Colorimetric Method for the Determination of Cobalt in Stainless Steel HARRY RI. PUTSCHE AND W. FRANCIS MALOOLY, Research hboratories, Rustless Iron and Steel Division, The American Rolling Mill Company, Baltimore, Md.
A colorimetric method is described for the determination of cobalt in stainless steel employing Vogel’s reaction. The steel is obtained in a slightly acid medium and a single zinc oxide separation is made. A n aliquot portion of the solution is reduced with sulfurous acid, and a strong sodium thiocyanate solution is added. After dilution to a definite volume, a measured amount of acetone is added, and the intensity of the blue complex, Na&o(SCK),, is measured by means of a photoelectric colorimeter. The method is applicable to all cobalt-bearing steels as well as to numerous other alloys.
A
*
cobaltous nitrate was precipitated once as potassium cobaltinitrite. The potassium cobaltinitrite was decomposed with nitric acid and the solution was fumed with sulfuric acid, then standardized by precipitating the cobalt with a-nitroso-@-naphtho1 and igniting to ColO,. The strength of the solution wa8 adjusted to equal 0.001 gram of cobalt per milliliter.
LITERATURE survey failed t o reveal the publication of a colorimetric chemical procedure for the quantitative determination of cobalt in stainless steel employing Vogel’s reaction. The usual method of determination requires anitroso-0-naphthol as a precipitating agent, and the process is completed gravimetrically. Although this is an umpire method, it is cumbersome, costly, and time-consuming. About 1873 H. W. Vogel discovered what is now known as Vogel’s reaction. Theoretically, this reaction forms a blue complex, sodium cobaltothiocyanate, KanCo(SCN)r, when sodium thiocyanate is added to a cobaltous solution. Upon dilution it turns pink. Amyl alcohol, amyl alcohol and ether, acetone, or ethyl acetate imparts a blue color to the cobaltothiocyanate. Kolthoff (6) and Tomula ( 7 ) have substantiated these facts and advocated a strong thiocyanate solution and equal amounts of acetone and test solution. Kickel does not form a similar complex. Kolthoff also suggests that the cupric ions be removed. Ditz (1) and Feigl (3) have found acetone to be the most sensitive of the group that imparts the color to the cobalt complex. Dorrington and Ward (t?) suggest the use of potassium isocyanate to avoid the interference of ferric ions. In the procedure recommended, a zinc oxide separation is made to remove iron and other interfering elements. If this separatiQn is made with cold solutions, little if any cobalt will be precipitated according to Heymans (4). However, Hoffman (6) indicates that temperature variations of the solution in which a zinc oxide separation is made have little influence on the amount of cobalt retained by the precipitate. He further concludes that results for cobalt tend to be low if only a single zinc oxide precipitation is made. Young and Hall (9) recommend tribasic sodium phosphate as a precipitating agent for iron. A variation of the original reaction is the use of a hydrochloric acid solution saturated with ammonium thiocyanate, and the removal of the ferric thiocyanate with stannous chloride as suggested by Vorontzov (8).
APPARATUS
h Klett-Summerson hotoelectric colorimeter to measure thy
color densities of the soktions. Corning red glass filter KO.2424, 3.51 mm. thick, and Corning blue glass filter No. 4308, 3.08 mm. thick. RECOMMENDED PROCEDURE
Transfer 0.2500 gram of the sample to a 250-ml. Erlenmeyer flask, and add 5.0 ml. of hydrochloric acid, 5.0 ml. of nitric acid, and 2 drops of hydrofluoric acid. Heat, gently until the steel is in solution, then boil for several minutes. Cool somewhat, add 10 ml. of perchloric acid, and heat until all the chromium is oxidized to chromic acid and fumes of perchloric acid are freely liberated. Cool, dilute with 25 ml. of distilled water, and add dilute ammonium hydroxide (1 to 1) until the solution is nearly neutral. Cool. Make a zinc oxide separation in the conventional manner (cf. Hoffman, 5 ) by adding the chilled solution to a 250-ml. volumetric flask and introducing zinc oxide, suspended in water, until precipitation is complete. Dilute to the mark at the proper temperature, return the solution to its original flask, and allow to settle. Carefully pipet or Glter off a 25.0-ml. aliquot and transfer to a 125-ml. Erlenmeyer flask. Add 5 ml of sulfurous acid. Heat to boiling and boil off the excess sulfurous acid. Cool. To a 50-ml. volumetric flask add 15 ml. of 4070 sodium thiocyanate solution. Add the chilled sample quantitatively t o the volumetric flask and dilute to the mark with water. Mix thoroughly. Insert the proper glass filters in the colorimeter. Set the dial of the colorimeter to read 20 and set the galvanometer to 0 (using a colorimeter tube that is.graduated into 5 and 10 ml. and contains distilled water). T h ~ elirmnates s using the extreme lower region of the scale during the sample readings. Rinse the tube severd times with the test solution. Add exactly 5.0 ml. of the solution to the tube, adjusting the volume with a capillary tube if necessary. Dilute to exactly 10.0 ml. with distilled water and mix thoroughly. Measure the color density of the ions present with the colorimeter and record the reading, A . Discard the solution in the tube and again rinse the tube with the test solution. Once again add 5.0 ml. of the test solution to the tube, but this time dilute to exactly 10.0 ml. with acetone and mix thoroughly. The blue cobalt complex. sodium cobaltothiocyanate will form. Allow the mixture t o age not less than 5 minutes and not more than 10 minutes. Read the color density due to the cobalt complex present, and record the reading, B.
REAGENTS AND STANDARD SOLUTIONS
Hydrochloric acid (specific gravity l.lQ), c.P., analytical grade. Nitric acid (specific gravity 1.42), c.P., analytical grade. Hydrofluoric acid (+%), c.P., analytical grade. Ammomum hydroxide, 1 to 1. Zinc oxide. Prepare a sufficient quantity of powdered zinc oxide suspended in distilled water to make a free-flowing suspension. Sulfurous acid. Distilled water freshly saturated with sulfur dioxide gas. Sodium thiocyanate (40%). Forty grams of reagent dissolved in a small amount of distilled water and diluted t o 100 ml. Acetone, reagent quality. Standard cobalt, solution. The cobalt in a C.P. grade of
Periodically, blanks should be determined to ascertain the color density of the solutions used. Proper blanks should be deducted from both the initial and final readings. Correctior blank- may be determined as follows: 236
V O L U M E 19, NO. 4, A P R I L 1 9 4 7 Table I.
237
Comparison between Certified Cobalt Values and Colorimetric Results
Sample
Type
Certificate Cobalt Value
%
78
N.B.S. 55a N.B.S. 55a N.B.S. 73a N.B.S. 73a N.B.S. 111 N.B.S. 133 N.B.S. lOlb N.B.S. lOlb N.B.S. lOlb N.B.S. 121 N.B.S. 126 N.B.S. 153
Ingot iron 0.008 Ingot iron 0.008 14 Cr steel 14 Cr steel ... S.A.E. 4615 ... 14 Cr-0.6 Mo steel 0 : 078 18 Cr-9 Ni steel 0.078 18 Cr-9 Ni steel 0.078 18 Cr-9 Ni steel 18 Cr-9 Ni-Ti steel 0.08 0,008 36 Xi steel 8.45 Mo-Co steel
...
Total Cobalt Present
Cobalt Added
l:oo 2:io 5.00 4.00 3160 6.00
1:oo
..
Cobalt Found Colorimetrically
%
%
0.008 1.008 0.000 2.50 5.00 4.00 0.078 3,578 6,078 0.08 1,008 8.45
0.008 1.00 0.000 2.50 5.02 4.04 0.08 3.60 6.08 0.08 0.99 8.43
Table 11. Comparison between Gravimetric and Colorimetric Cobalt Results
Sample
1 2 3
Cobalt Found Gravimetrically
Type of Alloy 23 Cr-8 Ni-6 Co-1 &io-1 Cb 22 Cr-12 Ni-3 Co-2 Mo 21.5 Cr-19.5 Ki-19.5 Co-3 Mo-2 W-1
Cobalt Found Colorimetrically
%
%
5.73 3.02
6.70 3.05
19.56 18.86 21.96 28.70 44.24 19.69 25.28 25.41
19.5 18.8 22.0 28.7 44.4 19.6 25.3 25.5
9.92 12.25 20.31
9.90 12.20 20.4
AI
10.00 45.00 9.42
10.10 25.0 9.40
AI
10.01 30.42 3.42 0.96 39.39 3.42 20.99
10.00 30.5 3.44 0.96 39.2 3.42 21 .o
0.18
0.18
4
5
6 7
8
9 10 11 12 13 14
18Zr-8 Ni-12 co-3 Mo-3 0 - 0 . 3 Ti 17 Cr-20 Ni-20 Co-1 Mo-3 A1-0.5 Ti 17 Cr-18 Ni-10 Co-4 Mo-2 F - 2 Ti-0.5
15 16 17
17 Cr-15 Ni-25 co-5 M o 17 Cr-15 Ni-9.5 co-4 Mo-2 W-2 Cb 17 (31-15 Xi-10 CO-4 Mo-2 W-2 Ti-0.5
18 19 20 21 22 23 24
-
Add 15 ml. of the 40% sodium thiocyanate solution to a 50-ml. volumetric flask and dilute to the mark with distilled water. Use this just as the test solution is used in the procedure. Setting the colorimeter to 20 as described above, take two readings: (a) one diluting 5.0 ml. of the solution t o 10.0 ml. with water, and (a) one diluting 5.0 ml. of the solution t o 10.0 ml. with acetone. Subtract 20 from each of the readings. These two readings are considered as blanks. Blanks a and b must be deducted from readings A and B, respectively, in the procedure. Calculations. Subtract corrected reading A from corrected reading B and consider the result as the difference reading. The percentage of cobalt may be determined by: 1. Applying the difference reading directly to a calibration curve obtained by plotting the difference readings vs. percentages of cobalt; or 2. Multiplying the differencereading by an established factor, dividing by the weight, and multiplying by 100. Sample Calculation. [(Reading B
- blank b) -
(reading A weight of sample
- blank a ) ] x
factor
Although the removal of iron is recommended, the authors have had success in employing a procedure in which all the iron IS retained. Sulfurous acid reduces iron to a near colorless state and selective filters are used in the colorimeter. Both these factors tend to render iron and its red complex harmless. Kevertheless, it was found beneficial t o precipitate this element, since it made the acetone solution ’of the sample less stable. The removal of cupric ion is recommended. I n this procedure the oxidized copper is removed practically quantitatively by the zinc oxide separation, thus eliminating an extra step to prevent the interference of copper. Sulfurous acid is added primarily to reduce the cobalt to the cobaltous state; but it also reduces any trace of iron, chromium etc. The excess is boiled off to ensure a near-neutral condition of the solution. .1 strong sodium thiocyanate solution is recommended. 11) this procedure the amount of thiocyanate is not too critical, providing that it is not less than the amount stated in the method The method uses 15 ml. of o 40% solution of sodium thiocyanate when diluting the sample to 50 ml. before taking the colorimetric readings. However, successful determinations were obtained when 10 ml. or even as high as 40 ml. were used. .i minimum aging time of 5 minutes is recommended for the acetone solution of the sample, since it drops a point or 80 o n the colorimeter scale in the first 5 minutes. After this time has elapsed, it is stable. Beer’s law is obeyed, since a plot of the color density of the solution against its concentration givep s -traight line. If the cobalt is present in the sample only in a residual amount, a more accurate determination may be made by increasing the size of the sample, either by starting with a larger sample-for example, 1.0 gram-or following the procedure, using an equivalent aliquot (100 ml. instead of 25 ml.), and boiling it down tc, the proper volume. Conversely, a sample containing a very high percentage of eohalt may be analyzed by using a smaller sample than is recommended in the procedure, either decreasing the size of the sample or increasing the volume and/or decreasing the size of aliquot in the procedure. Best results were obtained when the solution, read in the colorimeter, had a limiting concentration of 16 micrograms of cobalt per ml. per 1.3-cm. cell depth. The analyst may stay m-ithin this limit by proper dilutions and aliquots as explained above. \$’hen dilutions and/or aliquots other than those mentioned 11, the procedure are used, the calculations may be based on a factor for such dilutions and aliquots or, more simply, by using the same factor throughout and substituting a “corrected weight of sample”. For instance, if a dilution of 500 ml. were used instead of 250 ml., it would be the same as submitting an 0.125-gram sample to the proper dilution. Likewise, if a 5.0-ml. aliquot were taken instead of 25.8 ml., it would be the same as taking the proper aliquot when an 0.05-gram sample was weighed, etc The whole procedure should not take more than one hour. h working curve or factor may be obtained by submitting various steels, of definitely established cobalt values, to the recommended procedure. The pure standard cobalt solution alone or various amounts of the standard cobalt solution added to steels of nil or known cobalt content may also be 100 = % Co used as the sample. When various readings are obtained for known cobalt percentages, it is a simple matter to draw a working curve or calculate a factor. Table I shows a comparison between certified and colorimetric values. A comparison between gravimetric and colorimetric replilts is shown in Table 11.
X
[(160.5 - 14) - (26 - 5 ) ] X 0.00006115 X 100 = 3.09% Co 0.25 THE COLOR REACTION
To prevent iron from forming the red ferric thiocyanate complex, it is best to remove this element by an ether separation cupferron, or, more easily as the method suggests, a zinc oxide separation.
SENSITIVITY, ACCURACY, AND PRECISION
Sensitivity. Each scale division of the colorimeter is equal t o approximately 0.04% cobalt when a 0.25-gram sample is used
.
238
ANALYTICAL CHEMISTRY
Accuracy. When various amounts of standard cobalt solution were added to numerous types of stainless steel, etc. (cf. Table I), results had a maximum deviation of *0.04% cobalt from the total amount present. On high-cobalt steels (20 to 6OyG), the sensitivity and accuracy necessary are decreased somewhat, since smaller samples and aliquote must be used. Precision. If the method is carefully applied, the difference readings should not vary more than one scale division. As a check on the reproducibility of the procedure, one 60 co-30 Cr6 &Io-3 Xi alloy was chosen for pxperimentation. Five individually weighed 0.1000-gram samples, when separately analyzed by an analyst familiar with the procedure, gave identical readings throughout. As a further check on the precision, three analysts who had no previous experience with the method obtained the following rwilts on four “unknown” samples: Cobalt Found Gravimetrically
% 3.00 8.43 12.25 20.31
Analyst A
Cobalt Found Analyst B
4PPLICATION
The method as written is applicable to any known type of stainless steel, ingot iron, mild steels, numerous ferrous alloys, and probably many nonferrous alloys and other materials not yet investigated. The procedure has been used successfully on stainless steels containing from 0.008 to as high as 60.0% cobalt. Experiments have proved that there are few, if any, interfering elements. Alloys investigated contained large amounts of iron, chromium, nickel, manganese, copper, titanium, carbon, aluminum, silicon, molybdenum, vanadium, tungsten, columbium, selenium, and numerous residual elements (cf. Table 11). ACKNOWLEDGMENT
The authors are deeply indebted to the Rristless Iron and Steel Division of The American Rolling Mill Company, Raltimore, hld., for permission t o publish this paper. LITERATURE CITED
Analyst, C
%
%
%
2.99 2.95 8.45 8.40 12.2 12.1 20.4 20.4
3.03 2.99 8.46 8.30 12.2 12.2 20.4 20.3
3.03 2.86 8.44 8.42 12.3 12.0 20.3 20.6
(1) Ditz, H., Chem.-Ztg., 46, 121-2 (1922). (2) Dorrington, B. J. F., and Ward, A. M., Analyst, 54,327-32 (1929). (3) Feigl, F., and Stern, Rosa, 2. anal. Chem., 60, 1-43 (1921). (4) Heymans, J. IT., Satuurzc. Ti?dschr.,11, 151-3 (1929). ( 5 ) Hoffman, 3. I., Bur. Standards J . Research, 7 , 883-92 (1931). (6) Kolthoff, I. M., Mikrochemie (N.S.) 2, 176-81 (1930). (7) Tomula, E. S., Acta Chem. Fennica, 2,72-80 (1929). ( 8 ) Vorontzov, R. V.,J . Applied Chem. (U.S.S.R.), 8 , 555 (1935). (9) Young, R. S., and Hall, A . J., Ind. Eng. Chem., 18, 262-6 (1946).
Analysis of Silica-Alumina Cracking Catalysts Spectrographic Determination of Contaminants R. A . BURDETT AND L. C. JONES,
JR.,
Wood River Research Laboratories, Shell Oil Co., Znc., Wood River, I l l .
A spectrographic method for the determination of iron, sodium, vanadium, nickel, chromium, and copper in silica-alumina cracking catalysts incorporating a novel photometric technique is described. The procedure is very rapid and correlates well with the conventional chemical analyses.
T
HE use in the petroleum industry of silica-alumina type
cracking catalysts has become common in recent years. Inevitably it has been necessary to analyze these catalysts for contaminants introduced in their manufacture and in subsequent use in the cracking process. Nost of the available analytical methods are very time-consuming and in many cases are of inadequate sensitivity. Others involve analytical techniques of such complexity that the results obtained by any but highly skilled technicians are subject to large errors. However, it has been found in this laboratory that the application of the spect,rograghic method to the analysis of these products yields a rapid procedure of adequate accuracy which is easily mastered by semiskilled personnel. Iron, sodium, vanadium, chromium, nickel, and copper are determined simultaneously by this method. The spectrographic analysis of catalysts presents several difficulties which are not met in the analysis of steels or other slloys, notably the lack of certified standard samples. The selection of suitable internal standard lines also leads to certain difficulties. Table I shows typical silica and alumina contents of Four commercial cracking catalysts produced by four different manufacturers. It will be noted that silicon is the natural choice for the internal standard element when the products of different manufacturers are compared, since the extreme variation is only 4 . 6 S on a total silicon basis. Aluminum on the other hand
shows a spread of 33:;. Unfortunately silicon has a very limited number of spectral lines of suitable jntensity and all these are grouped in the region 2460 to 2540 A., so that it is not possible t o use internal standard and analysis lines of very nearly the same wave length, as is the usual practice. This difficulty has been overcome, however, by a novel photometric technique which is described below. I n the comparison of catalysts of substantially the same alumina content it is possible to use aluminum lines as internal standards. I n the determination of sodium the application of the dilution factor principle (7)permits the use of an aluminum internal standard for samples with a wide range of aluminum concentrations. However, this technique does not work for certain other contaminants, notably nickel and iron. The high streaming velocity electrode system of H a s h and
Table I.
Silica and Alumina Contents of Commercial Cracking Catalysts
Catalyst
8 1 ~ 0 8 ,%
1 2 3 4
13.5 14.4 10.5 10.4
54 86.4 85.5 89 4 89 5
Y102,
