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Vol. 17, No. 7
Of 0.052 ml. for jar 1, and an average capacity Of 2.56 ml. with an average deviation of 0.028 ml. for jar 111. The most satisfactory device was a square of plate glass with a central hole about 5 mm. in diameter. In use this perforated plate is placed on a jar with the tip of the buret a t the top of the hole and water is run in until i t just reaches the bottom of the hole: Care must be taken that no air bubbles are trapped under the plate, and it is sometimes necessary to tip the assembly slightly in order to bring trapped air bubbles to the hole. Observation is much facilitated if the water is colored with a suitable dye, and accuracy is increased if the rim is greased slightly with stopcock grease before the perforated plate is placed on the jar.
Again ten trials on each jar were made. By comparing these 6 t h those under Method E, their superiority in both accuracy and precision will be evident. For routine work, this volumetric method is sufficiently rapid, and the results arp PUToses. Of the methods amp1e in accuracy for here described it is probably to be preferred for routine control purposes.
In Table I under Method F appears a series of results obtained by this means, a microburet being used to increase accuracy.
The technical assistance of Florence Pines in the preparation of this paper is hereby gratefully acknowledged.
ACKNOWLEDGMENT
Polarographic Determination of Iron and Zinc in Phosphate Coatings JACOB KNANISHU AND THOMAS RICE Rock Island Arsenal, Rock Island, 111. A polarographic method for the simultaneous determinations of iron and zinc in commercial phosphzte coatings has been developed. The analyses are conducted in a supporting electrolyte containing 0.3 molar ammonium oxalate. The calibration charts for iron and zinc are presented in order that in similar analyses the iron and zinc content can be calculated from them. Complete analysis for iron and zinc can be completed in less than one hour.
T
HE need for a method for determining the composition of phosphate coatings on ferrous surfaces has long been recognized. Present methods of appraisal of phosphate coatings are based on accelerated breakdown tests or microscopic examination. Salt spray tests are being used to compare the relative protection of phosphate coatings, but are not indicative of the composition of the coating, and are not necessarily a reliable index of the thickness of the coating. Microscopic examinations are difficult, time-consuming, and limited in application by many factors.
by which a simultaneous determination of iron and zinc could be obtained on a sample of a few milligrams. The use of a polarographic method appeared feasible. An examination of a table of half-wave potentials of inorganic substances ( 1 ) revealed that zinc has a half-wave potential in 0.3 molar ammonium oxalate at -1.30 volts, and iron (both ferrous and ferric) has a half-wave potential a t -0.24 volt in 1 molar potassium oxalate. It was decided, therefore, to investigate the possibility of polarizing phosphate coatings containing iron and zinc in 0.3 molar ammonium oxalate solutions. Some previous work with iron and zinc in 0.3 molar ammonium oxalate solution in this laboratory had shown that sharp curves should be obtained for both iron and zinc without interference due to the presence of both metals. The half-wave potential of iron in this electrolyte was found to be -0.305 volt, and of zinc -1.406 volts. The iron remained in solution in the slightly ammoniacal ammonium oxalate solutions used in the experiments.
Table Curve No,
I. Analysis of Standard Zinc Solutions
Zn in 50 MI.
(Shunt, none) Galvanometer Deflection Microampere
M8.
2 3 4 5 6
Figure I. Typical Zinc Curve
Although the determination of the composition of phosphate coatings will not give the thickness, density, or relative protection of the films, such analyses will give the per cent by weight of iron and zinc phosphates in the coating. By removing the coatc ing from a unit area, the relative amounts of the iron and zinc phosphates in different phosphate coatings can be compared by the method described in this paper. In order to obtain a sample for the analysis of the phosphate coating on a metal article such as a machine gun link, it is necessary to scrape a small amount of the unoiled phosphate crystals from the surface. As only a few milligrams of coating will be available for analysis in many instances, a method was desired
7 8 9 10 11 12
0.04 0.08 0.12 0.I6 0.20 0.24 0.28 0.32 0.36 0.40 0.50
4 7 10.7 14.4 17.8 21.3 24.5 28.1 31.5 35 43.7
0.08 0.14 0.21 0.288 0.356 0.426 0.49 0.562 0.63 0.70 0.874
Half-Wave Volts -1.36 -1.38 -1.375 -1.41 -1.4 -1,425 -1.42 -1.4 -1.45 -1.45 -1.4
Table II. Analysis of Standard Iron Solutions Curve No.
Fe in 60 M1.
(Shunt, none) Galvanometer Deflection Microampere
Mo. 14 15 16
17 18 19 20 21 22 23
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
4.2 6.7 8.6 12.0 14.0 16.1 18.7 21.3 23.3 26.0
0.084 0.134 0.172 0.240 0.280 0.322 0.374 0,426 0.466 0.520
Half-Wave Volt -0.300 -0,285 -0,285 -0,300 -0.315 -0,310 -0.300 -0,330 -0.300 -0,320
ANALYTICAL EDITION
July, 1945
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A 5-ml. aliquot was placed in the polarizing cell; one drop of 0.05y0glue and sufficient mercury to cover the bottom of the cell were then added, and oxygen-free nitrogen was bubbled through the cell for 7 minutes. The solution was polarized from 0 to - 1.6 volts a t 77" F., using a drop rate of 6 seconds. The equipment used was a Leeds & Northrup Electro-Chemograph. I n order to obtain maximum sensitivity, no shunt was used in polarizing the standard solutions. The cathode capillary was made from Corning lead glass marine barometer capillary tubing with a lumen of 0.02 to 0.03 mm. The phosphate coatings were carefully scraped from five 0.50 caliber machine gun links. Samples of each coating weighing 4 mg. were dissolved in 20 ml. of the 0.75 molar oxalic acid, neutralized with ammonium-hydroxide using 2 drops methyl red indicator (adding 2 drops of ammonium hydroxide excess), and diluted to 50 ml. The solutions were polarized as described in the preceding paragraph, RESULTS
The results obtained from curves 2 through 12, polarograms of zinc, are shown in Table I. Curve 9 (Figure 1) is illustrated 89 typical of these zinc curves. Curve 1 (Figure 2) wm plotted from the data in Table I. The results obtained from curves 14 through 23, polarograms of iron, are shown in Table 11. Curve 20 (Figure 3) is demonstrated as typical of these iron curves. Curve 13 (Figure 4) was plotted from Table 11. The polarograms for both zinc and iron were well defined in these oxalate solutions. Plotting the milligrams of the elements against the galvanometer deflections gave straight lines-via., curves 1 and 13. Although curve 13, milligrams of iron per 50 ml. versus the galvanometer deflection, is a straight line, it
Figure 9 .
Figure 3.
Zinc Curve
Typical Iron Curve PROCEDURE
I n order to obtain standard curves for iron and zinc, solutions were prepared containing 0.05 mg. of iron and 0.04 mg. of zinc per ml., respectively. The iron solution was prepared by dissolving 25 mg. of iron wire in 3 ml. of concentrated nitric acid, evaporating to dryness dissolving in a few milliliters of 0.75 molar oxalic acid, and diluting to 500 ml. The supporting electrolyte was prepared by dissolving 95 grams of oxalic acid (C2H2O4.2H20)in a volumetric flask and diluting to 1 liter with distilled water. After 20 ml. of ttus solution are neutralized with ammonium hydroxide and diluted to 50 ml., a 0.3 molar solution of ammonium oxalate is obtained. The required amounts of iron and zinc solutions were measured into 50-ml. volumetric flasks and 20 ml. of the 0.75.molar oxalic acid added .to each. Tables I and I1 show the milligrams of iron and zinc In all the solutions polarized. Two drops of methyl red were added and the solutions neutralized by the dropwise addition of concentrated ammonium hydroxide. Two drops of ammonium hydroxide were added in excess. The solutions were then diluted to 50 ml. and mixed thoroughly.
Figure 4.
Iron Curve
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Table 111.
Curve No. ’ Link
Vol. 17, No. 7
Analysis of Phosphate Coatings from 0.50 Caliber Machine G u n Links (Weight of sample. 4 mg. shunt, 200,000 ohms) Galvanometer Deflection
Microampere
Fe
Fe(POd.2HgO
HalfWave
1.983 1.436 1.609 1.526 1.551
-0.31 -0.31 -0.30 -0.30 -0.29
MO. 24 25 26 27 28
A
B
c
D
E
Table IV.
Curve No.
Link
15.4 11.1 12.5 11.8 12 1
0.616 0.444 0,500 0.472 0.484
0.6923 0.4288 0,4808 0.4558 0.4650
1’011
Analysis of Phosphate Castings from 0.50 Caliber Machine G u n Link (Weight of sample, 4 mg. shunt, 200,000 ohms) Galvanometer MicroDeflection amperes Zn Zna(POd.4HnO
Mg. 24 25 26 27 28
Figure 5.
A B
C D
E
32.0 18.7 30.6 26.0 31.2
1.600 0.748 1.224 1.040 1.248
0.9153 0,4275 0.6994 0,5943 0.7131
Half-
Wave
volt8
2.136 0.999 1.634 1.388 1.666
- 1.366 - 1,376
-1.336 -1,350 -1.300
coatings removed from the five machine gun links are shown in Tables I11 and IV. The milligrams of zinc and iron were calculated from the data in Tables I11 and IV by reference to curves 1 and 13. Curve 24 is illustrated (Figure 5) as typical of these phosphate-coating curves. This polarographic method has not been compared with any wet methods of analysis for iron and zinc in phosphate coatings, and whether the results obtained are in complete agreement is not known to the authors. It is hoped, however, to make a comparison at a later date.
Typical Phosphate-Coating Curve
does not pass through zero, probably owing to the presence of some iron in the distilled water used in the experiments. The results obtained from the polarograms of the phosphate
LITERATURE CITED
(1) Kolthoff, I. M., and Lingane, J. J., “Polarography”, New York
Interscience Publishers, 1941.
Ozonizer Capable of Producing a Constant Amount of Ozone FRED L. GREENWOOD Division of Agricultural Biochemistry, University of Minnesota, St. Paul, Minn.
By maintaining a constant transformer primary voltage and a constant oxygen flow, and by cooling the water in the Berthelot tubes, the conventional laboratory ozonizer can b e designed to maintain ozone production constant to 0.1% ozone (by volume) over a period of 19 hours or longer.
IN
CAItRYIKG out quantitative ozonolysis studies, an OZQnizer which would produce a constant amount of ozone would make possible easy and accurate determination of the amount of ozone introduced into the reaction mixture. Briner et al. (8) claimed to have constructed such an ozonizer, but stated (2) that the constancy claimed in the former paper could not be reproduced. A t the time of the construction of the author’s apparatus, the laboratory ozonizers described in the literature (4-7, 9) used the Berthelot tube to effect the conversion of oxygen to ozone. As it was reported that these ozonizers gave satisfactory yields of ozone, an attempt was made to modify one of them so that it would yield a constant amount of ozone. [Since the construction of this apparatus, Henne and Perilstein (8) have described a new design for an ozonizer which yielded 5.1% ozone by weight (3.5% by volume), with an oxygen flow of 10 liters per hour and a
voltage of 22 kv. This ozonizer is constructed of Pyrex (for good ozone production Berthelot tubes should be made of soft glass), but the yield of ozone is inferior to that obtained with an apparatus of the type described by Smith (9).] In order to achieve constant ozone production a constant rate of oxygen flow through the ozonizer and a constant voltage are obviously necessary. In the ozonizers described in the literature provision wm made for cooling the large volume of water in the battery jar, but it has been found much more important to control the temperature of the small volume of water inside the Berthelot tubes, By controlling voltage, gas flow, and the temperature of the water in the Berthelot tubes, it is possible to maintain the ozone concentration in the ozonized oxygen cons t m t within 0.1% ozone by volume. DESCRIPTION OF APPARATUS
Except for some modifications necessary for maintaining constant the previously mentioned factors, the apparatus constructed was essentially that described b Smith (9) but with Berthelot tubes of the type described by Jmith and Ullyot (11). A constant transformer pnmary voltage was obtained by means
