Colorimetric Determination of Nickel in Steel G. R. MAKEPEACE AND C. H. CRAFT Metallurgical Laboratory, Menasco Manufacturing Company, Burbank, Calif. The method of Murray and Ashley for the colorimetric determination of nickel in steel is outlined and criticized. Experimental data are presented on the stability under various conditions of the red color of oxidized nickel dimethylglyoxime. A modification of the method is described which gives a highly stable and readily repro-
ducible red color, and i s particularly suitable for routine work because of its rapidity and manipulative simplicity. Copper and cobalt interfere only slightly; the other elements ordinarily found in steel d o not interfere. The accuracy of the method is comparable to that of routine gravimetric procedures.
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except difficultly soluble rhromium-nickel steels for which a mixture of equal parts of nitric and hydrochloric acids is suggested. Since many of the st,eelsanalyzed in this laboratory contain about 17; chromium and do not dirsolve completely in nitric acid or rapidly in hydrochloric acid, a method was tried of decomposing first in an "acid mixture" (133 ml. of 1.82 sp. gr. sulfuric acid and 167 ml. of 8575 phosphoric acid per liter of solution) and then completing dissolution by adding 1 to 1 nitric acid. For 170 rhromium steels the method was more rapid than decomposing with hydrochloric acid and more nearly complete tha,n dissolving in nitric acid. S o effect, on the development of the nickel color was observed.
I-RRAl- and Ashley ( 1 ) have presented a method for dcveloping quantitatively the red color of oxidized nickel dimet hylglyoxime. The sample is dissolved in dilute nitric acid (in the case of difficwltly soluble chromium-nickel steels, a mixture of nitric and hydrochloric acids is used). To a n aliquot of suitable size the following additions are made: citric acid to prevent' iron prccipitation a t the final pH, bromine water to oxidize nickel, ammonium hydroxide in sufficient quantity to bring the pH to 8-9, and dimethylglyoxime in the form of 1% solution in alcohol to develop the red nickel color. The solution is diluted to known volume and the color is compared. A wave length that has bcen recommended for spectrophotometric reading is 530 mp. Although t,he method of Murray and Ashley has been wed with some succcss in this laboratory and elsen here, it has certain undesirable characteristics. The color developed in the nickel solution shows continuous change from the instant, of addition of dimethylglyoxirne, tending for the first few minutes to become more int'ense, then to fade. Murray and Ashley note the phenomenon of the color intensity cliange a t 530 mp with time, and present a .*et of curves illustrating its characteristics. At no time is there :t period of constant t,ransmittance in the transmittance 1's. time cwve of sufficient duration to give a time margin for truly reproducible reading. Bot,h the slope and the time of the minimum of the traiismitt m c c PY. time curve appear to be changed by c-hanges in the nickel concentration of the solution used for making the curve. Small differences in p H a t t,he time of adding dimethylglyoxime to otherwiqe similar. solutions were found to have pronounced effects on the shzipe of the transmitt,ancae 1's. time curve, altering bot,h thc 4ope : i d tht, position of the minimum to :I marked dcgrce.
I n the effort to produce a nickel color that would be quickly formed, stable, and reproducible without elaborate control of the ctonditions of development, various other basic substances were tried in place of ammonium hydroxide. Among them were sodium carbonate, sodium tetraborate, sodium orthophosphate, potassium pyrophosphate, and sodium hydroxide, all in the concentrations required to produce the proper pH range for color development. S o n e proved satisfactory. Other oxidizing agents such as sodium perborate, potassium chlorate, potassium iodate, and hydrogen peroxide were tried in place of the bromine. Only ammonium peroxydisulfate in the presence of silver ion showed promise, but the desired degree of color stability was not achieved with ita The effects of solution temperature on color development were studied, but no worthwhile modification utiIizing temperature control was found. Color development a t high ammonium hydroxide concentration followed by reduction of t h e .solution pH to 8.5 to prevent iron precipitation proved unworkable.
It was realized at thi.; point t'hat tartaric acid will hold iron in solution at a considerably higher pH than will citric acid. Accordingly, the citric acid n-as replaced by tartaric. Upon experiment it was discovered that, while iron will develop an interfering color in tartrate solutions made basic with ammonium hydroxide nearly as quickly as in citrate solutions under the same conditions, the interference and any precipitate that may form can be cleared up rapidly by increasing the pH still further with sodium hydroxide. This is not true of citrate solutions. While the nickel color develops ,slowly and incompletely in very dilute sodium hydroxide solutions of pH 8 to 9 and not a t all in more concentrated solutions, the color, once developed, remains stable over long periods of time even in rather strongly basic sodium hydroxide solutions. In view of these fact?, the folloning approach wax tried:
If 807; of the recommended amount of ammonium hydroxide ih used, no color a t all develops (pH 6.8). (All pH measurements were made with a k e d s & Northrup potentiometric pH meter using a standard glass electrode and a calomel-saturated potassium chloride reference electrode.) With 90% of the amount required (pH 8.1), the rate of color development is slow and rate of fading is high. If the concentration of ammonium hydroxide is high, the rate of color development is high and the rate of fading slow, solutions retaining substantially all their color for several hours. If as milch as four times the recommended amount of ammonium hydroxide is used, the color is completely developed in about 30 seconds. A second effect enters, however-a rapidly increasing interfcwnce of the iron in the solution. Ultimately iron hydroxide is precipitated under these conditions. Critical examination of the pH range within which the Murray-.Miley method is ~rorkableshows it t,o be less than 1 pH unit, pH 8.2 to 8.7. I t waa found that the amounts of tartaric acid, bromine water, and dimcthylplyoxime solut'iori were not' critical above a minimum value if sufficient extra ammonium hydroxidc was added to neutralize increases in the acid content.
The sample n-as treated with tartaric acid and bromine and made strongly ammoniacal. Then the dimethylglyoxime solution was added. 17nder these conditions the rolor developed rapidly. hfter 1 minute, sodium hydroxide was added. .%iter 2 to 3 minutes the increase in iron interference which had taken place in ammoniacal solution cleared up completely and the color hecame stalile. S o furthvr change in color and conrequently in transmittanc-e at ,530mp ~vaqobserved. The reading a t 24 hours was identical n-ith the reading 5 minutc3 after adding dimethg-lglyoxime.
I t \va- concluded that the unmodified method of Mui~rayand Ashley, while suitable for many purposes, is not sufficiently reproducible (at least without elaborate precautions) to meet the need3 of this laboratory. I t does, however, have the virtue of being rapid. I t was in a n effort to reta.in its speed while improving its accuracy that this research was undertaken. The original method recommends dissolving thc sample in 1 to 1 nitric acid,
PROCEDURE
Since these results seemed very encouraging, a procedure was devised to make use of them and was used in all the studies hereafter reported. High-purity reagents must be used. Particular
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INDUSTRIAL AND ENGINEERING CHEMISTRY
attention should be paid to the witability of the tartaric acid and of the dimethylglyoxime. Decompose a 0.25-gram sample in 20 ml. of an acid mixture (133 ml. of 1.82 sp. gr. sulfuric acid and 167 ml. of 85% orthophosphoric acid per liter of solution). Steels containing little chromium may be dissolved directly in 8 N nitric acid. Stainlesstype steels may require the use of hydrochloric acid. Cautiously add 10 mi. of 8 N nitric acid and boil to expel oxides of nitrogen. Transfer the solution to a volumetric flask of suitable size and dilute to the mark. Transfer by pipet to a 100-ml. volumetric flask an aliquot of the diluted solution containing between 0.05 and 0.3 mg. of nickel. Add to it, mixing after each addition, 5 ml. of a 20% tartaric acid solution, 5 ml. of saturated bromine water, 10 ml. of 0.90 sp. gr. ammonium hydroxide, and 5 ml. of a 1% solution of dimethylglyoxime in methyl alcohol. (Occasional difficulties in development of color and in fading upon addition of sodium hydroxide have been traced to impure or partially decomposed tartaric acid and dimethylglyoxime. C . P . reagents are not uniformly satisfactory in this respect. Impure tartaric acid interferes with development of color upon addition of dimethylglyoxime, in extreme cases preventing any color formation a t all. Impure dimethylglyoxime results in a fading of the color upon addition of sodium hydroxide. The fading may take place very rapidly or slowly, depending on the degree of impurity. Both difficulties can be overcome by special treatment. Addition of a second 5-ml. portion of bromine water after introduction of the dimethylglyoxime ensures complete color development. Difficulties with the dimethylglyoxime reagent can be overcome by acidifying the alcohol solution with dilute sulfuric acid and adding enough bromine water to color it yellow. This should be done 15 to 30 minutes before it is used. Occasional small further additions of bromine water are necessary to keep the solution yellow. The treated reagent is usable for only a few hours.) After 1 minute add 10 ml. of 6 N sodium hydroxide solution and dilute to the mark. After 5 minutes, transfer the solution to the optical cell and compare the transmittance a t 530 mp with that of pure water. Two transmittance us. wave-length curves were prepared, one from National Bureau of Standards Bessemer steel 10d containing substantially no nickel, and one from a nickel solution made from C.P. nickel nitrate. They are shown in Figure 1as read on a Coleman Universal spectrophotometer. A B c k m a n spectrophotometer, with a much narrower wave band than the Coleman, gave a similar curve for the nickel solution; the positions of the maxima and minima were identical. Appreciable interference of the blank (Bessemer steel), however, did not occur on the Beckman instrument until wave lengths as short as 470 mp were reached. From a study of these curves it was decided that the most satisfactory wave length for reading the transmittance of the nickel color in steel on a Coleman Universal spectrophotometer, which has a wave band width of 35 mp, is 530 mp, the wave length originally suggested for the method of Murray and Ashley. This selection was made on a basis of minimum interference of the blank and maximum interference of the nickel color. For instruments with a wave band much narrower than 35 mp, such as the Beckman photoelectric spectrophotometer, a lower setting such as 480 or 490 mp seems to be preferable, since the lower value is closer to a minimum of the nickel curve and since iron interference is small in this range with such an instrument. Presumably, it would be possible to use for both spectrophotometers a wavelength setting a t the 470 mp minimum of the nickel curve, if a blank prepared from a steel free of nickel were used as the reference solution, thus canceling the effect of iron interference. For colorimeters using filters one probably would do best to select a filter with a rather sharp cutoff a t about 450 mp, passing no light of shorter wave length. This selection is indicated to eliminate interference due to the color of the iron present. Increased sensitivity can be obtained by further restriction of the wave length of light used for comparison to the range of maximum interference of the nickel color. Samples were taken from a series of National Bureau of Standards nickel-containing steels and mixtures of these steels t o cover in small steps a series of nickel percentages from 0.002 to 5.12.
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Wovdenyth Figure 1.
I 550 /k m p
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6.50
Per Cent Transmittance vs. Wave Length
Curves lor Beuemer steel blank and C.P. nickel nltratc solution as read on
Colenlan Universal spectrophotometer (35 mp band .widlh)
They were prepared according to the method previously outlined, and the points so obtained were plotted on semilog paper with the per cent transmittance of the sample compared with water a t 530 mp as the ordinate and the per cent of nickel in the steel as the abscissa. I n this fashion three transmittance us. concentration curves were obtained, for 0 to 1%, 1 to 2.5%, and 2.5 to 5.5% nickel steels. A total of 27 concentrations was used in obtaining the three curves. The color was found t o follow the Beer-Lambert law very exactly when read a t 530 mp; therefore the transmittance US. concentration curves, when plotted as described, were straight lines over the entire range utilized. No one of the 27 points determined from the National Bureau of Standards samples deviated from the transmittance us. concentration curves plotted from them by more than 2% of the total nickel present in the steel in the case of steels containing more than 1% nickel. No one of the points deviated from the transmittance US. concentration curve by more than 0.02% nickel (2 “points”) based on the total analysis of the steel in the case of steels containing less than 1% nickel. These points were established by single determinations, not by averages of groups of determinations. The differences between the high and the low values on the National Bureau of Standards reports sent with each steel are as great as 1.77, of the total nickel present in the steel in the case of steels containing more than 1% nickel. Differences of as much as 0.021?4 of nickel (2 “points”) based on the total analysis of the steel are listed in the case of steels containing less than 1%nickel. Reproducibility was checked by making five complete determinations by the authors’ method on National Bureau of Standards nickel steel 33b containing 3.48% nickel. Using the transmittance US. concentration curve established for steels containing 2.5 to 5.5% nickel, the average value of the five determinations was 3.48% nickel. The maximum deviation from the average value was -0.05% nickel. High and low values on the National Bureau of Standards report are 3.51 and 3.46% nickel, respectively. The method appears to be capable of the high order of reproducibility neressary for precise steel analysis.
ANALYTICAL EDITION
June, 1944
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SENSITIVITY
Tests were made to determine the sensitivity of the method to small variations in procedure. It was found that the tartaric acid, bromine water, and dimethylglyoxime added could be decreased 25y0 or increased 100% without affecting the results. The ammonium hydroxide may be decreased 20% or increased 507, without effect. Approximately the same range of values holds for the sodium hydroxide. It was determined that the time elapsed between adding the dimethylglyoxime and the sodium hydroxide i. not ciitical SO long ar it exceedq 1 minuteLe., color development iq complete in less than 1minute. Three identical samples equivalent to a standard steel containing 0.60% nickel were prepared, using for each sample 0.125 gram of National Bureau of Standards Bessemer steel 10d and 0.125 gram of nickel-chromium stee1.32~. To sample 1 the sodium hydroxide, which arrests color development as well as preventing iron precipitation, was added 30 seconds after adding dimethylglyoxime; to sample 2,jminutesafter adding the dimethylglyoxime; and to sample 3, 10 minutes after adding the dimethylglyoxime. In each case the solution was diluted to the volumetric mark and mixed immediately after adding the sodium hydroxide. Transmittance readings in each case were taken 10 minutes after adding the sodium hydroxide. The results, expressed in terms of the analysis of the steel as read from the 0 to l.y0nickel curve, averaged 0.599% nickel with a maximum deviation from the average value of 0.0057, nickel. INTERFERENCES
The method wab tested for interference by copper, cobalt, tungsten, molybdenum, chromium, and vanadium. The small amounts of copper (less than 0.2%) present in the usual steels did not interfere. Copper, when present to the extent of 0.50(7, in the steel, caused a positive nickel error of 0.027,. Cobalt, tvhen added to thr sample equivalent to 2.57, in the steel, caused a positive error of 0.037, nickel. Both elements were tested for interference on a
