ANALYTICAL EDITION
DECEMBER 15, 1937
When two asphalts are within one-half a stain number they must be considered as having the same staining propensity, since this is the limit of accuracy of the test as determined by duplicate runs.
Acknowledgment The authors wish to acknowledge the assistanceof R. N. Traxler, P. R. Smith, and R. E. Thurn in preparing the manuscript, and the cooperation of Peter J. Schweitzer, Inc.
565
Literature Cited (1) Proc. Am. Soc. Testing Materials, Part I, 933 (1936); Tentative Method of Test D202-36T. (2) Schweyer, H. E., Coombs. C. E., and Traxler. R. N.. Ibid.. 36, 11, 531-43 (1936). (3) Traxler, R. N., Chem. Rev., 19, 119-43 (1936). (4) Traxler, R. N.7 and Coombs, c. E., J. PhUs. Chem., 40, 1133-47 (1936).
RH~CEIVED August 13, 1937.
An Electrometric Method for the Determination of Silver H. ROBINSON AND H. HUGG, Tiffany & Co., Newark, N. J.
I
N THE potentiometric determination of silver, using potassium iodide as the precipitant, the end-point error due to adsorption of the reagent by the precipitated silver iodide has been eliminated by the method of Lange and Berger (1). However, the length of time required renders the method unsuitable for industrial analysis. The method described here eliminates this error, and is fast and accurate. The silver, in solution as nitrate, is titrated with potassium iodide, using a pair of electrodes which are connected to a microammeter. One electrode is gold, the other is carbon saturated with nitric acid. The first excess of potassium iodide generates sufficient current through oxidation by the nitric acid-carbon electrode to read directly on the microammeter, neither amplifier nor balancing circuit being required.
Apparatus and Reagents _ _
A microammeter with a range of 0 to 100 microamperes is required, pyeferably with a zero center scale.
FIGURE 1. CARBON ELECTRODE
The gold electrode may be of any convenient size. The one used by the authors is 2 mm. square by 100 mm. long. Larger gold electrodes allow a larger current flow a t the end point, but one of shorter duration. The metal should be well annealed, and cleaned by rubbing with moist pumice powder. The carbon electrode may consist of a piece of coreless arclight carbon 12 mm. in diameter. However, if much work is to be done by this method, an electrode may be constructed as illustrated in Figure 1. The carbon electrode is saturated with nitric acid which has been treated with nitrogen peroxide, made by passing nitric oxide through concentrated nitric acid in an open vessel. Potassium iodide (0.1 N ) is used for the titration.
Procedure The carbon electrode, if solid, should be allowed to soak in nitric acid containing dissolved oxides of nitrogen for about 5 minutes. If the hollow electrode is used, the stopcock from the nitric acid reservoir is opened until the electrode is saturated. I n either case, the electrode is prepared for 20 sterling silver assays without further treatment with nitric acid. The carbon electrode may be saturated with ammonium persulfate instead of nitric acid. Crystallization of the salt renders it less desirable. The electrodes are connected directly to the microammeter, the carbon electrode being positive a t the end point. One gram of the silver alloy is weighed and placed in a 250-cc. beaker with 5 cc. of 1to 1nitric acid, and heated until the metal has dissolved and oxides of nitrogen have been expelled. An excessive amount of nitric acid must be avoided. After cooling, the solution is diluted to 120 cc. and placed under the electrodes. At first a momentary current of lOOpa is generated, the meter rapidly returning t o zero and remaining there. The mechanical stirrer is started and the titration is begun by allowing the 0.1 N potassium iodide solution to run at the rate of 0.5 cc. per second into the beaker. The microammeter will not start t o fluctuate until the titration has proceeded t o within 1cc. of the end point. At this stage, the addition of each drop will cause the needle t o fluctuate, but after each fluctuation it will return to zero, until 0.05 cc. of 0.1 N potassium iodide solution in excess has been added. At this point the meter will indicate about 40pa. This flow of current will be constant for 4 minutes with the gold electrode 2 mm. square immersed 50 mm. in the solution. An excess of 0.1 cc. will generate 120pu, increasing with added potassium iodide until at 0.7 CC. excess the current generated is 1000pa. Further addition of the precipitant will produce increasing current flow, but with decreasing gain in proportion to reagent added.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Interfering Elements
Gravimetric Chloride
VOL. 9, NO. 12 Eleotrometric Method
%
%
92 99 93 01 93 00
93 00 93 04 93 03
Lead, copper, cadmium, nickel, cobalt, and ainc in concentrations as high as 55 per cent have been found not to interfere with the end-point potential nor the accuracy of the method. Suspended metastannic acid, and traces of antimOnY, aluminum, and colloidal gold have no effect- The Presence of iron does not lower the accuracy 01 sensitivity, but in solution as ferric nitrate it will cause a current flow in the same direction as that generated a t the end point, and proportional to the amount of iron present. However, when iron is present the needle of the microammeter may be returned to zero by a balancing current and the titration carried out as usual. Palladium must be absent, as this element is precipitated as iodide in weak acid solutions and is precipitated completely before the end point. The presence of sulfates renders the end point indefinite, If the sample consists of sulfates or if the sample has been dissolved in concentrated sulfuric acid, the carbon electrode must be saturated with 0.1 N ceric nitrate. Results will be satisfactory if the concentration of sulfuric acid is about 2 N .
A new method for the determination of silver incorporates the following features: It is accurate, precise, and rapid. With the exception of palladium none of the usual elements Present in silver alloys interferes. No amplifier, balancing circuit, nor reference cell is required. (In the exceptional Case where iron is Present, the balancing Circuit must be used.)
Accuracy
Literature Cited
by this method were checked by the gravimetric chloride method, using 1-gram samples of sterling silver. Some typical results are given.
The
Obtained
The method is accurate up to 500 c, Above this temperature the rapid oxidation produces violent fluctuation of the microammeter a t the end point, with consequent shortening of the duration of the currentflow a t the end point, At temperatures below 250 c. the end-point reading of 4oPa will remain for 4 minutes.
Summary
(1) Lange, E., and Berger, R., 2. Elektrochem., 36,980 (1930). RECEIVED September 27, 1937.
Determination of Carbon and Hydrogen Content by Combustion MARY W. RENOLL, THOMAS MIDGLEY, JR., AND ALBERT L. HENNE Ohio State University, Columbus, Ohio
T
HE purpose of the present paper is to show that carbon
and hydrogen determinations by combustion can be performed with a degree of accuracy considerably higher than that which is generally obtained. These improved results are obtained by slight modifications of the ordinary combustion setup, and above all by adequate technic in the burning of the sample.
Apparatus The equipment used in this laboratory (see Figure 1) follows the design of that described by Smith, Taylor, and Wing (3). It has no rubber connections, only three ground joints, and is made entirely of glass. The purifying train consists of tubes sealed in series and containing, 1, copper oxide heated by an external electric resistance; 2, concentrated sulfuric acid acting as a bubble counter; 3, Ascarite; 4,Dehydrite; and 5, phosphorus pentoxide. This train ends in a male half of a 20-mm. standard Pyrex ground-glass joint, 6, and it is mounted on a board to permit its connection to the combustion tube by sliding it in place as one unit. The air which it purifies is received through a calcium chloride tower from a 20-ljter bottle from which it is displaced by water. Up to the purifying train, rubber connections are used, but not beyond it. The combustion h b e , 8, is of the shape described by Smith (3) but is of Jena Supremax glass (obtained from the Fish-Schurman Co., New York, N. Y.). It contains plain copper oxide held between two rolls of copper gauze. The reason for this change is that combustions are always better regulated with air than with oxygen; however, when air is used, a higher temperature is needed to burn completely the high molecular weight
materials used in this laboratory. At the high temperature needed, the life of a Pyrex tube is only one or two combustions, and it frequently adheres t6 the heating units, thereby ruining them. (New Pyrex glass No. 172 has recently become available; its specifications are very close to those of Jena Supremax, but the authors have not tried it.) The Jena glass is sealed by means of a ring of uranium glass, 7, to the female half of the Pyrex joint, 6, connecting to the purifying train on the one side and to the Ushaped water-condensing- tube, 9. meceding- the water absorber, 10,-on the other. The absorbers are light tubes containing Dehydrite and phosphorus pentoxide, 10, to absorb the water. and Ascarite with a little Dehydrite and phosphorus pentoxide, 11, to absorb the carbon dioxide. They are connected by small ground-glass joints, coated with vaseline, which is carefully removed with ether before weighing; the body of the absorber is washed with ether and wiped with a clean rag before each weighing.
FIQURE1.
DIAQRAM OF APPARATUS
