ANALYTICAL CHEMISTRY
412 Table 11.
Sample NO.
1 2
Comparison of Ultraviolet and Gravimetric Methods for Stack Gas Analysis Weight of Ppt., hIg. Gravimkric Method 54.3 9.6
1,3-Diphenylurea in Ppt., M g . , Ultraviolet Method 14.0 8.1
in which the gas flow was slowed down to rates such as those encountered in practice, the recoveries are ahove 97.5%. This is considerably better than could be espected from the gravimetric method. Acetyl chloride is very easily hydrolyzed and in the presence of water only a small amount reacts with aniline to form acetanilide. This compound s h o m an absorption maximum a t 240 mp in methanol (Figure 4). Chloroacetyl chloride quantitatively forms a-chloroacetanilide in aqueous aniline. It also shows an absorption maximum at 240 mp in methanol. APPLICATION TO STACK GAS ANALYSIS
The precipitation of materials other than 1,3-diphenylurea may cause high results by the gravimetric procedure. This TTas
emphasized by the analysis of two samples from a titanium chlorinator stack. The phosgene was precipitated as 1,3diphenylurea according to the procedure of Olsen and coworkers (’7) and the precipitate was weighed. The precipitate was then dissolved in methanol and the 1,3-diphenylurea was determined by the ultraviolet method. The results are shown in Table 11. -4s diphenylurea in the precipitate must inevitably be revealed by its absorbance, it is apparent that the gravimetric method mas giving high results on these samples, particularly in the first case. LITERATURE CITED
Biesalski, E., Z.angew. Chem. 37, 314 (1924). Fieldner, 8. C., Oberfell, G. G., Teague, M . C., Lawrence, J. X , J . Ind. Eng. Chem. 11, 519 (1919). Flesser, L. A , , Hammett, L. P., Dingwall, A., J . Am. Cheni. SOC.
57, 2103 (1935). Jahresber. (?hem.-Tech. Reichsdnstalt 5, 11-20 (1926) ; 6, 57-63 (1927).
Kling, A , Schmutz R., Compf. rend. 168, 773, 891 (1919). Kutnler, W.D., Strait, I,. A., J . Am. C h e n . SOC.65, 2349 (1943). Olaen, J. C., Ferguson, G. E., Sabetta, T’. J., Scheflan, L., IND. ENG.CHEX.,.\XAL ED.3, 189 (1931).
Schroeder, W.A, Wileox, P. E., Trueblood, K. S . ,Dekker, A . O., i i s a ~CHEM. . 23, 1740 (1951). Tischler, A. O., Howard, J. N., Xatl. Advisory Cornm. Aeronautics, A.R.R. No. E5H27a (1945). RECEIVED for review July 2 5 , 1955. Accepted Sovember 25, 1955.
Determination of Water Content of White Fuming Nitric Acid Utilizing Karl Fischer Reagent M. L. MOBERG, W. P. KNIGHT, and H. M. KINDSVATER Aerojet-General Corp., Azusa, Calif.
A method which utilizes Karl Fischer reagent has been developed for the direct determination of the water content of white fuming nitric acid. The w-eighed acid sample is first neutralized by the use of pyridinedimethylformamide solution to prevent reaction with the reagent. .4n excess of Fischer reagent is then added and a standard methanol-water solution is used for the back-titration. The method has shown a relative accuracy within 1% between the calculated and experimentally determined values in the absence of dissolved metallic salts. Approximately 5’70 variation is found in the presence of the solids. Nitrogen dioxide concentrations of less than 1.5% do not interfere with the determination.
A
DIRECT method for the determination of the water content of white fuming nitric acid, using Karl Fischer reagent, was developed in this laboratory in 1950. Mitchell and Smith (2) had reported the use of this method for the determination of water in concentrated nitric or sulfuric acids, but the method had not been extended to the white fuming nitric acid system. Later, Eberius (1)used the Karl Fischer reagent for the determination of water in mixed acid. The following procedures were developed for the determination of the water content of white fuming nitric acid containing small quantities of nitrogen dioxide and dissolved metal salts. APPARATUS AND REAGENTS
Apparatus. Covered tall-form Berzelius beaker with openings for two burets, stirrer, and end-point indicator.
Dead-stop end-point indicator (3). Reagents. All reagents were chemically pure or better. White fuming nitric acid was prepared by reaction of concentrated sulfuric acid and sodium nitrate. Distillation was conducted under moderate vacuum, and dry nitrogen was passed through the distilled product to remove nitrogen oxides. Acids of 0.08 to 0.6% water content were made by this method; in the presence of phosphorus pentoxide, acids of so-called negative water contents have been prepared which contain small quantities of nitrogen pentoxide. Karl Fischer reagent. To prepare 2 liters of Karl Fischer reagent, use 538 ml. of pyridine, 169.4 grams of iodine, 1334 ml. of absolute methanol, and 90 ml. (128 grams) of sulfur dioxide. Any of the materials Tyhich contain over 0.1% water by aeight should be dried and distilled before using. Methanol-water solution, prepared by adding weighed quantities of nater to anhydrous methanol. Pyridine-dimethylformamide solution, 2 to 1 bg volume. Standard sodium hydroxide solution, checked with standard hydrochloric acid solution standardized against sodium acid phthalate. Standard ceric sulfate solution, prepared from ceric ammonium sulfate and standardized against arsenous oxide. Standard ferrous sulfate solution, prepared from ferrous ammonium sulfate and standardized against ceric sulfate solution. Liquid nitrogen dioxide, 98.0%, Matheson Co., Inc. Ferric nitrate, Fe(SO& 9HzO. Chromic acid. Xickel nitrate, Ni( NO8)*,6H20. Aluminum nitrate solution,, prepared by dissolving aluminum (99% pure) in white fuming nitric acid of known composition. PROCEDURE
The classical method for the analysis of the distilled white fuming nitric acid was utilized. Nitric acid was determined by direct titration with standard base, nitrogen dioxide by reaction with excess ceric sulfate and back-titration with ferrous sulfate
V O L U M E 28, NO. 3, M A R C H 1 9 5 6
tion technique described above and analyzed by the Karl Fischer method. Results are shown in Table I and Figure 1. Statistical analysis of the data, using Youden’s procedures ( 4 ) ,gives a standard deviation of the intercept equal to 0.0975 and a t value for the intercept (7 degrees of freedom) of 1.200. The 5% critical value for t ( 7 degrees of freedom) is 2.365; therefore, there is insufficient evidence to maintain that the intercept differs from zero by more than can be attributed to the analytical errors. Likenise, the standard deviation of the slope (8 degrees of freedom) is 1.744. The value of 1 obtained from this computation is less than the critical value of 2.306 (5% probability level), so that the variation of the slope from unity is not considered significant. These results indicate that no blank is required in the mater titration, and that the Karl Fischer method can he used for accurate analysis of nater in Thite fuming nitric acid a t a11 concentration levels. To ascertain the effect of nitrogen dioxide upon the titration, liquid nitrogen dioxide was Iveighed into samples of acid of knon-n water content, and the samples nere analyzed for water by the Karl Fischer procedure. Results are presented in Table 11. I t is apparent that for concentrations of nitrogen dioxide below 1.5% no interference occurs. Unconfirmed data indicate that this method is applicable up to nitrogen dioxide concentrations of 2.5 to 3.0%. Above this concentration low results for water are obtained.
t LT w
5 c w K
h
PERCEYT WATER
BY CALCULATION
Figure 1. Comparison of calculated water to water found in white fuming nitric acid by Karl Fischer method
Table I.
413
Water Content of White Fuming Yitric l c i d
(Comparison of calculated water content with that found by Karl Fischer method)
52
Water by Calculation .
.
I
1.07 2 01 2.30 3.28 9.63 14.40 25.00 29.55
%
Table 11. Determination of Water in White Fuming Kitric Acid Containing Various Concentrations of Nitrogen Dioxide NO? Added, Wt. %
I‘alcd.
Found
0 50 1 54
0 51 0 98
0 51 0 97
Water, Wt %
Water by Analysis 0.56 1.05 2.00 2.28 3.31 9.76 14.70 25.10 29.87
solution, and water by difference. Weighed quantitiea of water were added to the frozen acid to prepare the more dilute solutions. Acids of higher nitrogen dioxide content were prepared by adding nitrogen dioxide to the white fuming nitric acid. The compositions of these prepared acids were computed from the available weight relationships. The water equivalence of Karl Fischer reagent was obtained by permitting weighed quantities of water to react with excess Karl Fischer reagent and back-titrating with methanol-water solution, using a dead-stop end-point indicator. The ratio between Karl Fischer reagent and methanol-water solution waq established daily. The white fuming nitric acid solutions (1 to 2 grams) were neutralized with excess pyridine-dimethylformamide solution (10 ml.) to prevent interference with Karl Fischer re. agent. Caution must be used in adding the acid to the base to avoid violent reaction. A slight excess of Fischer reagent is added to the neutralized acid solution in the water analysis, and the excess is back-titrated with methanol-water solution to the deadstop end point. Blank determinations on all reagents used in the analysis are performed, and the results are corrected accordingly. RESULTS AND DISCUSSION
Because Mitchell and Smith had shown that the Karl Fischer method was accurate for the determination of water in solutions containing 70% nitric acid, it was believed that the accuracy of the determination in more concentrated solutions (where available techniques were not completely accurate) could be established by proving a straight-line function. Accordingly, eight acid solutions of known water content were prepared hy the dilu-
The effect on the Fischer method of metallic salts commonly found dissolved in commercial white fuming nitric acid was investigated by the addition of these compounds to acid samples of predetermined water content. Water analysis for these solutions are recorded in Table 111. Although differences between calculated and experimental values are higher than desired, the results are not unreasonable in view of the hygroscopic nature of the solids. Because the quantities of solids used were several times the concentration normally encountered in commercial acid, it is concluded that theqe materials offer no interference in the analysis.
Table 111. Determination of Water in White Fuming Nitric Acid Containing Dissolved Salts Compound Water. Wt. % Added. Compound wt. % Calcd. Found Si(N03)2.6HsO 3.42 1.58 1.73 Cr(NOa)a. 9Hz0 2.52 1.31 1.83 4 23 5.05 5.09 Fe ( N o d I . 9 Hz0 Mixed solids4 2.60 2.68 2.51 SI(S0s)a 0 106 5.15 5.01 *Composed of 90.35 wt.% Fe(S08)3 QHtO, 5 37% 0 0 8 , 4 34% Ni(N0a)z 6Hz0.
LITERATURE CITED
(1) Eberius, E., Angew. Chem. 64, 195 (1952). 12) M t c h e l l . J., Jr., S m i t h , D. M., “ A q u a m e t r y , ” Interscience. Xew York, 1948. (3) W e r n i m o n t , G., H o p k i n s o n , F. J., IXD. ENG.CHEX, ANAL.ED. 15, 272 (1943). (4) Y o u d e n , 157. J., “Statistical M e t h o d s for Chemists.” Wiley, S e w Y o r k , 1951. RECEIVED for review January 22, 1954. Accepted r\-ovember 25, 1955,
