proceeds normally, and stoichiometric values of chloride are obtained. Table I1 lists results obtained on known compounds. The best available commercial material was used. There should be no difficulty in applying the method to any organic material that is not too volatile to weigh. Except for the formation of thiosulfate noted above, no interference was found. Alternate use of the equipment for sulfur and halogen analysis has caused no problems. After running catalyst samples, the exit end of the combustion tube in the hot zone was heated briefly before returning to sulfur analysis to remove condensed volatile portions of the catalysts which would interfere with sulfur analyses. Both chlorine and bromine can be estimated in the same material. Each has its own equivalence point, and by noting the volume of titrant a t the characteristic point, quantitative fi,vures for each can be obtained. Some work was done with inorganic materials, but this aspect was not fully explored. Nearly quantitative results were obtained with ferric chloride and sodium chloride. Samples containing barium, calcium, and zinc chlorides were also analyzed with good results. The outstanding application to inorganic material is the analysis of refractory material, such as re-forming catalysts. Table I11 compares results with those obtained by an acid extraction pro-
cedure. The high-temperature results are always higher and probably better in view of known uncertainties in the extraction steps, and the method is much faster. Grinding is not required, and relatively large samples (up to 5 grams) can be used when lorn concentrations are being determined. Certain residues containing complex cyanides were easily analyzed for chloride content when other methods gave unreliable answers. Table I11 shows agreement with results obtained on unknown organic samples by another method.
dual combustion furnace, together with an automatic titrimeter for the potentiometric titration of the chloride and bromide, two determinations can be made in 1hour. I n this laboratory, the sodium biphenyl (4) procedure for halogen has proved satisfactory for simple organic material. However, for analyzing insoluble materials, acidic compounds, residues, emulsifying systems, and inorganic catalysts, the high-temperature equipment, where already available for sulfur analysis, provides an extension of the scope of chlorine and bromine determinations.
DISCUSSION
No limit was found to the upper range of the high-temperature method. The lower range is limited by the volatility of the material and the permissible sample sizes that can be handled. The resistance combustion apparatus has an upper sample limit of 0.4 gram for many materials. Thus, t o extend the range below a few tenths of 1%, more sensitive methods, such as colorimetric procedures, must be used to determine halogen content, or multiple burns must be made. Larger samples can be used on high-melting solids and refractory materials. The use of gelatin capsules is recommended for weighing volatile samples. The method is rapid for analyzing unknown materials and by using the
LITERATURE CITED
(1) Agazzi, E. J., Peters, E. D., Brooks, F. R., h A L . CHEX.25, 237 (1953). 1~, 2 ) ASTM Standards on Petroleum Prod-
ucts and Lubricants, Appendix 11. November 1956. (3) Belcher, R., Ingram, G., Anal. Chirn. Acta 7,319 (1952). (4) Liggett, L. M., ANAL.CHEY.26, 748 (1954). (5) Siederl, J. B., Siederl, V., “Micromethods of Quantitative Oreanic Analysis,” p. 16O;Riley, New York, 1942. (6) Pregl, F., Grant, J., “Quantitative Organic Microanalysis,” p. 85, Blakiston, Philadelphia, 1946. ( 7 ) Safford, H. W., Stragand, G. L., ANAL.CHEM.23, 520 (1961). (8) Sundberg, 0. R., Royer, G. L., Ibid., 18, 719 (1956). RECEIVED for review February 14, 1958. ilccepted June 6, 1958. Division of Petroleum Chemistry, 133rd Meeting, b C S , San Francisco, Calif., ilpril 1958.
Determination of Oxide Films on Tin Plate A. R. WILLEY and D.
F.
KELSEY
Research Division, American Can Co., Barringfon, 111.
A rapid coulometric procedure for the determination of oxide films on tin plate is described. Its validity was established b y an independent analytical procedure based on a chemical solution principle. This coulometric method or close modifications thereof have been universally adopted b y laboratories associated with the tin plate industry. Although designed primarily to study commercial problems posed b y oxide films on electrolytic tin plate, the procedure is also useful in related research investigations.
T
IN OXIDE films
on tin plate formed during production at the mill, or developed during warehousing, have long been associated with adhesion failures of organic coatings, yellow surface discoloration, and soldering dif1804
ANALYTICAL CHEMISTRY
ficulties. Although a visual method for classifying the extent of surface oxidation on tin plate was early established, substantial progress in oxide film control required the development of a convenient and accurate analytical method. Among different investigators there is still a difference of opinion as to the exact chemical composition of the oxide, but data obtained in this study indicate that it is essentially stannous oxide. Kerr ( 7 ) and MacKaughton (8)used weight loss during cathodic reduction t o determine film thickness on very heavily oxided tin surfaces. The apparent ease of reduction of surface oxide by cathodic treatment suggested a method based on Faraday’s law for evaluation, and Miley (9) and llIiley and Evans (11) used such a method for
studying oxide films on iron. Later Miley(l0) demonstrated that the method was applicable to measurement of oxide films on copper. Similar studies were made by Price and Thomas (12) on silver, copper, and silver-copper alloy. Salt and Thomas ( I S ) and Katz (6) reported quantitative procedures for measurement of oxide on pure tin. The first application of the electrical reduction procedure for estimation of oxide films on tin plate appears to have been made by Donelson (3). Recently Frankenthal, Butler, and Davis (4) described a coulometric method for tin plate, as did Britton and Bright (1) who also studied the nature of the films. The technique described is based on the earlier methods used, but is improved by better circuitry and the use of a recording potentiometer to eliminate manual plotting of data. It rvas
established that the exclusion of oxygen and the use of a n acid hydrogen bromide electrolyte, rather than a neutral potassium chloride one, are required for reliable results, and a n appropriate reduction cell was devised. An alternative method based on chemical solution and analysis was developed t o confirm the accuracy of the electrolytic procedure.
Table I. Comparison of Oxide Film Thickness Measured in 2 Electrolytes
Visual Discoloration None . ~ ~
M.C.E./Sq. Inch HC1 HBr 16
.
15
Light 39 40 Heavy 526 126 Extremely heavy 70" 247 There n-as visual evidence that film sloughed off. 5
COULOMETRIC METHOD
Reduction Cell. T h e reduction cell is illustrated in Figure 1. A 2.26inch-diameter tin plate disk (4-squareinch surface area) customarily used as t h e cathode is placed inside t h e cap, and a Tygon gasket having a n open center area of 3 square inches pressed in over it. The disk is vapor degreased over boiling acetone a n d dried prior t o use. T h e threaded glass cylinder is then screwed down tightly. Electrical connection with the cathode is made by a pressure contact through n hole in the cap. The anode, reference electrode, and electrolyte filling or introductory tubr are fitted into a large rubber stopper. The reference electrode is a silver wire which forms a silver-silver halide half cell in the electrolyte. The anode is designed for current entry through a salt bridge t o prevent oxidation products a t the anode from diffusing to the cathode. This is of no consequence with lighttransparent films, but it becomes of considerable importance with heavy films \Then reduction time Jvould otherwise be sufficient for such products to reach the cathode. Electrolyte. A dilute hydrogen bromide solution (1 ml., 48%, in 5 liters of ivater) was found t o be superior t o a n y of a large number of other electrolytes investigated. This solution is without detectable solubilizing effect o n the oxide of test specimens held in i t for various times prior t o electrolysis. It gives better defined time-potential curves, and only with it do results correlate well with visual grades on oxide-discolored plate. The potassium chloridp solution used by most investigators frequently gave low and poorly reproducible oxide valucs on all but lightly coated, relatively new plates, principally because the film was undermined and sloughed off. A very dilute hydrochloric acid solution gave bettrr rmults, but values were still lower and more irregular than would be predicted by visual grading. Often plates given storage tests in humid atmospheres gradually increased oxide as measured in hydrochloric acid and then showed a decrease as time went on, although the visual grade steadily increased. Table I compares results in the tn-o halogen acids. It is essential that the electrolyte be deaerated by bubbling tank nitrogen through the stock solution which is siphoned directly to the cell. I t is unnecessary to flush the cell with nitrogen before filling. Circuit and Recorder. The circuit
CONTACT
Figure 1. films
Cell for measuring tin oxide
circuit t o adjust sensitivity, and to position the curve on the chart. A chart speed of 30 seconds per inch on paper with 10 squares per inch makes a convenient system for calculation of millicoulomb equivalents of oxide per square inch of plate (h4.C.E. per square inch) by inspection of the recorded curves, as indicated by the following calculations: M.C.E./sq. inch =
time (see.) X current (ma.) area (sq. in.) M.C.E./sq. inch = squares X 3 seconds X 1 = squares
H2 EVOLUTION
3
I20
Figure 2. Tracing of tin oxide reduction curve
Time-Potential Curves. T h e curve shoxTn in Figure 2 was produced from a plate showing light yellow discoloration equivalent t o 43 M.C.E. per s q . inch and is mounted as viewed on t h e recorder. Significant points are noted on t h e graph. The meaning of t h e peak in potential immediately after starting is not clear. It is not observed with slouer responding instruments, but does not appear to be caused by over-run due to high pen speed. Voltage figures on the horizontal axis are only approximate, as the voltage divider between the cell and recorder is not calibrated, and is adjusted in specific cases to give the best shaped curve The curves from both electrolytic and hot dipped plate are similar t o those produced by pure tin surfaces.
employed is not unusual. A regulated direct current power supply furnishes current t o t h e cell. Variable and fixed resistors in series with i t make possible adjustment of t h e current from 0.1 t o 2 ma. With t h e 4-squareinch disk (3-square-inch exposed surface) a constant current of 1 ma. was found t o be best. Voltage a t this current is 254 volts. High resistance (over 100 kohms) in series with the cell is necessary to prevent current fluctuation as the resistance of the film is decreased during reduction. A Leeds & Korthrup Speedomax 0-Bvolt potentiometer having a 2-second full scale pen speed is connected to the specimen cathode, and to the reference electrode, to record the time-potential curve. A voltage divider and a small bias potential are also included in this part of the
Amounts of Oxide Normally Encountered. Freshly produced electrolytic tin plate exhibits between 6 and 20 L1.C.E. per sq. inch of oxide, depending upon t h e surface treatment during manufacture. Tin plate cathodically cleaned in 0.5% sodium carbonate solution, rinsed quickly, dried. and run immediately ill s h o x 4 t o 6 M.C.E. per sq. inch formed by t h e exposure of moisture and air. As plate ages, t h e amount of oxide increases a t a rate depending on storage conditions, and the quality of the original film. Experience indicates that plates do not begin t o sho~vyellow discoloration visually until the film exceeds 30 to 40 1I.C.E. per sq. inch. Above 50 N.C.E. per sq. inch of oxide, enamel adhesion problems arise.
I
30
0
OXIDE REDUCTION LEVEL
,_I 1.1
POTENTIAL-VOLTS
15
VOL. 30, NO. 1 1, NOVEMBER 1958
1805
~~~
Table II.
Type of Plate No. 25 electro.
No. 25 electro.
No. 50 electro. Hot dipped Hot dipped, baked 30' at 390" C. Pure tin
Comparison
of Coulometric and Chemical Methods
Chemical Treatment Dichromate, electrcchemical Chromic acid immersion h'one None Kone Kone
CHEMICAL SOLUTION METHOD
The coulometric technique for measurement of oxide on tin plate described was developed during the period 1943 t o 1951, but until 1956 no method was found suitable for a check of its accuracy. The amalgamation system of Silverman and Gossen (IC) proposed for oxide on pure tin was found inapplicable to oxide on tin plate. Brummet and Hollweg (2) described a method for the determination of oxide on nickel by taking advantage of the difference in the solution rates of nickel oxide and the underlying metal in potassium cyanide solution. Tests on the behavior of a number of solutions on tin-tin oxide surfaces indicated that 0 . l M oxalic acid would quickly dissolve the oxide and more slowly attack the tin. -4cell was, therefore. constructed, in which immersion and reimmersion of the specimen in oxalic acid solutions was possible without contact with oxygen. The solution could be quickly removed or replaced by fresh material a t any desired time. The stock solution of oxalic acid was deaerated by bubbling with oxygenfree nitrogen, and nitrogen was also bubbled through the test cell continuously. Tin plate disks from the central area of a sheet n-ere exposed to four successive &minute treatments with the oxalic acid solution and each solution was analyzed for tin polarographically, by the method of Godar and Alexander (6). Figure 3 shows results for two tin plate surfaces and a pure tin surface. The first point is the average total tin found on three or more specimens. Succeeding points show tin dissolved after adding average increases per &minute reimmersion, for the same number of samples, to the previous average level. The vertical line indicates the spread in the values. Practically all of the oxide is dissolved during the first minute and extrapolation back to zero time gives the amount of oxide tin within the accuracy of the analysis. The slope of the tin solution
1806
0
ANALYTICAL CHEMISTRY
Plate Age, Months
Coulometric M.C.E./ Sq. Inch
Chemical hl.C.E./ Sq. Inch
12
6 1 0
6
18
28 i- 2
22
9 12
..
34 4 37 & 1 56 16
34 35 59
60
38 =t2
35
*
coupled with visual observation of nonuniformityof color on the baked material, indicates the spread reflects some spotto-spot variation, as well as degree of analytical reproducibility. Micrograms of tin per square inch of surface found by chemical solution were converted to millicoulomb equivalents, assuming stannous oxide to be the oxide. These data listed in the last column of Table I1 show R close agreement with the coulometric results, and are interpreted as a validation of the electrolytic reduction method. OTHER APPLICATIONS
35r
k
Besides application to commercial operating difficulties, the coulometric method has proved to be a useful tool in the collection of fundamental information on types and rates of oxide growth, stabilization of the oxide film, and effect of such films on container manufacture and performance. Specific areas of study have included surface films on tin plate produced by chemical and electrochemical treatments, storage factors influencing film growth, and influence of oxide films on solderability and corrosion-resistance of tin plate. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance given by R. W. Pilcher and L. P. Gotsch.
I 5
0
IO
15
20
MINUTES IN OXALIC ACID
Figure 3. Determination of tin oxide by chemical solution and analysis 1. 2. 3.
Low oxide on No. 25 electrolytic plate Pure tin
Medium oxide on hot dipped plate
rate line is influenced by the galvanic tin-steel couple. COMPARISON OF COULOMETRIC AND CHEMICAL SOLUTION METHODS
The two methods for tin oxide were compared by analysis made on adjacent disks taken in parallel rows of three each, near the center of 30 X 30 inch sheets of five lots of tin plate and one lot of pure tin. Because the chemical method ascertains total oxide on both sides of the sample, it was necessary t o run both sides of the plate separately by the coulometric procedure, thus involving six determinations on the three disks. The results are shown in Table 11. The spread in values is s h o w by the i figure, and the variation is greater on some plates than on others. This,
LITERATURE CITED
(1) Britton, S. C., Bright, K., Metallurgia 56, 163 (1957). (2) Brummet, B. D., Hollweg, R. hl., A x . 4 ~ CHEM. . 28 887 (1956). 13) Donelson, J. , Drivate communication, 1940. (4) Frankenthal, R. P., Butler, T. J., Davis, R. T., h A L . CHEM. 30, 441 (1958). ( 5 ) Godar, E. M.,Alexander, 0. R., IND. ENQ. CHEM., ANAL. ED. 18, 681 (1946). ( 6 ) Katz, W., Stahl u. Eisen 76, 1672 (1956). ( i ) Kerr, R., J . Soc. Chem. Znd. 57, 405 (1938). (8) Kerr, R., hIacNaughton, D. J.,
6..
Intern. Tin Research Develop. Council Tech. Pub. A, No. 48 (1937 j. (9) Milev, H. A., Iron SteelInst. Curnegie
Schol. &em. 25, 197 (1936). (10) Miley, H. A., J . Am. Chem. SOC.59,
2626 (1937). (11) Miley, H. A,, Evans, U. R., J . Chem. SOC.1295 (1937). (12) Price, L. E., Thomas, G. J., Trans. Electrochem. SOC.76,329 (1939). (13) Salt, F. W., Thomas, J. G. S., Nature 178, 434 (1956). (14) Silverman, L., Gossen, W., Anal. Chim. Acta 8,436 (1953).
RECEIVED for review October 25, 1957. -4ccepted J u n e 5, 1958.
