.95%
=
5
quantitatively the solution of 2.9400 grams of potassium iodate, washing the beaker thoroughly with cold water. Agitate the Erlenmeyer flask s t once in order to prevcnt an," prolonged Cont&CtOf the iodine that is formed Won the strip and the precipitated antimony or copper. Add starch solution and titrate with 'the dilute potassium iodate solution ( 3 grams per liter) to
30.
mn..arl ...".I",
2,9400 __
7 .94nn .... 3 0015 grams KIOa
__
the usual blue end point. If available, pass carbon dioxide gas into the Erlenmeyer flask from the moment the rubber stopper is re-
ANALYTICAL RESULTS
The data in Table I Bhow that results obtained in actual analysis by the potassium iodate method agree very well with those of the indirect method where tin is calculated by difference.
( I ) Kallmann, S., IND.n n ~ . .HE.^., n n R E C E W E ~ October
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lyy-l
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28, 1949.
p-Alanine (p-Arninopropionic Acid)
Contributed by J. KRC, JR., AND W. C. McCRONE, Armour Research Foundation, Illinois Institute of Technology, Chicago 16, Ill.
foundation'sfileof crystal data there INareCO'iXECTIOSwiththe inquiries for crystallographic data on the following cornpounds: aureomycin, acewacetsnilidr, diethyl carbamate, and heavy metal salts of fat$" acids. It would be appreciated if anyone having unpublished and even fragmentary data on any of these compounds would send the information to W. C. McCrone, who will send it to the interested parties.
Axial Itiltio. a:b:c = 0.714:1:O.44lL Interfacial Angles (Polar). 011 A 011 = 132" 24'; 101 A 101 = 116" 36'. Cleavage, olo strang; lol
x-nruD
~ D~~~
Cell Dimensions. a
slight, ~
~
~
= 9 .86A.; b = 13.81A.; c = 6.009.
Formula Weights pcr CelI. 8. Formula Weight. 89.00. Density. 1.412 (pyenam eterj; 1.418 (x-ray).
CRYSTALUIGRAPHIC DATA FOR &ALANINE
Principal Lines
H2X-CH2-CH1-COOH Structural formula of &alanine Excellent crystals of 0-alanine are obtained from nrpropyl
p = I.i)Y,
-_ h.
*
d
I/I,
6.88
0.25
4.83
1.00
".""L;
y =
I.0""
=
U.""'.
...,DI...D..Y..."p.r.
Fusion preparstion. Dark areas are @-alanineand decomposition products. Lighter ereas contain neodlelikc sublimate of 8-.d*..i"e
projection of 8 typical palanine crystal is shown in Figure 2. Pronounced 010 cleavage, together Kith the fact that n is parallel t o b? indicates that the molecules lie in the 010 plane. No evidence of polymorphism was observed.
CRYSTAL MORPHOLOGY Crvstnl System. Orthorhombic. Pdrm and Habit. Tablets lying on brachypinnroid showing bipyramids (1111 and macropinaeoid 11001.
10101
Figure 2. Orthographic Projection of Typical Crystnl of P-.tlnninr
~
