166
ANALYTICAL CHEMISTRY
per thousand. This has been done only to compare the prrcisiori of the colorimetric and volumetric methods more closely. Actually, the average colorimetric value is very nearly within t h e experimental error involved in the oxalate standardizations DISCUSSION
A very inviting possible application of differential colorimetry would be in the determination of a base metal in an alloy, the content of which n-as known to be close to loo%, or in the analysis of a “pure” metal. Here a very short range graph could be prepared in the vicinity of zero on the optical density scale. It might also be possible to obtain an accuracy of 0.1% transmittancy over such a short range. All these factors would make for extreme accuracy. CONCLUSION
Differential colorimetry with the present Model DU Beckman spectrophotometer is capable of yielding results as accurate as most gravimetric and volumetric methods of analysis. The problem of finding colors that are stable and reproducible cnough to utilize needs investigation. ACKNOWLEDGMENT
The authors wish to thank Joan Stewart and Frank Bassani for preparing the graphs shown in this paper. 4DDENDA
Some time after the data in Table I11 were taken, the ultraviolet source employed burned out. The lamp, together with its reflector which showed signs of wear, was replaced and the ultraviolet data were rechecked a t a sensitivity equivalent to that previously employed. The results are given in Tahle I X .
Table TX. Ultraviolet Accuracy Increase in Values with New Lamp Wave Length, mw
Slit Aperture for Distilled Water
225
0.50
A8
Corresponding t o 2.0-Mm. Slit
1.14 1.31 1.42
Accuracy Increase Using Formula 5 6
3.6 4.0 4.3
7 8 9
As anticipated, the accuracy increase values are greater than those given in Table 111, the most appreciable differences being a t wave lengths below 260 mp. Based on these results, differential colorimetry mould be feasible even at the shorter wave lengths. The data given should merely be taken as indicative of the performance to be expected from a new lamp, because these lamps probably vary in their light output. LITERATURE CITED
(1) &res, G. H., XSAL.CHEM.,21,652-7 (1949). (2) Bastian, R., Zbid., 21, 972-4 (1949). (3) Beckman National Technical Laboratories, BuZl. 91E,pp. 3, 10 1947. (4) Cary, H. H., and Beckman. .1.0..J . Optical SOC.A m . , 31, 682-9 (1941). (5) Hawes, R. C . , Katiorial Technical Laboratories, private communication. (6) Hiskey, C. F., Trans. S. IT. Acad. Sci., 11, 223-9 (1949). (7) Hodgman, C., “Handbook of Chemistry and Physics,” p. 1695, Cleveland, Chemical Rubber Publishing Co., 1947. (8) Kolthoff and Sandell, “Textbook of Quantitative Inorganic Anal3 sis.” p . 667, New York, Macmillan Co., 1943. (9) Mellon, M. G., Bx.4~.CHEY.,21,4 (1949). (10) Natl. Bur. of Standards, Provisional Certificate of A4nalysisof Standard Sample 40e, Sodium Oxalate. RECEIIEDMarch 5 , 1949.
Chromium and Manganese in Steel and Ferroalloys Simultaneous Spectrophotometric Determination JAMES J. LINGANE AND JUSTIN W. COLLAT Harvard ilniversity, Cambridge 38, Mass. The spectrophotometric method of Silverthorn and Curtis has been re-examined to determine whether the empirical calibration with standard steel samples is necessary. A study of the various optical and chemical factors demonstrates that Beer’s law is obeyed and the manganese and chromium contents can be computed from the densities observed at two appropriately chosen wave lengths. Corrections have been established for the effect of other elements commonly present in ferroalloys. Results obtained are equal in precision and accuracy to values derived from empirical calibrations.
S
ILVERTHORX and Curtis (6)described a spectrophotometric method for the simultaneous determination of chromium and manganese in steel, which inherently is of great practical utility, The method is based on a persulfate oxidation of the two elements to dichromate ion and permanganate ion, in the presence of phosphoric acid to decolorize the ferric iron. From measurements of the optical density of the solution a t two appropriate wave lengths the chromium and manganese contents can be computed. Under the particular conditions that they selected, Silverthorn and Curtis found that it was necessary to calibrate the method empirically with a series of standard steel samples. It is not clear from their description whether this apparent failure of
Beer’s law was due to the chemical aspects of their procedure or whether it was instrumental in nature and a reflection of the spectrophotometric technique which they employed. The purpose of the present investigation was a critical study of the various aspects of the Silverthorn and Curtis method. I t was found that under the proper chemical conditions, and nThen the absorption measurements are made with a spectrophotometer that operates with a relatively narrow spectral band width, Beer’s law is obeyed and it is not necessary to resort to empirical calibration. In the improved procedure periodate ion is used in addition to persulfate ion to ensure complete oxidation of the manganese and to eliminate the fading of the permanganate which occurs when persulfate is used alone. The influence of
V O L U M E 2 2 , NO. 1, J A N U A R Y 1 9 5 0 other elements commonly present in steels and ferroalloys has also been invrstigated and the appropriate corrections have been estahlished. EXPERlMENTAL TECHNIQUE
Absorption spectra were measured from 220 to 1100 mp with a Beckman ;\lode1 DE quartz spectrophotometer, using 1-em. quartz cells which had been matched against each other. (The less expensive Corex glass cells may be used in the actual analyses, inasmuch as measurements do not need to be made below 440 mp.) The slit widths n-ere adjusted so that the spectral band widths were smaller than 5 mp a t all wave lengths, and in the range from 300 to 700 mp it wvas usually possible to employ a band width as small as 1 mp when care was taken t,o maintain the optical system in optimum focus. The absorption spectra shown in Figure 1 were measured against the same concentrations of sulfuric and phosphoric acids in the reference cell. Later i t was found that a sulfuric-phosphoric acid solution does not absorb over the range of wave lengths used in the analysm, and hence water may be used in the reference cell. A standard 1 millimolar solution of +2 manganese was prepared determinately from pure manganous oxalate dihydrate ( S ) , by decomposing a 179.0-mg. sample of the salt in 5 ml. of hot concentrated sulfuric acid and finally diluting to 1 liter. Standard dichromate solutions were prepared determinatelj. from pure potassium dichromate. Standard solutions of pervanadyl ion-nickel, cobalt, copper, and iron-were prepared from purified ammonium vanadate, nickel ammonium sulfate hexahydrate, cobaltous sulfate heptahydrate, cupric sulfate pentahydrate, and Bureau of Standards ingot iron. ;\11 rragenta xere of analytical reagrnt quality. Measured volumes of these solutions were carried through thr oxidation procedure described below to obtain the solutions used for the drtermination of the extinction coeficirnta. RESLLTS ~ Y D DiscussioN
*
Absorption Spectra. Judged by the absorption spectra in Figure 1, an optimum vave length for the measurement of puir dichromate solutions would he at the peak in the near ultraviolet a t 350 mp. However, even in the presence of phosphoric acid, ferric iron begins to absorb 60 strongly below about 425 mp that shorter wave lengthh than this are inaccessible in qteel analysri, and i t is necessary to employ the weaker broad band at 440 nip. Even a t 440 my the correction for ferric iron absorption corresponds to 0.0005% chromium for each 1% of iron, and hence amounts to 0.03 to 0.05% chromium with most steels. From the curves for dichromate and permanganate in the region 400 to 450 mp it is seen that 425 mp a t the minimum in the permanganate curve would be the optimum wave length for measurement of the mixture, for a t this point the ratio of the extinction coefficients of dichromate and permanganate is maximal. However, because the correction for iron adsorption is greater a t 425 than a t 440 mp, measurement at the latter wavr length is the best compromise. Silverthorn and Curtis ( 5 ) measured the permanganate extinction a t 575 mp and a t this wave length the extinction due to dichromate ion is negligibly small. The present authors prefe1 to measure a t the absorption maximum a t 545 mp, which provides greater precision, and to apply the small correction for dichromate absorption. Because the apparent extinction coefficient a t the sharp absorption maximum decreases with increasing band width, the analytical measurements must be made with the same slit width used to determine the extinction coefficient. The apparent extinction coefficient a t 545 mp decreased by 2.4% when the slit width was increased from 0.01 to 0.2 mm., corresponding to an increase in the band width from 0.8 to 5.3 mp. A t 575 mp an increase in band width from 0.9 to 3.1 mp caused the apparent extinction coefficient to increase by 3.9%. At 440 rnp the apparent extinction is, of course, much less sensitive to band width. Validity of Beer’s Law for Dichromate. The fact that potassium dichromate solutions do not obey Beer’s law in very dilute (0.005 N ) sulfuric arid, but show an increasing extinction
167 coefficient with increasing concentration, is well known from the excellent study by Kortum ( 2 ) . This apparent deviation a t relatively small hydrogen ion concentrations is doubtless partly due to the shift of the chromate ion-dichromate ion equilibrium with changing total chromium Concentration.
H
k2.0 0
Y
m
21.5 z 0
L1.0 0 2
-
c
50.5
0
300
350
400 450 500 MlLLlMiChONS
550
600
650
Figure 1. Absorption Spectra 1 millimolar permanganate ion, dichromate ion, a n d pervanadyl ion, a n d 100 millimolar nickelous ion, i n 1 hl sulfuric acid-0.7 M phorphorir acid
I n solutions of high hydrogen ion concentration Beer’s law is accurately obeyed, a5 shown by the data in Table I. In this table, D is the observed optical density (extinction), defined by D = log(Io/l) = €IC, where E is the extinction coefficient in the units cm.-’ (millimoles per liter)-1, 1 is the cell length (cm.), and C is concentration (millimoles per liter). The value of t is constant to *0.5% over the concentration range which can be measured with a 1-em. cell. The extinction coefficient of dichromate ion decreases with increasing concentration of sulfuric acid, but above 0.5 ‘iA acid the rate of change is not very great-for example, with 1.667 millimolar dichromate ion in 0.5, 1.0, 2.0, and 3.0 Jf sulfuric acid the observed densities at 440 mp were seriatim 0.611,0.610,0.604, and 0.597. Multicomponent determinations rest on the assumption that the substances concerned contribute independently to the total optical density at a givrn wave length. This was verified for the
Table I.
Extinction Coefficient of Dichromate Ion
(Measured at 440 mp in a 1-om. cell with a band width of 0.5 mfi. Solutions were 1 M in sulfuric acid and 0.7 M in phosphoric acid ) KaCraOy Millimolar D e 0.166 0.062 0.373 0.417 0.153 0.367 0.308 0.370 0.833 1.250 0.459 0.367 1.662 0.609 0.367 1,544 4.17 0.370 .4v. 0.369 * 0.002
Table 11. Test of Additivity Principle with DichromatePermanganate Mixtures at 440 mp (Measured density of the dichromate solution alone was 0.627 and that of the permanganate solution was 0.376. Both solutions were 1 .l4 in sulfuric acid.) Dichromate Permanganate D D Obsd. Calcd. Soln., M1. Soln., M1. 50 5 0.607 0.604 25 5 0.588 0.585 25 10 0.558 0.555 25 25 0.502 0.502 10 25 0.448 0.448 5 50 0.397 0.399
ANALYTICAL CHEMISTRY
168 present case by preparing separate solutions of dichromate ion and permanganate ion and measuring the density of each a t 440 mp. The solutions were then mixed in various ratios from 0.1 to 10 and the optical density of each mixture was measured a t 440 rnp. The data in Table I1 demonstrate that the observed densities of the mixture agree with those calculated from the individual densities and the mixing ratio, and hence the additivity principle is valid in this case. Calculational Technique. rlssuming for the moment that absorption a t 440 and 545 mp is due only to dichromate and permanganate ions, the following relations are valid for a 1-cm. light path:
where the subscripts Cr and Mn denote dichromate ion and permanganate ion, and concentrations are expressed in millimoles per liter. Simultaneous solution of these two equations yields
itt 440 than at 545 mp. The spectrum of cobaltous ion (not shown in Figure 1) comprises a broad band with a maximum a t 510 nip. When the percentages ~ i nickel, ' cobalt, arid vanadium are known, the corresponding corrections are most conveniently made in ternis of the optically equivalent chromium and manganese percentages computed once and for all from the experimentally determined extinction coefficients. These corrections are listed in Tables I11 and IV, as the equivalent chromium or manganese percentages corresponding to each 1% of the element in question whirh must be subtracted froni the apparent chromium and manganese percentages computed by Equations 6 and 7 to ohtain the true values. Expressed in this way, these correctionare independent of the sample weight. In all cases the corrertions on the chromium are much larger than on the manganese, which tends to render the chroniium tifltvrmination somewhat less precis? than that of the mangancsr.
Table 111.
Chromium Corrections at 440 m p
(Valuea listed under Correction are equivalent percentages of chromiurr, to be siihtracted froni apparent chromium perrentage for each 1 % ' of element in question.) Substance E Correction, 7 0.369 0 . 098
0.00481 0.0018
0.0008 0.0001
When the values of the various extinction coefficients (Tables 111 and IV) are introduced, Equation 3 brcomc~r
Table IV.
,.... 0,490 0,0266 0.0072 0.0039 0.0005
Manganese Corrections a t 345
m p
(Values listed under Correction are equivalent manganese percentages t o b t Rubtracted from apparent manganese percentage for Pach 1% of the e l e i ~ ~ i ~ n i in question.) Substance E Correction, pc
Conibining Equation 4 with Equation 1 yirlds for the concentration of dichromate ion
ccr = 2.71 nrro
- 0 . 110 Di,j
(5)
Correspondingly, the percentages of manganese and chromium in a sample of W grams in a volume of V ml. are
Manganese,
%
(0.426 Lj,,,, - 0 013
=
o*olT '
Chromium, % = --
(2 71 /14au
- 0.110 D545)
(6)
(7)
where the constants 0.00549 and 0 01040 are 10-4 multiplied b j the atomic weight of manganese and tu ice the atomic weight of chromium, respectively. Of the other elements commonly present in ferroalloys, vanadium, cobalt, and nickel, exhibit extinction coefficients a t these wave lengths which are sufficiently large so that corrections must be subtracted from the apparent manganese and chromium percentages computed from the foregoing equations when moderately large amounts of these elements are present. Correction for iron itself is also appreciable a t 440 mp. The absorption spectra of pervanadyl ion and nickelous ion are shown in Figure 1. The nickel spectrum corresponds to a concentration 100 times larger than the other elements. Shsorption by pervanadyl ion is still appreciable at 440 mp but negligible at 546 nip. Absorption by nickelous ion is also more significant
Living and Parsons ( 1 ) recent13 demonatrated that apparelit extinction coefficients measured with ten different Beckman Model DU spectrophotometers differed slightly but significantl) from each other, even though the sevrral instruments and mea+ urement conditions were apparently identical. The differenceare sninll enough so that the corrections for nickel, cobalt, vanatiium. and iron listed in Tables I11 and IV may safely be used with different Beckman Model DU instruments, and probahl? they may also be used with the simplified Model B Beckman spectrophotometer. I t is advisable, however, to determine the extinction coefficients of dichromate ion and permanganate ion at 440 and 545 mp with the particular instrument used, eniploying solutions of known concentrations of dichromate ion and permanganate ion prepared according to the procedure d e w i h r d in thr following section. Procedure. Persulfate (peroxydisulfate) ion in the presence of silver ion a< a catalyst is one of the few oxidants which quantitatively oxidize both chromic ion a i d manganous ion to dichromate ion and permanganat? ion in arid solution and it ha. long been a favored reagent for this purpose ( 4 ) . The oxidatiorl may be performed in solutions containing (either or both) sulfurii and nitric acids. The optimum acidity is stated (4)to be hi.tween 0.5 and 1.5 molar sulfuric x i d : n-ith too lon an acid ( W I I centration manganese dioxide tends to precipitate, and the 0x1dation of chromium is inhibited at greater acidities. .Ilthough the persulfate oxidation of chromium is invarittbl! satisfactory, oxidation of manganese occasionally is erratic and incomplete. In the hot acid solution required persulfate ion drcomposes iapidli by ouidizing n:tter, 2S&-2H20 = 0.
+
-
169
V O L U M E 2 2 , N O . 1, J A N U A R Y 1 9 5 0
+
4S04-- 4H+, and v:irious substances can catalyze this reaction a t the expense of the manganous ion-persulfate ion reaction. Furthermore, permanganate solutions prepared by a persulfate sida dation are not stable, but, fade rather rapidly, and hence the riir:r*urrnients must he made inimediately after the oxidation. I n ti typical instanw the optical density a t 545 mp of a pernian,variate solution pwparrd 1)). pclrsulfate oxidation in 1 JI sulfuric acid and 0.3 M phosphoric acid decreased by 2.4% per hour. These difficulties are eliminated by the combined use of perw l f : r i c ~ ion and periodate ion. The superiority of periodate ion for thr osidation of niang:tirese was originally demonstrated by R'i1l:ird and Greathouse ( A ) . Permanganate solutions prepared ID!. pcriodate oxidation :ire stable indefinitely. Because periodate ion doc^ not oxidize rhromiuni quantitatively, persulfatr inn is ret:iinrd for this purpose i n the present, procedure. H), appropriate choive of smiple m i g h t and final volume of the 't\-itlizetl solution thr, following procedure may he' applird to utwls, ferroalloys, : t r i i l rwpp,rr-nic'krl alloys iv-ith manganese cont m t s t)rtn.cen about 0.1 and 5 % and chromium contents from a t'c.n. triiths of 1% up ro 20% or niorcr. Thr sample wright and tinnl volume should tw cho~enPO that the optical (lc~iisitiesat 440 :ind 545 niH fall in thcA r:inge 0.1 to about 1.5. For an c.scdlent di.sc~nssiori( i f v:rrious methods of tliswlvirig 4iromium steris the niotiograph of Lundell, Hoffman, and Bright $ I should be consulted. 111 lll
