580
A N A L Y T I C A L CHEMISTRY Table I
Analysis Element Be
Line,
Matrix Bi
co
Bi
Cr
Bi, Pb-Bi
Fe
Bi, Pb-Bi
Mn
Bi, Pb-Bi
MO
Nb
Bi, Pb-Bi a
Bi, Pb-Bi Ta
a
Ti
Bi
V
Bi
W
b
A. 3131.072 2650,781 3443.641 3474.022 3021.558 2766.540 3593,488 3605.333 2739.546 2599.396 2626.585 2794.817 2801.064 2810.200 2816.154 3132.594 3028.443 3032.768 3194.977 2950.878 2981,651 3054.316 3050.819 2675.901 3214.750 3465,562 3186.454 3254,250 2880.030 2952.075 2944.395 2946.981
Range, P.P.M.
0.5-0.06 5-0.6 100-10 100-10 100-10 100-10 20-2 20-2 100-10
100-10 100-10
10-1 100-10 100-10 100-10 10-5 100-10 ’ 100-10 100-10 100-10 1000-100 1000-100 100-10 1000-100
1000-100 1000-100 100-10 100-10
100-10 400-50 400-50
2659,454 2929.794 2997.967 b
CALCULATIONS
The photographed spectra were all photometered using an ARL film densitometer. Partial background correction was made by substraction of values reIated to the true intensity values through the plate calibration curve. Intensity ratios for analytical purposes were calculated for the wave-length pairs of Table I. The use of the platinum lines a t 2659.454, 2929.794, and 2997.967 A. was dictated by interference factors noted. OccasionaIly the 3132.594 A. molybdenum line was used as a refcrence line for the 3131.072 and 2650.781 -4. beryllium lines.
100-10
P t (reference lines)
.z
former. The sample was in the positive electrode of the polarized spark which crossed a 2-mm. gap. Spectra were photographed in the 2480 to 3700 A. region with a Baird 3-meter grating spectrograph. A 50p slit was employed. In most cases, duplicate spectra were recorded on SA-1 film, which was processed under carefully controlled conditions in the conventional manner. Some consideration was given to the effect on the excitation of polarity in the analytical gap. A slight increase in sensitivity with the sample in the anode was noted. This is in agreement with the observation of Jolibois ( 2 ) . A 0-2 ampere r-f ammeter was installed in series with the analytical gap for use during porous electrode excitation. This meter provided sensitive electrical indication of conditions in the analytical gap. I t was observed that concentrated salt Rolutions were excited with increased electrical stability.
Pib, Ta, separated from matrix. Excited in solution containing H I S O I . W, separated from matrix. Excited in presence of fluoride ion.
using them to fill 10-ml. volumetric flasks to which 75 lambda of a 1%platinum solution had previously been added. The sohtion, ready for excitation, therefore contained 75 p.p.m. of platinum (weight-to-volume) as an internal standard. Samples containing elements requiring special treatment were handled in a manner analogous to that employed in the bismuth matrix samples.
DISCUSSION
Intensity ratios obtained from duplicate excitations of samples agreed within 20% for lines having transmission values from 5 t o 90%. In most cases, the agreement was within less than 5%. The authors’ experience employing the porous electrode technique substantiates the claim of Feldman ( 1 ) for an inherent precision of the method of about 2%. The requirements of their application were not, however, rigorous enough to demand limiting precision; and the authors took over-all advantage of the situation in a manner which permits them to claim only the precision given above. In many instances, for the analysis of samples from special experimental work for specific elements they obtained precision of the order claimed by Feldman. LITERATURE CITED
( 1 ) Feldman, C., ANAL.CHEM., 21, 1041 (1949); 5. S. Atomic Energy EXCITATION
The solutions were excited in porous cup electrodes according to the method proposed by Feldman (1). Excitation was furnished by the high voltage section of an ARL high precision source unit with 60 volts to the primary of the high voltage trans-
Commission, AECD-2392. (2) Jolibois, P., Compt. rend., 202, 400 (1936). RECEIVEDAugust 12, 1950. Presented before the Division of Bnalytical Chemistry a t the 118th Meeting of the . ~ M E R I C A K CHEMICAL SOCIETY, Chicago, Ill.
Differential Spectrophotometric Determination of High Percentages of Nickel ROBERT BASTIAN, Sylvania Electric Products, Inc., Kew Gardens, N . Y .
R
ECENT papers (2-4) have emphasized the great accuracy and precision which are possible in colorimetry when a solution of high absorbancy (for nomenclature see S , 5 , 6 ) is compared to a similar standard rather than to the solvent. The terms ‘‘differential colorimetry” or “method of transmittance ratios” have been applied to this procedure. A general mathematical treatment has been given by Hiskey ( d ) , and a treatment applied specifically to the Model DU Beckman spectrophotometer has been made by Bastian, dWeberling, and Palilla ( 3 ) . Using a reference standard in place of the solvent, the absorbancy (optical density) scale on the Reckman instrument is set
for zero by making the slit aperture wider than usual. Somewhat more concentrated solutions are then read against this standard. For systems obeying Beer’s law at the wider aperture, the resultant absorbancy scale readings are directly proportional to the difference in concentration between the zero point standard and the solution in question. The term A: is used here for the absorbancy scale reading obtained against the zero point standard. Then:
A:
1
log -
T:
V O L U M E 23, NO. 4, A P R I L 1 9 5 1 It is difficult to determine nickel accurately in materials containing 98 to 100% nickel, by conventional methods. A differential colorimetric procedure is presented by w-hich nickel is determined to about *0.05%in several samples, includingelectronic nickel, spectrographic nickel, and synthetic mixtures containing copper, chromium, iron, and cobalt. Simultaneously, cobalt can be approximated and small amounts of chromium determined. To determine nickel, the green color of this metal contained in
where T: =
Tsolution
Twro point standard
This follows the nomenclature for the conventional case, because
where T , =
Teolution -
Taolveat
\ h e n readings are taken a t a wave length of maximum absorption, A: will be less than the difference between A , for the given solution and A , for the zero point standard, because of the wider slit aperture used in the differential case. This discrepancy will become greater as A , for the zero point standard increases; for many systems it will not be appreciable below A , values of 2.
581 perchloric acid solution is used. Concentrated nickel solutions are read on the Beckman Model DU spectrophotometer, using a similar standard in the comparison cell; the procedure for determining the optimum concentration of the standard is given. Under the selected conditions, 13 to 14 times the precision obtainahle by normal colorimetry is theoretically possible. Where sufficiently stable colors are available, a similar approach should be applicable to the determination of other base metals.
Excellent results can be obtained by differential colorimetry when the absorbancy of the unknown differs from that of the zero point standard by a reasonable amount ( 3 ) . However, for systems that obey Beer's law, if A , for the zero point standard equals or exceeds 0,434, optimum results will be obtained when the absorbancy of the unknown is identical with that of this standard (3,4). This condition can be approached in the determination of nickel in certain grades of nickel. In the following work the material is assumed to contain 98 to 1 0 0 ~ nickel, o a condition which includes many grades used in the electronics industry plus certain special alloys. Because the amount of nickel varies by a limited amount, a very short range graph can be prepared and AS made to approach 0. The analysis is of practical value because the direct determination of nickel in such materials to better than a feR parts per thousand is not simple even by the oxime procedures. The following work utilizes the green color of the nickel ion in perchloric acid solution. This was chosen because of the successful determination of copper in that medium ( 2 ) , although that work was hardly done under optimum conditions. SELECTION OF WAVE LENGTH
For the curves in Figure 1, 1.000 gram of spectrographic metal was dissolved in nitric acid, fumed with 8 ml. of 60% perchloric acid, and diluted t o 100.0 ml. Readings were taken in 1-cm. cells against the solvent. Nickel has a sharp peak a t 395 mp and a band of lower absorbancy with a maximum a t about 720 mp. To avoid confusion, curves for 99.85y0 iron, 99.99+yo cqpper, and C.P. chromic acid in this medium (2) have been omitted. Instead, the A , value of an equal concentration of the metal has been compared t o that of nickel a t 395 and 720 mp and, for a later purpose, t o that of cobalt a t 510 mp. When a very low reading was obtained in the 1-cm. cell, one tenth of the reading obtained in a 10-cm. cell was employed. Values for sexivalent chromium a t 350 and 510 mp were computed from much lower concentrations using Beer's law.
Figure 1.
Spectrophotometric Curves and Relative Absorbancies
A t 395 mp 1 X i + + = 0.009 C r + + + + + + = 21 c o + + > 1000 cu++ A t 720 m p 1 N i + + = 28 C o + + = 29 F P + + + = 0:29 &+e > lo00 Cr++++++ At 510 mp 1 C o + + = 90 Ni++ = 130Cu++ = 0.13 C r + + + + + + > 30 F e + + +
Although the absorbancies of the metals relative to nickel cannot be taken to represent their extent of interference directly, they serve as approximations. At 395 mp even a trace of sexivalent chromium would cause serious interference. The interference of cobalt appears greater a t 395 than a t 720 mp. Bo figure is given for iron a t 395 mp because inconsistent results were obtained; in addition, the absorbancy index (extinction coefficient) appears to decrease sharply with the weight taken. Even for small amounts, however, the interference is probably greater than a t 720 mp. The latter wave length was chosen in spite of the interference of copper, because this can be separated from nickel by a very simple method. To keep the weight of nickel required from becoming excessive, 10-cm. cells were used in the following work. DETERMINATION OF OPTIMUM CONCENTRATION OF NICKEL
For systems obeying Beer's law, the accuracy possible by differential colorimetry increases as the absorbancy of the zero point
ANALYTICAL CHEMISTRY
582 standard increases. Eventuallj-, however, because of the rombined effects of broadening band widths of light emitted by the widening slit apertures, and excessive concentrations, an optimum concentration is reached beyond which decreased accuracy results (5). For a given plot of A , or At against concentration, the accuracy obtainable a t any given scale reading is directly proportional to,the slope times t,he concentration a t the given point and inversely proportional t o AA,, the minimum difference in -4: or A , that can be detected a t t,hat point ( 3 ) . The concentration can be replaced by the number of grams of substance, G, if the slope, S , is expressed as ( f A 8 ! d Q or da.*/dc, because the volume terms cancel out. If all the concentration ranges studied are diluted to a constant volume, S will usually fall (if we work a t an absorption peak) as the weight of the zero standard increases, because of the increasing slit aperture, but the product of S and G a t any given scale reading will tend t o increase.
To determine the optimum value for the differential zero standard, a series of graphs could be plotted over the concentration range t o be covered in practice, using varying weights for the zero standard, and determining when the average value of (S x G ) / A A , was a maximum. However, to carry out such a process over the short concentration range contemplated would be tedious. For the present purpose it was decided to determine relative accuracies a t a value of A: = 0. Assuming that the value of A A , is independent of slit aperture, the accuracy is proportional t o S t,imes G for the differential zero standard. If it is further assumed that all the plots are straight lines, the value of 8 will be constant for each range and can be obtained by measurement of a single fairly distant point. Although this assumption will not be valid for very wide slit apertures, it is sufficient for a first approximation. Varying amounts of 99.91% nickel differing by 0.2 gram were dissolved in nitric acid, fumed with 8 t o 10 ml. of 60% perchloric acid by the procedure described later, and diluted in 100-ml. volumetric flasks. Each weight of nickel was read against the lower weight beginning with pure solvent. The readings were made in 10-cm. cells and were corrected for the difference betm-een the cells determined by using the solvent. These readings are shown in Table I, column 4, above the double line. Assuming that, all plots are straight lines over t,he range covered, the slopes are given in column 5. These continually decrease. The product>s,S X G (at a scale reading of zero), are given in the nest column. These rapidly increase to a maximum, then decrease slowly. Table I.
Selection of Optimum Nickel Concentration
Nickel, Grams/100 Mi. To set To scale obtain zero reading 0.0000 0.2002 0.4002 0.6006
0,8000 1.0003 1,2000 1.4000 0.6008 0.7000 0.8000
0,2002 0.4002 0.6006 0.8000
1,0003 1,2000 1,4000 1.6005 0.7000 0,8000 0.9000
Slit Aperture 0.034 0.078 0.187 0 439 0 864 1.26 1.62 1.69 0.436 0.630 0 855
Absorbancy (Optical Density) Scale Reading
S
735 714 711 629 561 405 341 278 0 334 0 328 0 289
3.67 3.57 3.55 3.30 2.80 2.03 1.71 1.39 3.37 3 28 2.89
0 0 0 0 0 0 0 0
Rels-
‘2rG A: = 0
o:iij
1.42 1.98 2.24 2.03 2.05 1.95 2.02 2 30 2.31
2;racy 1 4 4
8 8 12.3 13 9 12 6 12 8 12 1 12.6 14,3 14.4
It is interesting t o compare the accuracy obtainable by the differential procedure with that which can be obtained by normal colorimetry. I n the latter case the reading cannot be made at zero on the scale because a t that point the error is infinite. For colors that obey Beer’s law, the optimum reading is 0.434. To obtain the accuracy of the differential procedure compared to the normal procedure we can divide the product of S and G a t zero for the differential case by the product of S and G a t 0.431 for the normal case. (The normal product a t 0.434 is 0.434.) We must then multiply by 2.7, because the absorbancy scale can be read about 2.7 times as accurately a t zero as a t 0.434 (S).
The results of these relative accuracy calculations are shown in column 7. The differential figures given in columns 6 and 7 are minimum values, because the slopes were measured over much longer concentration ranges than that to be used in practice, and deviations from linearity will occur with the higher concentrations. Measurement of 0.1-gram ranges a t the bottom of the tahle illustrate the point. The values in columns 6 and 7 are slightly higher than those for the 0.2-gram ranges employing zero standards of corresponding might ; the discrepancy becomes greater as the weight of the zero standard increases. When the range is compressed to that to be used in practice, the accuracy will probably show a further small increase a t weights above 0.8 gram. Xevertheless it apprars unwisr to increase the weight of the zero standsrd above 0.7 gram. The small increase in accuracy which is theoretically possible above this point would hardly justify the additional interference from other elemcnts (especially cobalt) which probably would be introduced by going to a wider slit aperture. In a:ldit,ion, the chances of obtaining a straight-line working curve even over a short range, a very desirable feature in analytical work, decrease its the sample weight is unduly increased. A maximum value for the accwracy increase of any weight can be computed by multiplying the given weight by the previous slope, because the initial slope of any range is not likely to exceed the average slope for the former range. Such a calculation yictlds a maximum increase of 17 times normal at a weight of 1.0 grain. Even if this unlikely value co:ild be achieved, the lower w i g h t would be chosen for the inJicated reasons. The procedure presented here for choosing the optimum concentration is believed adequate for most purposes. The more theoretically sound procedure of working over the short concentration range to be actually used would necessitate preparing a greater number of solations and Lvorking carefully, because a small error a t any step would cause a relztively large error in slope. (Accuracy increase values calculated from the slopes of the later analytical curves varied from about 13 to 15.) The selected zero standard has an A, value of 2.5 by measurement and 2.4 by calculation from slit apertures (5). [The method of computing accuracies from A , values (Formulas 5 and 6 in S ) will show large errors for higher weights of samples because the assumptions on xhich the formulas were based no longer hold. A4t the selected weight,, however, the check is still fairly good; the accuracy increase is 15 to 16, depending upon whether the calculated or observed ‘4,value is used, as against about 14 in practice. ] In the analytical work a zero standard of 1.40 gram.: per 200 nil. was employed for greater dilution accuracy. AT varied from 0.00 t o 0.05. At 0.05 the accuracy increase will be about, 13 rather than 14. Assuming a photometric error corresponding t o 0.27, transmittancy, a concentration error of about 0.4 part per thousand is to be anticipated over the indicated range. s
ANTICIPATED ACCURACY AND PRECISION
To estimate this more closely, the photometric reproducibility was checked by the following procedure on many of the solutions analyzed. After insertion of the solution in the comparison cell, a friv A: readings were taken t o the nearest 0.0008, and averaged to the nearest 0.0001. After all solutions had been so read, the entire process was repeated and a second average was obtained. These two results mere used to compute an average deviation. Over the range of 0.00 to 0.05 the average reproducibility based on 27 such duplicate and 11 triplicate determinations was *0.00078, which is slightly better than *0.2Y0 transmittancy. (The maximum deviation for a single solution was +0.0019, and for an entire series on any given day ==0.0013.) Using this average and the average slope of the experimentally determined curves, the anticipated error is 10.33 part per thousand. Thie.
V O L U M E 23, N O . 4, A P R I L 1 9 5 1 is only the photometric error, and the other manipulations must also be considered. INTERFERENCE O F COPPER, IRON, AND COBALT
Interferences indicated in Figure 1 are only approximate, because, among other considerations, the band width of light emplol-ed in the actual analysis is greater. The interference of copper, cobalt, and iron was therefore determined dirwtly. series of nickel standards was carried through the procedure, accompanied by others to which varying amounts of the given element were added; sufficient quantities were taken to give readings between 0.01 and 0.04. From these tests it was calculated that, each 0.2‘2wo iron, 0.22VGcobalt, and 0.0026c7, copper caused an error of O.Olyoin the nickel determination. The interference of copper is such that unless its content is very low and known, it must Iw removed. The interference of iron is not serious; amounts present in several electronic grades would introduce an error of no more than 0.01 % in the nickel result. The interference of cobalt is more serious, because lyOis likely to be present, which would cause an error of 0.04 to 0.05% in the nickel result. Hon-ever, with types of nickel analyzed in this paper, it is possible to approximate the cobalt content on the same solution used for the nickel analysis, by reading it against t h r nickel zrro rtandard at 510 mp, using the 10-cm. cells (see Figuw 1 ). For this above purpose a plot of absorbancy against cobalt concentration was made in a medium similar to that used in the nickel analysis, using a constant concentration of 1.40 grams of nickel per 200 ml. in all Tolutions, inc.luding that uqrd to set the scale zero. .4 straight line was obt,aiiied up to 20 mg. of cobalt per 200 ml. (highest concentration tried), which gave an absorbancy rmding of about 0.084. The varia.tion in scale reading due to a 2 s var,iation in nickel cont,rnt in the samples is very small, the correction for cell discrepancies being more important. The following procedure was used. The zero standard was read against itself, followed by the highest nickel standard carr i d i n the work. The average of these t%-oreadings was then deducted from the readings taken on the samples. When this procedure k followed, the error introduced in the cobalt by variation in nickel content will be no greater than 0.01%. The interference of copper and iron is negligible in the cobalt estimation (Figure 1 ) . However, sesivalent chromium has about 8 times the absorbancy of anequal amount of cobalt at 510 mp, and if present, must be corrected for. This is easily accomplished, owing to the great absorbancy of sexivalent chromium a t 350 mp. It is sufficient simply to read t,he solution against the nickel zero standard at. 3.50 m p and to deduct 1/49 of this reading from the reading at. 510 mp. This correction was obtained on solutions containing varying amounts of sexivalent chromium and 1.40 grams of nickel: 1.1 mg. of sexivalent chromium gave an absorbancy reading of about 1.54 a t 350 mH, which is about the limiting amount one would care t o handle in the 10-cm. cell. Excellent adherence to Beer’s law was obtained up to this point. If it is desired actually to determine the amount of chromium, the reading procedure used for cobalt is adequate. The error due to variation in nickel content will be less than 0.001% chromium: 0.300/, iron introduces about the same error, and copper and rohalt have negligible effects. INTERFERENCE OF OTHER METALS
Relative absorbancies (Figure 1)indicate that the interference of sexivalent chromium is especially low a t 720 mp. Similar measurements indicate that 2Vo of aluminum or magnesium should not interfere. A solution containing 2 grams of lead per 200 ml. gave no reading in a 10-cm. cell a t 720 mp. Attempts t o prepare pure solutions of manganese in perchloric acid resulted in either precipitation of manganese dioxide or colloidal suspen-
583 sions. Some tests were therefore conducted on a nickel sample t o which o.5y0manganese was added; no interference could be detected. Titanium also has a tendency to precipitate when fumed with perchloric acid. Ordinarily, however, no more than a few? hundredths of lCr, are present, and analysis on a later sample shows no evidence of trouble from this source. The only difficulty silicon might introduce is that precipitated silica might adsorb some nickel. In the following samples the silicon content, was so low that this w a ~not considered. In samples containing larger amounts silicon might have to be filtered off and washed, or perhaps volatilized with hydrofluoric acid and perchloric acid and the nickel recovered. STANDARD AND “UNKNOWN” MATERIALS
Nickel of 100.00% purity is not available. The spectrographic mat,erial which approaches this is too expensive for ordinary use. As a standard material, nickel shot (General Chemical Co., Code 2009) were selected. These were submitted to spectrographic analysis. and analyzed chemically for all significant metals, carbon, and sulfur. The results are shown in Table 11. By difference, the purity of the metal is 99.91%, and this figure wm used in future work. For an “unknown,” A.S.T.M. electronic nickel sample H-1400 was selected. The results obtained on this sample and the nickel content obtained by difference are also shown in Table 11.
Table 11.
Standard arid “Unknown” LZiaterials Kickel Standard 0 021 0 000
H-1400 A.S.1‘ AI, Sample 0 . 043a 0.80 0.123 0,039
Carbon, % Cobalt, % Manganese. Yo 0 000 Iron,-% 0 033 0 029 0,015 Silicon, % 0 0044 0.0081 Copper, % 0 0008 Uagnesiurn, Yc 0.042 Chromium, 7o
