Ultraviolet Spectrophotometric Determination of Nitrate Application to Analysis of Alkaline Earth Carbonates ROBERT BASTIAN, RICHARD WEBERLING, and FRANK PALILLA Chemistry Laboratory, Sylvania Electric Products / n c , Flushing,
N. Y.
b A rapid and accurate method for determining nitrate in alkaline earth carbonate mixtures used as cathode coating materials in the electronics industry utilizes ultraviolet absorption of nitrate ion in dilute perchloric acid solution. The system is sensitive and stable and adheres to the usual absorption laws. The interference of a number of anions and cations is considered. Because of the high purity of the carbonates with respect t o metal ions, no separations or corrections need ordinarily b e employed. However, methods for dealing with the interference of common metal ions are considered with a view toward possible wider application of the method.
P
mixtures of carbonates of barium and strontium (tlouble carbonates) or barium, strontium, and calciuni (triple carbonates) are iniportant as cathode coating materials in the electronics industry. Such mixtures are usually prepared by btarting with the alkaline earth ni-
4
RECIPITATED
mg. N0;/100
mi.
Figure 2. Analytical curves Corrected for absorption of perchloric acid, Jyhich as less than 0.005 absorbance unit a t 210 and 220 mp and 0.03 unit at 203 mp. Slits were 0.9 mni. a t 220 mp and 1.2 mm. at other wave lengths. Curves run at 210 mp with slit near maximum aperture showed no significant difference
trates. The final products are extremely pure 11-ith respect to metals but usually contain 0.1 to 1%nitrate. Colorinietric methods for the determination of nitrate have been reviewed by JIacDonald ( 5 ) . Many of these methods are conducted in the presence of sulfuric acid which makes prior separation of barium desirable, I n many cases the absorption laws are not strictly obeyed, and color stabilities leave something to be desired. The present method consists of dissolving the samplc in dilute perchloric acid, adjusting the acidity to about 5 nil. of free perchloric acid per 100 nil., and reading a t the appropriate nave length. APPARATUS AND REAGENTS
A Beckman Model DT- spectropho-
WAVE LENGTH ( m p ) Figure 1. Absorption spectra of nitrate ion u 0.675 mg. of nitrate and 5 ml. of perchloric acid per 100 ml. b 0.675 g. of nitrate and 5 ml. of perchloric acid per 100 ml. Curves corrected for absorption of perchloric acid
tometer equipped with a photomultiplier attachment, ultraviolet accessories, and 1.00-cm. cells was used. Standards were prcpared from reagent grade sodium nitrate, dried for several hours a t 110" C. All other chemicals werc reagent grade. Perchloric acid v a s 70 to 7270. CHARACTERISTICS OF SYSTEM
Figure 1 shows absorption spectra of nitrate ion in dilute perchloric acid. VOL. 29, NO. 12, DECEMBER 1 9 5 7
1795
Dolance and Healy (3) used the 300+-mp region to determine nitrate in plating solutions, but the lower region, in which nitrate is about 1000 times as sensitive, has apparently not been used. The lower ultraviolet peak (checked on two DU spectrophotometers) appears to be a t 203 nip, although a t such a short wave length no great absolute accuracy is claimed. Buck, Singhadeja, and Rogers ( 2 ) reported a peak at 198 mp in the absence of acid. Figure 2 shows that the system obeys the normal absorption l a m a t 220, 210, and 203 mp. It was decided not to work a t 203 mp because many interferences are larger than a t 210 mp, and the gain in sensitivity is small-interference of chloride ion a t 203 nip is about 20 times that a t 210 mp, and the perchloric acid background is more than five times as great a t the shorter wave length. I n addition, stray light may reach a sufficient level on some instruments to cause apparent absorption lan- deviations below 210 mp. Although 210 mp is probably the optimum wave length, the method was tested at 220 mp to ensure application to all DU instruments. Increasing acidity decreases the absorption of nitrate slightly a t both wave lengths. This was determined using a nitrate ion concentration of 0.9 mg. per 100 ml. a t 220 mp and 0.45 mg. per 100 ml. a t 210 mp. The effect is approximately linear over the range 0 to 10 ml. of perchloric acid per 100 ml. The choice of 5 ml. m-as made arbitrarily; a variation of 1 ml. causes an error of about 5 parts per thousand at 210 mp and 6 parts per thousand a t 220 mp. The system is extremely stable. Solutions allowed to stand for one week showed no significant change in absorbance. The effect of temperature is small. An increase of several degrees caused no detectable change in absorbance a t either wave length. At 210 mp the method is about three times as sensitive as the brucine method of No11 (6) and comparable in sensitivity to the phenoldisulfonic acid method (4). INTERFERENCE OF OTHER IONS
Tables I and I1 show the interference of several other ions due to their own absorbance. These values were determined in the absence of nitrate and do not include interaction effects. Metallic ions were derived from purist grade metals or salts and fumed with perchloric acid prior to reading, to expel nitrates. I n many cases corrections were applied for traces of iron, which were determined by reading a t 260 mp ( 1 ) . Anions were derived from dried sodium salts. To determine if barium, strontium, or calcium affected the absorption of
1796
ANALYTICAL CHEMISTRY
Table 1.
Ions Having Litt!e Absorbance % Ion Having
Absorbance Equivalent t o O . O O l ~ oNitrate 2 2 0 m ~ 210mr
Ion C a t + Ba++ S r + + jJf& xaC pJi+i! Al+f)+,S H , + a ' >lo0 >lo0 Zn++ 21 21 Co++ 16 4 Rln +b 4 1 Cr+++c 0.39 0.20 c1>100 38 so,- 60 90 Pod--c103Ac-
16 1.4 0.17
19 0.49
0.21 Obtained by neutralizing NHaOH with HC104. * From Mn( ClO&. 6HzO (G. Frederick Smith Chemical Co.). By reduction of Cra+lyith methanol. Concentrations of ions measured varied from 0.5 mg. t o 1 g. per 100 ml. 5
Table II.
Ions Having More Significant Absorbance
yo IOn Having Absorbance Equivalent to 0.1 % Nitrate Ion 220 mp 210 mfi 0,089 0.20 Fe+++ Cut+ 0.47 0.59 Pb++ 0.44 0.30 &foe+ 0.127 0.26 Cr6 + 0 146 0.194 v5 0.132 0.257 Ti4+ 0 88 1.2 I0,068 0 39 SCN 0.114 0.23 KO*0 .30a 0 3Sa s20;- 0.61 1.08 Br3.9 0.52 a For freshly prepared solution. Because of decomposition absorbance increases with time. +
Table 111. Analysis of Low Purity Synthetic Triple Carbonate Containing 0.250% Nitrate
yo Kitrate
Found 220 210 mp mr
Impurities Added 1% Na, 0 . 5 % C1-, 0 . 5 % A c - , ~0.4% SHa+, 0.260 0.01% &Ig++,0.01% 0.261 Al+++.0.004% Fe+++. 0.256 0.004% Pb +i,'O. 0003% Mn++, 0.000270 Cu++ Av. 0.259 Corrected for Ac-, Pb++, Fe+++b 0.251 Ordinarily absent. Using factors in Table 11.
0,258 0.258 0.252
0.256 0.250
Q
nitrate ion, several solutions containing nitrate plus the alkaline earth perchlorates were compared with standards containing nitrate alone. These solutions contained the equivalent of 0.5 to 1 gram of the alkaline earth carbonate per 100 ml. and nitrate corresponding to 0.09 to 0.10%. Each alkaline earth metal was tested separately and in a mixture cor-
responding to 1 part of calcium carbonate, 4.5 parts of barium carbonate, and 4.5 parts of strontium carbonate. The results checked the theoretical within 0.001% nitrate, indicating no interaction. Chloride was tested to see if it interacted with nitrate. .4t a level of 1.2 grams of chloride and 0.45 mg. of nitrate per 100 ml., no interaction mas detected. Table I shows that a large number of ions have a relatively low absorbance. None is ordinarily present in carbonate materials in sufficient amounts to cause interference. Traces of sulfate cause turbidity due to formation of barium sulfate. If this occurs, a portion of the solution should be decanted through fine filter paper prior to reading. At 220 mp iron(II1) and iodide have absorbances greater than that of nitrate itself (Table 11). Nitrite interferes in the determination of nitrate. Of the ions listed in Table I1 iron, copper, and lead are most likely to be present in carbonate materials, but normally in such trace amounts that they have little or no effect. The effect of potentially interfering cations can be eliminated. ANALYSIS OF SYNTHETIC SAMPLE
A synthetic triple carbonate sample containing nitrate and considerably higher levels of impurities than ordinarily present was prepared by additions to a solution of the alkaline earth perchlorates corresponding to a base composition of 1 part of calcium carbonate, 4.5 parts of strontium carbonate, and 4.5 parts of barium carbonate. Table I11 shows that results on production batches of carbonates should be accurate to about O . O l ~ , nitrate without prior separations or corrections. ANALYSIS OF CARBONATES
Recommended Procedure. Place a sample not exceeding 1 gram in a 100ml. beaker fitted with a watch glass. Cover the sample with n-ater and add perchloric acid dropwise until just dissolved. Wash down the sides of the beaker with water and swirl to aid in the expulsion of carbon dioxide. To the solution or a suitable aliquot add 5.0 ml. of perchloric acid, transfer to a 100-ml. volumetric flask, dilute, and mix. If turbid, decant through Khatman 42 filter paper into the absorption cell prior to reading a t 210 or 220 mp. Run blanks and standards with each batch of samples. Analytical Results. Table I V shows the results on several actual triple carbonate samples. For the brucine method the samples were dissolved in acetic acid and passed through a cation exchange resin [Amberlite 1R120(H)] to remove the alkaline earths, and then aliquots were analyzed (6).
Table IV. Analysis of Miscellaneous Triple Carbonate Samples
% Nitrate Ion exchange, Ultraviolet brucine Method Sample SO. method 220mp 210mp 1 0.40 0.39 0.38 0.39 2 0.29 0.28 0.28 0.29 3 0.60 0.59 0.60 4 0.96 0.98 0.98 0.97 5 0.81 0.79 0.79 0.79 6 0.21 0.23 0.21 0.22 7 0.16 0.15 0.15 0.15 8 0.47 0.48 0.47 0.48 9 0.55 0.57 0.57 0.56
The brucine values are averages of from three to six determinations. Ultraviolet values are averages of three determinations, except for the second set a t 220 mp, which were run in routine fashion by a second analyst and represent one or two determinations. At 210 mp the maximum average deviation on a single sample was i 0.01170 nitrate, and the average of all such deviations mas =t0.005% nitrate. At 220 mp (considering only values based on three determinations) the corresponding results were ~t0.017 and =t0.0077,. For the brucinemethod the values were 10.019 and 1 0 . 0 1 0 ~ .
Table V.
Elimination of Cation Interferences on Sample Containing 0.5070 Nitrate
Nitrate Found, yo Direct Triple Carbonate Method Sample Containing 220mp 210mp 0.73 0.227GFe+++,0.225& Cu++, 0.84 0.73 0,22% Pb+0.85 0.74 0.86 -4v. 0.85 0.73 Corrected for Fe+++,Cu++, Pb + + a 0.51 0.51 Using factors in Table 11.
Difference hlethod 220mp 210mr 0.50 0.51 0.50 0.50 0.52 0.51 0.51 0.51
Ion Exchange Separation, 210mp 0.50
0.50
0.50
0.50
0
ELIMINATION
OF CATION INTERFERENCES
T o make the procedure more generally applicable, several variations can be used to eliminate or correct for interfering cations. The general method is to pass a weakly acid perchloric acid solution of the material through a cation exchange resin t o remove cations, adjust the acidity, and read the solution directly. The resin should be thoroughly mashed with dilute perchloric acid immediately before use and the washings tested to make sure the ultraviolet absorption is negligible. -4second method is to take t x o equal aliquots of the sample solution, fume one thoroughly with perchloric acid to expel nitrates, and use it as a background for the other. This “difference method” mag fail if the oxidation state of the metal ion is changed in the process as, for example, with chromium(II1). If iron(lI1) is the main interference, the solution may be read a t 260 mp (1) and a correction applied.
The first two procedures are illustrated in Table V on a synthetic sample Tvith a triple carbonate base composition and iron, lead, and copper impurities. Methods for eliminating the interference of anions are beyond the scope of this paper. However, certain anions could be precipitated with silver which, in turn, could be precipitated with chloride. LITERATURE CITED
\
,
Bastian, R., Sf7eberling, R., Palilla, F., ANAL.CHEX. 28, 459 (1956). Buck, R. P., Singhadeja, S., Rogers, L. B., Zbid., 26, 1240 (1954). Dolance, A.. Healv. P. W., IND.ENG. C H E ~ANAL. . “ED.17. 718 (1945). Komarmy, J. M., Brokch, SV. J., Testerman, RI. K., Anal. Chim. Acta 7, 349 (1952). LIacDonald, A. M. G., Znd. Chemist 31, 515 (1955). S o l l . C. A.. IND. ENG.CHEM..ANAL. ED. 17, 426 (1945).
RECEIVED for review May 27, 1957. Accepted August 23, 1957.
Determination of Carbon-14 Steroids on Paper Chromatograms DAVID L. BERLINER, OSCAR V. DOMINGUEZ, and GARTH WESTENSKOW Departments of Anatomy, Biological Chemistry and Radiobiology, University o f Utah College o f Medicine, Salt take City 7 2, Utah
b The major points of loss in the chromatographic determination of radioactive steroids are the steps involving elution and plating. Steroids may b e quantitated directly on paper chromatograms by using @ray selfabsorption factors and measuring radioactive areas recorded on a strip counter. The method makes possible the study of physiological quantities of steroids from biological tissue and fluids.
P
Chromatography is one of the most widely employed analytical techniques in tracer studies of steroid metabolism. The removal and quantitation of steroids from paper chromatograms require several steps (elution, plating, and counting), which are often accompanied by loss of material. Therefore quantitation of steroids directly from the chromatogram would be advantageous. The purpose of this study was to deAPER
velop a more sensitive method for determination of steroids following paper chromatography. A technique for determining steroids directly on a paper chromatogram, a more sensitive counting apparatus for evaluating the radioactivity, and a method for determining correction factors for self-absorption of low energy beta rays have been developed. The method has been successfully applied to the determination of steroids VOL. 29, NO. 12, DECEMBER 1957
1797
