Rapid Detection of Chromium In Stainless Steels, Other High-Chromium Alloys, and Plating JOSEPH A. CALARZARI, Laboratory, Medical Section, New York General Depot, Brooklyn, N. Y.
C
HROMIUM in high-chromium alloys and plating may
tensity and the duration of the spot increase with the chromium content of the specimen.
be detected rapidly by excellent chemical methods, utilizing the s-diphenylcarbazide reaction with chromate ion (2-4, 7). The rapidity of the tests is limited, however, by the time required for solution in acids, oxidation to chromate ion, and the removal of interfering ions.
Apparatus and Test Solutions
An expeditious electrographic test for chromium Tlating is described by Arnold (I) and by Glazunov and Ki-ivo ahv9 (6), in which the chromium is dissolved, anodically, as chromic acid by means of an electric current. A characteristic spot is formed on filter paper moistened with a solution of an electrolyte and benzidine or s-diphenylcarbazide, a t the surface adjacent to the plating where the current enters the paper. The test has been applied to the detection of chromium in steel and cast iron (6, IO). Two electrographic tests for the detection of chromium in high-chromium alloys and in plating, employing a characteristic reaction of chromium-namely, the formation of blue perchromic acid-have been developed by the author. Elements, such as molybdenum, manganese, and mercury, which interfere with the s-diphenylcarbazide test, do not produce interfering colors. The tests may be performed in approximately one second, with little effect upon the surface examined. One of the tests responds to chromium in plating only. The second test responds to chromium in high alloys and in plating. Since the initial sensitivity of the latter test occurs near the minimum chromium content of the stainless steel group, it is particularly useful in the identification of high-ferrous alloys. ANODICSOLUTION OF ACTIVE AND PASSIVE CHROMIUM ALLOYBAKD PLATING.Polyvalent elements in the active state always dissolve anodically to form ions of the lowest valence, while anodes of the polyvalent elements which become passive, enter solution as ions of a higher valence than when in the active state. Active chromium yields C r + + upon anodic solution, while passive chromium yields CrOl-(8, 9). The author has found t h a t the chromium in active and passive high-chromium iron and nickel alloys and in plating behaves in the same manner and t h a t the high-chromium alloys can be made active or passive by the influence of chloride or nitrate ions, respectively. Chromium in plating remains passive in the presence of either anion, under the conditions of the test, and dissolves as CrO4--. The iron and chromium components of the stainless steels in the active state enter solution as F e + + and C r + + and, in the passive state, as F e + + + and CrO4--. An e. m. f. of 6 to 9 volts, a current density of 0.5 to 1.5 amperes per sq. cm. of anode, and 30 per cent solutions of sodium chloride and sodium nitrate have been employed in the tests. FORMATION OF BLUEPERCHROMIC ACID. When chromate ion enters a solution containing hydrogen peroxide, it reacts with the peroxide ion to form blue perchromic acid in the acidic medium adjacent to the anode. By replacing the solution with filter paper moistened with the solution, the sensitivity of the test is increased as the diffusion of the perchromic acid is retarded. The v a d of moistened filter paper is placed between the anode and the cathode in such a manner as to permit the flow of electric current through the paper. When chromate ion is formed upon solution of the anode, a purpleblue spot is found on the side of the paper adjacent to the anode. The spot begins to fade, after a few seconds, a s the perchromic acid commences to decompose. The initial in19
The apparatus consists of six large 1.5-volt dry-cell batteries of the bell-ringing type connected in series, a cylindrical graphite rod about 7.5 cm. (3 inches) lon and 0.6 cm. (0.25 inch) in diameter, two lengths of insulated ffexible copper wire with suitable connectors, and “ashless” quantitative filter paper, 7.5 to 10 cm. (3 to 4 inches) in diameter. The graphite rod is made the negative electrode and the test metal, the anode. Filter paper moistened with solution, placed between the graphite rod and the test metal anode, completes the electric circuit. TESTSOLUTION A (for chromium plating). Dissolve 30 grams of reagent grade sodium chloride in sufficient 3 per cent hydrogen peroxide solution, U. S. P., to make 100 ml. and filter. TESTSOLUTION B (for chromium in plating and high-chromium alloys). Dissolve 30 grams of sodium nitrate, reagent grade, in a quantity of 3 per cent hydrogen peroxide solution, U. S. P., sufficient to make 100 ml. and filter. The test solutions should be slightly acid. Hydrogen peroxide solution, U. S. P., usually contains enough mineral acid to make a suitable test solution; 0.8 ml. of 1 N nitric or hydrochloric acid should be added if a neutral hydrogen peroxide solution is used in preparing the reagent. The test solutions made in 100-ml. quantities have remained useful for periods of more than one month. Solutions older than 2 weeks, however, should be checked frequently. Test solution A is the least stable.
Method A sheet of filter pa er is folded in half and the foldin is continued in the same Jrection until a wad, approximateyy 0.175 cm. (0.7 inch) wide and having a length equal to the diameter of the pa er, is made. One end of the wad is dipped into one of the test soyutions and the moistened section is applied to the bright surface of the metal specimen (anode). A momentary contact is made with the graphite cathode to the free side of the moistened end of the aper wad. The paper adjacent to the metal surface is examinel. A purple-blue spot obtained with test solution B indicates that chromium is present as plating or as a component of a high alloy. If the purple-blue perchromate spot is obtained with test solution A, the chromium is present as plating only. An e. m. f. of 5 to 6 volts has been found to be the most suitable when testing for plating and an e. m. f. of 7.5 to 9, when testing for alloyed chromium. A firm contact is made with the graphite electrode, lasting from 0.5 to 1 second. The pressure and duration of the contact are easily determined by experimenting with a specimen of stainless steel or chromium plating. The technique is acquired in a few minutes and no further reference to known metals need be made. If the blue perchromate color appears as a ring, too long a contact is being made or too high a voltage is being employed. The color of the spot is best observed in daylight, since the light from an ordinary incandescent lamp is relatively deficient in the blue end of the spectrum. After a few seconds the spot begins to fade, since the perchromic acid is unstable, but the time of duration is more than sufficient for purposes of identification. The initial temperature of the solution and the anode should preferably be below 30’ C., since higher temperatures cause a more rapid decomposition of the perchromic acid and, hence, a more weakly colored spot.
Discussion A positive test for chromium has been obtained using test solution B with ferrous, nickel, and cobalt-tungsten alloys containing more than 12 per cent chromium and with chromium plating. A series of ferrous alloys ranging from 0.6 to 26.5 per cent chromium which have been tested with test solution B has shown t h a t the point of initial sensitivity of the test lies between 9.75 and 12.38 per cent chromium and is probably closer to the lower value.
20
INDUSTRIAL AND ENGINEERING CHEMISTRY
Test solution A has given a positive test with chromium metal and chromium plating and a negative test with the stainless steels, stainless irons, and the chromium-nickel alloys. A chromium-cobalt-tungsten alloy with a chromium content of 25 to 31 per cent is the only alloy which has given a positive result with both test solutions, the reason being that this alloy remains passive with either test solution and the chromium dissolves as Cr04--. Numerous other metals and alloys have been tested with both solutions, and negative results have been obtained in each case: nickel-silver, solders, brasses, white metals, 15 per cent silicon steel, bronzes, copper-nickel alloys, carbon steels, nickel, copper, manganese, molybdenum, tantalum, tungsten, mercury, cadmium, aluminum, tin, zinc, vanadium, silver,
Vol. 13, No. 1
and lead. Vanadium gives a red spot with both solutions and silver yields a black spot with test solution B.
Literature Cited (1) Arnold, E., Chem. Listy, 27, 73 (1933). ( 2 ) Chazeneuve, P., Compt. Tend., 131, 346 (1900). (3) Feigl, F., 2.angew. Chem., 44, 739 (1931). (4) Feigl,
F., and Krumholz, P., Mikrochem., Pregl-Festschr., 77
(1929)
( 5 ) Glazunov, A,, and Kiholahv$, J., Chem. Obzor, 8, 176 (1933). Glarunov, A., and KEivolahv$, J., paper presented at International Founding Congress, Prague, 1933. (7) Heller, K., and Krumholr, P., Mikrochemie, 7 , 213 (1929). (8) Hittorf, W., 2.physik. Chem., 30,481 (1899). (9) Ibid., 34, 395 (1900). (10) Jirkovsky, R., Mikrochemie, 15, 341 (1934). (6)
Determination of Cobalt as Trioxalatocobaltiate G. H. CARTLEDGE
AND
PARKS RI. NICHOLS', University of Buffalo, Buffalo, N. Y.
I
N CONNECTIOX with the work on complex compounds in this laboratory t'he authors have developed a new spectrophotometric method of determining cobalt which is much more rapid than the gravimetric or electrolytic procedures. Numerous colorimetric methods are available (S), but certain of these require time-consuming separations, others are applicable only to minute amounts of cobalt, and others have only fair accuracy. The basis of this procedure is the measurement of the absorption a t 605 m p due to the trioxalatocobaltiate ion, Co(CzO&---, which is produced by oxidation of a cobaltous solution by lead dioxide in a weakly according to the reacacid solution of potassium oxalate (4, tion
+
++
++
2COSOa 4HCzHaOz 7KzCzOr PbOp 2KsCo(Cz04)s PbCz04 4KCzHaOz
+ 2KzSO4 + 2H20
A small amount of lead remains in solution, presumably as acetate. The trioxalatocobaltiate anion produced has a dark emerald-green color with a maximum absorption at 605 m p ( 5 ) . The intensity of the absorption is used as a measure of the cobalt content. A Bausch & Lomb visual spectrophotometer was used for estimating the absorption, and with a maximum cell length of 5 cm. the solutions actually measured may contain from 2 to 50 mg. of cobalt per 100 ml. Since the volume of the final solution need not exceed 50 ml., the method may be applied to any sample or aliquot containing as much as 1 nig. of cobalt. Xickel, iron, and chromium may be present in considerable amounts; copper and manganese must be previously removed, if present. Blthough the authors have used a spectrophotometric procedure requiring no standard solutions for comparison, the same method could be adapted for use with a photoelectric colorimeter with a suitable red filter. The analysis can be completed in about 15 minutes from the time the aliquot of cobaltous salt solution is measured out. The accuracy is better than 2 per cent of the quantity determined. It is necessary to have a standard solution of the pure salt to determine the extinction coefficient a t the wave length selected for measurement of the absorpt'ion. The preparation of a standard solution of potassium trioxalatocobaltiate is difficult, however, since the n-ater content of the crystalline salt is very variable. Furthermore, the complex is not entirely stable, so that' its purity needs to be established. K h e n the salt is prepared by a modification of the procedure of Jaeger and Thomas ( 2 ) its formula approximates closely t'o 1
Prfsect addrees, Lewiston, N. T.
K3Co(CL0&. 3.5H20, but the water content varies with the atmospheric humidity from day to day. T'ranek (5) measured the absorption of trioxalatocobaltiate solutions. H e standardized his solutions by electrolytic determination of total cobalt, however, which of course would give an erroneous extinction coefficient if the salt had decomposed to a cobaltous compound to an appreciable extent. Since he gave no statement as to the history of his material, the authors redetermined the absorption a t 605 m p with solutions standardized by a procedure which they developed for estimating the complex cobaltic ion in the presence of cobaltous ions. Volumetric Determination of Trivalent-Cobalt Complex I n acid solution, potassium iodide does not reduce the trioxalatocobaltiate ion rapidly; ferrous sulfate reacts much more readily, but the formation of the greenish-yellow trioxalatoferriate ion in the solution obscures the end point. The authors found that ferrous sulfate may be used in an electrometric titration of the cobaltic ion, though the electrode is very sluggish until the end point is passed. The potential changes by about 150 millivolts a t the end point. I n the direct tit'ration with Mohr's salt the end point may be seen much more clearly if potassium fluoride is added near the end, but this is objectionable. Even so, the end point is reached slowly a t room temperature, and the authors therefore abandoned the attempt to make a direct titration of t rivalelit cobalt. The procedure finally developed consists in titrating the cobaltic solution with approximately 0.05 N Nohr's salt (20 grams per liter in 2 per cent, sulfuric acid) a t room temperature until the change from green to yellow shows that t'he end point has been slightly overstepped. The excess of ferrous ion is then titrated with standard potassium dichromate, using diphenylamine as the indicator. The back-titration of the excess ferrous salt is complicated by the induced reaction between dichromate and oxalic acid, however, so that i t is necessary to determine an empirical factor for the dichromate solution in each analysis. The solution of the trioxalato salt to be analyzed is prepared
so as to be nominally about 0.05 -11. To an aliquot (10 to 25 ml.),
add from a buret standard ferrous sulfate until about 0.5 ml. in excess is present. Let the mixture stand for 5 minutes, when the color change should be complete. Then add 1 ml. of a solution containing 10 ml. of phosphoric acid and 25 ml. of sulfuric acid per 100 ml., and dilute to about 75 ml. Add 3 drops of diphenylamine solution (1 gram per 100 ml. of sulfuric acid) and titrate with standard 0.05 N potassium dichromate. Not over 1.0 ml.
