behavior was found for several of the elements in the Ar-H2 flame. This type of flame has never been used before, and the results obtained indicate that it should be investigated further. Scattering curves for several elements were obtained. The scattering is probably due to molecular aggregates of compounds formed with flame gas products. One may have noted in Figures 2 and 3 that the analytical curves for a few of the elements show something other than a first-order dependence of fluorescence intensity on concentration. The reason for this disagreement with the theory (6) is not known a t this time. The shape of the analytical curves of Zn, Cd,
and T1 are different from those obtained previously (5) with line sources. The reason for this is not known a t this time, It is possible that the continuous source may be of considerably more utility to atomic fluorescence than to atomic absorption flame spectrometry. The apparently wide spectral regions over which scattering is observed may cause some elements to interfere in a practical analysis. Also, scattering might constitute an interference for elements exhibiting atomic fluorescence. LITERATURE CITED
(1) Corliss, C. H., Bozman, W. R., Natl. Bur. Std. ( U . S.) Monograph 53, July 20, 1962.
(2) . . Dubbs, C. A.. ANAL.CHEM.24. 1654 (1954). ‘ (3) Fassel, V. A., Mossotti, V. G., Grossman, W. E. L., Kniseley, R. N., Pitts-
burgh Conf. Anal. Chem. Appl. Spect., 1 Qfi.5.
(4) Jenkins, F. A,, White, H. E., “Fundamentals of 0 tics,” p. 460, McGrawHill, New Yorf, 1957. (5) Mansfield, J. M., Winefordner, J. D., Veillon, C., ANAL. CHEW 37, 1049 (196:). (6) Winefordner, J. D., Vickers, T. J., Ibzd., 36, 161 (1964).
RECEIVED for review September 15, 1965. Accepted November 24, 1965. Work supported by Air Force Grant AF-AFOSR1033-66 and a NASA traineeship (J.M.M.).
Preconcentration and Spectrographic Determination of Ultratrace Metallic Impurities in Potassium Chloride M. C. FARQUHAR, J. A. HILL, and M. M. ENGLISH Trona Research Laboratory, American Potash & Chemical Corp., Trona, Calif.
b
Ultratrace quantities of certain metallic impurities in chemical grade potassium chloride are critical in electrolytic potassium hydroxide cells. These impurities are preconcentrated by carrier precipitation with 8-quinolinol, thionalide, and tannic acid and determined spectrographically, similar to the method first proposed by Mitchell and Scott. However, in the present work, the method is extended in sensitivity and scope. A total of 39 elements is determined at low partsper-billion concentrations.
CONCENTRATIONS of certain E:oluble metallic impurities in potassium chloride brine cause excessive generation of hydrogen in electrolytic potassium hydroxide cells, similar to the effect reported by Angel and Lunden ( 2 , 3) in sodium hydroxide cells. Cell efficiency is reduced, and in addition the possibility exists for generating explosive mixtures of hydrogen in chlorine. 4 s a result, stringent purity requirements are imposed on chemical grade potassium chloride for these impurities. Target levels of about 6-p.p.b. NO,8-p.p.b. V, 10-p.p.b. Cr, 30-p.p.b. Co, and 50-p.p.b. Ni have therefore been indicated for this product. To achieve such purity, sensitive and precise analyses are required for monitoring production and for specification analyses of the potassium chloride product. The advantages of emission spectrography are apparent for the required multielement analyses. However, even
208
ANALYTICAL CHEMISTRY
the high sensitivity of spectrography is not adequate for determinations a t the concentrations mentioned earlier. It is necessary to preconcentrate the trace impurities before they can be determined. Several investigators have enumerated the advantages of preconcentration followed by spectrographic determinations. The preconcentration methods have for the most part utilized separations based on ion exchange, solvent extraction, or carrier precipitation, For example, Brody, Faris, and Buchanan ( 4 ) separated 0.1-p.p.m. quantities of impurities from plutonium and uranium by anion exchange prior to determinations by the copper spark spectrographic method. Van Erkelens (5) and also Koch (7) used solvent extraction for preconcentration of 0.1p.p.m. amounts of trace metals from biological ashes and from pure aluminum. The extracts were evaporated to dryness for subsequent spectrographic determinations, Carrier precipitation with organic precipitants was employed by Pohl ( I S ) for preconcentrating 0.1p.p.m. amounts of metallic impurities from aluminum prior to spectrographic determinations. Perhaps the best known preconcentration method is that of Mitchell and Scott (9, 10) for spectrographic determinations of trace constituents. Aluminum was used as a carrier in the precipitation with 8-quinolinol, thionalide, and tannic acid. The ignited precipitate was spectrographed to provide accurate determinations of 11 elements in soils and plant residues.
The method has the advantage that the impurities from large samples (for example, 50 grams of KCl) can be preconcentrated into 15 to 30 mg. of ignited precipitate. Heggen and Strock ( 6 ) , and Silvey and Brennan ( 1 4 ) used the precipitation method with indium as a carrier to determine 17 elements. Mitchell and Scott, as well as subsequent workers, found the method to be limited in sensitivity by impurities in the reagents. Below about 0.1 p.p.m., samples could not be differentiated from the blanks with sufficient accuracy. The present method provides for extensive purification of the reagents, and the blanks are sufficiently low in the critical elements to permit accurate determinations a t the required concentrations. The effect of reagent purification is illustrated in Figure 1, which is a photograph showing the spectra of blanks which were precipitated with specially purified reagents compared with those made with reagent grade chemicals. In addition, the present work has expanded the scope of the method to a total of 39 elements which can be preconcentrated and determined (Table I). EXPERIMENTAL
Preparation of Reagents. It is important that all reagents be as free as possible from metallic impurities. Methods are given here for preparation of reagents of the purity demonstrated in Figure 1. Purified reagents are always stored in polyethylene, rather than in glass, with the advantages pointed out by Thiers (15).
Spectra of reagent blanks (numbered from top to bottom)
Figure 1, 1. 2. 3. 4.
Iron spectrum ( 2 8 0 0 to 3 4 6 0 A) No purification of reagents All reagents purified except tannic acid and 8-hydroxyquinoline Reagents purified occording to the method
A11 water used in the procedure, including that used in making up reagents was purified by passing distilled water through a mixed bed deionizer such as a Crystalab Deeminizer (Crystal Research Laboratories, Hartford, Conn.). Hydrochloric acid was prepared by dissolving anhydrous HC1 (Matheson Co.) in purified water in a polyethylene container (the HC1 gas was first scrubbed through glass wool in plastic and glass equipment) to give a 6 N HC1 solution. Ammonium hydroside was similarly prepared by dissolving anhydrous XH3 (Matheson Co.) in purified water in polyethylene to give a 4N K H 4 0 H solution. Acetic acid was purified by distilling reagent grade glacial acetic acid in a glass apparatus and storing the distilled acetic acid in polyethylene. Ammonium acetate (214’) reagent was prepared by mising equal volumes of the specially prepared 4 5 ammonium hydroxide and 411’ acetic acid. Tannic acid solution was prepared by dissolving 15 granis of tannic acid in 7 5 ml. of purified water and passing the solution through a cation eschange resin column, 24 X 3/8 inches (Amberlite IR-120 is a suitable resin). The first 40 ml. of effluent were discarded, and the next 25 ml. collected and diluted to 50 ml. with purified water. The tannic acid reagent decomposes on standing and was made fresh each dag. Thionalide (1%) reagent was made by dissolving 1 gram of thionalide in 100 ml. of the undiluted distilled acetic acid. This reagent also decomposes on standing and was prepared fresh daily. The 8-quinolinol was purified by twice distilling in glass a t atmospheric pressure (267’ C.). The 5% reagent was prepared by dissolving 50 grams of the purified 8-quinolinol in 115 ml. of undiluted distilled acetic acid and diluting to 1 liter with purified n-ater. Possible contamination from filter paper was minimized by filtering on a 4.5-em. disk of Whstman KO. 542 filter paper in a Millipore Corp. filter holder. Indium-HC1 solution was prepared by dissolving 1.209 grams of Inz08 (Indium oxide, Spex Industries No. 1230, 5-9’s purity) in 333 ml. of purified hydrochloric acid and diluting to 1 liter with purified water. Fifteen milliliters of this solution contain 15 mg. of indium and 5 ml. of 6N HC1. Palladium chloride \vas dissolved in purified water and diluted to give a standard solution containing 0.025 mg. of Pd per ml. Strong standard solutions of the individuar impurity elements were -pre-
-
-
pared from reagent grade salts, purified hydrochloric acid, and purified water to contain the equivalent of 1 gram of element per liter. illiquots from the standards were combined and diluted to make a dilute standard impurity solution which contained the various elements in the approximate ratio espected in samples. The concentrations of the elements ranged from 0.15 pg. of Mo per ml. to 4.0 pg. of Fe per ml. -1ppropriate volumes-e.$., 1.0, 3.0, and 5.0 m1.-were used for preparation of working curves. The dilute standard solution was prepared fresh each time it was used. Preconcentration Procedure. Dissolve 50 grams of sample in 150 i d . of purified water and start a reagent blank. -1dd 15 ml. of the indiumHCl solution, 2.00 ml. of palladium standard solution, 10 nil. of 570 8quinolinol solution, and adjust the p H to 1.8 by dropwise addition of 4 S ammonium hydroxide. -1dd the following reagents with rapid stirring: 45 ml. of 2-Y ammonium acetate, 2 ml. of tannic acid solution, and 2 ml. of thionalide solution. Adjust the pH to 5.2 with 4&1‘ammonium hydroside and allow t o stand overnight. Filter the precipitate with vacuum and !\-ash thoroughly with cold, purified water. Ignite the precipitates in a porcelain or Vycor crucible a t 450’ C. overnight. Apparatus. A Baird 3 meter spectrograph with a 15,000 line per inch grating and a reciprocal linear dispersion of 2.8 -1.per nun. in the second order was used. Excitation was by a 10-ampere d.c. arc, and line densities were measured on a National Spectrographic Laboratories Xodel TX-102 spec reader. Lltra Carbon 101-L cupped graphite electrodes were used for the samples us. a 1/4-inch flat Ultra Carbon 107 graphite counterelectrode. Spectra were recorded on Eastman Kodak S A b l spectrum analysis 4- X 10-inch plates. A rotating sevenstepped logarithmic sector and a Baird calculating board were used. Spectrographic Procedure. Prepare an emulsion calibration curve for each batch of spectrographic plates using the two-step sector method (1). Experience has shown that plate eniulsions are uniform in any given batch of plates with the same emulsion number. Consequently, a single, carefully prepared calibration curve can be used for such an entire group of plates without serious error. Precipitates of standard impurity solutions are prepared in the same manner as the sample and blank precipitates (see Preconcentration Procedure). The arcing procedure is identical for all ignited impurity precipitates obtained
Table 1. Minimum Determinable Amounts of Ultratrace Impurities in Potassium Chloride
Element
Determinable limit, Wavelength, A. p.p.b.
Pd Ga Yb Be Bi 310 Ge Cr
v
co Xi
sc
Sn RLI Rh DY
Tm Lu Ag lln
Zr
Ti Eu Er
w
Cu
Th
Au Gd Ho Cd os Tb
Sm
Fe Pb Sb Zn
u
T1 A1
Hg Ce Ir La Nb & Hf Y Pr, Xb, Ac
3287.2 2943.6 3289.4 3130.4 3067.7 3170.3 3039.1 3021.6 3185.4 3453.5 3414.8 3353.7 2863.3 3428.3 3434.9 3407.8 3131.3 2911.4 32SO. 7 2801.1 3392.0 3199.9 2813.9 3264.8 2947.0 3274.0 2837.3 3122.8 3422.5 3416.5 3403.6 2909.1 3324.4 3306.4 3008.1 2833.1 3232.5 3345.0 2941.1 3229.8 3082.2 2967.3 3201.7 3220.8 3337.5
... ...
...
...
Q
1 1 1 2 3 5 5 5 5 5 5 5 3
5 5 5 5 8 8 10 10 10 10 20 20 20 25 25 25 40 50 50 50 80 100 100 100 200 300
. . .b . . .d . . .d ... . . .ef ... . . .D . . .ed
Internal standard. High blanks preclude the determination. S’olatilizes during ignition of precipitate. Absent: Ce (500 p.p.b.), Ir (250 p.p.b.), and La (50 p.p.b.). e X o suitable salt available for test. f Not added (suspected Pd interference). g No strong lines in spectral region 2800 a b 0
to 3460 A.
VOL. 38,
NO. 2,
FEBRUARY 1 9 6 6
209
in the precipitation procedure, including those from standards, samples, and reagent blanks. However, triplicate portions of standard precipitates are arced and results are averaged for precipitation of working curves. Similarly, duplicate portions of sample precipitates aye arced and average values are reported. Each ignited precipitate was blended in a Wig-L-Bug vial with Ultra Carbon, high purity, 200-mesh graphite in the ratio of 1.3 mg. of graphite to 1.0 mg. of precipitate. Precipitates usually weighed approximately 15 mg. and were blended with 20 mg. of graphite. For each spectrographic exposure, 8 mg. of the mixture were used. A11 spectra were made with the step sector. The arc was started a t 5 amperes, and the current increased to 10 amperes after 10 seconds. A 3-mm. gap was maintained throughout the burn, which was continued for 15 seconds after the carbon temperature was reached to ensure a complete burn (approximately 90 seconds for total burn time). The absorbance of the spectral lines of interest was determined a t the position indicated by the wavelengths shown in Table I. The absorbance of the palladium line was always determined on the unsectored portion of the spectrum where the palladium con-
centration and sample weight (in the electrodes) had been adjusted to give an optimum absorbance value of 0.2 to 0.4. All other absorbances were read on appropriate sectored portions of the element line where the values were between 0.1 and 1.0. Sector numbers were recorded with absorbance values for use in subsequent calculations. For the standards, log intensity ratios of the impurity elements us. palladium were plotted using the plate calibration curve and the calculating board. Permanent individual working curves were then prepared for each impurity element by plotting log intensity ratios us. micrograms of the impurity element. Nine log intensity ratios were averaged for each point (3 arcings for each of the triplicate standard impurity precipitates). These working curves are checked as necessary by running standards. The frequency of such checks will be dictated by individual requirements. I n our experience, however, working curves have shown little tendency to shift. It is believed that this consistency is one of the advantages of preconcentration. The impurity elements are determined spectrographically in a controlled matrix of indium oxide which is free of potassium or other alkali salts and which has an optimum concentration of internal standard.
With the extensive reagent purification, blanks have been observed only on Pb, Cu, Fe, and Nil as shown in Figure 1. To cover a wide range of impurity concentration by a single sample preparation, the step sector is incorporated (1W). For elements Pb, Cu, and Fe, which have a relatively high specification level, working curve calibration is made on a sectored portion of the element lines. Greater accuracy is thus achieved and no blank correction is necessary because the lines from the reagent blanks do not appear on the sectored spectra. I n the occasional case where a small blank occurs on an unsectored line, correction can be made by the method described by Nachtrieb (11). For the remaining elements in Table I, working curve calibration is made on the unsectored portion of the element lines. Plot the log intensity ratios of the elements from the samples on the appropriate working curves. Multiply the indicated micrograms of impurity elements by an appropriate sector factor (if the absorbance was read on a different sectored portion of the element line than that for which the working curve was prepared) to determine micrograms in the sample precipitate. Report the average of the two arcings for each sample. RESULTS AND DISCUSSION
Table II.
Element co
P.p.b. in
Amt. added, p.p.b.
Total amount detd., p.p.b.
