1182
ANALYTICAL CHEMISTRY
By using a scale factor of 256 or a total count of 4 X 25,600 counts, the average deviation should be reduced to approximately 0.23% barium sulfate. The total counting time would, holyever, be approximately doubled for each sample.
Table 111. Comparison of Chemical and X-Ray Spectrographic Analysis for Barium Sulfate % Barium Sulfate
ANALYSIS OF BARIUM SULFATE SAMPLES
Four samplcs for which chemical analyses were known were selected at random for comparison of chemical and x-ray spectrographic analysis, using the working curve established in Table I and Figure 1. The results of this analysis are given in Table 111. CONCLUSIONS
The working curve for barium sulfate established on the x-ray spectrograph follows a straight line in the upper concentration ranges, and the results are reproducible to nearly within statistical fluctuations in counting. Results based on the spectrographic method agree with wet chemical analysis to within 0.2 to 0.5% barium sulfate. Although no standard samples were available in the very low concentration ranges for barium FUIfate, the working curve should be able t o be extended to low concentrations of barium sulfate, perhaps t o values of a few tenths of 1% barium sulfate by changing the scale factor. The method should be useful for routine control analysis on large numbers of samples, because the time for an analysis is very short. For example, the total counting time for the 72.7% bar-
X-ray
Counts/Sec.
spectrograph
W e t chemical analysis
142.4 177.6 185.3 190.4
66 7 89 5 94.6 98 1
66.52 89.12 94.12 98.32
ium sulfate samples was 4 X 3200 f 148.5 seconds or 1 minute 16 seconds. Samples must be thoroughly dried before using, and preferably ground to -400 mesh, as the precision of the x-ray spectrograph decreases with larger particle sizes. Sample handling is a t a minimum, the only handling required being t o place the sample (approximately 30 grams) in the plastic sample holder. By keeping the surface of the sample a t the same level, and using the same surface area, smaller size samples (less than 30 grams) could also be counted. LITERATURE CITED
(1) Sorth American Philips Co., Inc., Kew York, “Operating In-
structions for the X-Ray Spectrograph,” 1952. (2) Shamsusaman, N., “Effects of High Atomic Numbered Elements and Particle Size on Working Curves Established by the X-Ray
Spectrograph,” M S . graduate thesis, Colorado School of Mines, Department of Metallurgy, 1954. RECEIVED for review December 23, 1954.
Accepted February 23, 1955,
Determination of Organic Substances by Standard Chromous Chloride Solution RUDOLPH S. BOTTEI and N. HOWELL FURMAN Department o f Chemistry, Princeton University, hinceton,
.i standard chromous chloride solution of exactly deter-
minate strength was utilized for the determination of a number of reducibIe organic compounds. These include anthraquinones, nitro, nitroso, azo, and acetylenic compounds. The anthraquinones were titrated directly with chromous chloride, while the other compounds were analyzed by adding an excess of reducing agent and back-titrating with standard ferric alum solution. The end points were determined potentiometrically.
C
HROMOUS salts have not been as extensively used for the
quantitative analysis of reducible organic compounds as titanous salts. Someya (4)reduced p-nitroaniline, picric acid, and p-nitrophenol with an excess of chromous chloride solution which was prepared by the incomplete reduction of chromic chloride by amalgamated zinc. The excess chromous chloride \vas titrated with standard ferric alum solution. Terent’ev and Goryacheva ( 6 ) had titrated quinone, axobenzene, and mand p-nitroaniline directly using methyl red as an indicator. Their precision for the determination of azobenzene was very poor. They prepared their chromous solution by dissolving chromous acetate in hydrochloric acid. Both of these methods necessitated the frequent standardization of the chromous solution. Recently, Lingane and Pecsok ( 1 ) have shown that it is relatively easy to prepare and maintain a standard chromouc: solution of exactly determinate strength. Since chromous solutions are stronger reducing agents than titanous solutions, and their reactions are generally faster than titanous salts in
N. J. that the reductions are usually carried out a t room temperature, it was decided to reinvestigate the use of chromous chloride for organic analysis. This paper presents the results of the use of this reagent for the determination of: o-nitrobenzoic acid, 2,4,6-trinitrobenzoic acid, 2,4,6-trinitroresorcinol,2,4-dinitrophenylhydrazineJnitroguanidine, p-nitrobenzeneazoresorcinol, nitroso R salt, anthraquinone 2,i-disodium sulfonate, and the monopotassium salt of acetylene dicarboxylic acid. All are quantitatively reduced, the nitrogencontaining compounds to the corresponding amines (rupture of the N--N link in the azo compound), the anthraquinone to the corresponding anthrahydroquinone, and the acetylenic compound to the corresponding ethylenic compound.
EXPERIMENTAL
Apparatus. The titration cell was a tall-form 200-ml. electrolvtic beaker, covered by a rubber stopper provided with a gas inlet tube, a saturated calomel reference electrode, a platinum Indirator electrode, a thermometer, if the reaction was t o be carried out a t an elevated temperature, and openings for a gas outlet and the delivery tips of two burets. If any of the openings were not used, they were closed by means of corks. If the solution was t o be heated, a beaker encircled with asbestoscovered heating 15 ire was used. The temperature was controlled by regulating the current flowing through the heating wire by means of a Variac. The solutions were stirred xvith a magnetic stirrer. The end point of the reaction was determined potentiometrically by measuring the voltage change by means of a Leeds and Sorthrup line-operated pH meter, Model 7664. Since the titrant could be added in very small increments (about 0.02 ml., if one
1183
V O L U M E 27, NO. 7, J U L Y 1 9 5 5 spins the stopcock rapidly and the buret' tip has a rather small opening), it was not necessary t o plot the voltage change. The voltage change at the end point in t,he back-titration of excess chromous chloride with ferric alum solution is about 500 mv., while the voltage change at the end point in the direct titration of the anthraquinone salts is only about 175 mv. The end point, fn the back-titration of chromous solution with ferric alum solution can also be determined using the derivative polarographic end point ( S ) , in which case t,he saturated calomel reference electrode is replaced by a second platinum electrode. The pair of platinum electrodes are polarized by a small constant current of about 2 pa. The apparatus used for storing and dispensing standard chromous solution x a s the same as that utilized by Lingane and Pecsok ( I ) , except that a 2-liter st'orage flask was used in place of the 1-liter one. I n this way enough chromous solution was available for about 35 separate determinations. Reagents. -4standard O.lOOOLVsolution of chromous chloride in 0.1.Y hydrochloric acid was prepared directly in the storage flask by the procedure described by Lingane and Pecsok (1). The 2-liter storage flask was filled about two thirds full with amalgamated mossy zinc, and about 1 liter of 0 . l O O O X chromic chloride in 0.1S hydrochloric acid was added. The reduction was usually allowed to proceed overnight. The solution was stored under pure hydrogen obtained from a Kipp generator. The hydrogen was freed from oxygen by passage through a bubble tower containing chromous chloride solution in l N sulfuric acid in contact with amalgamated zinc. The chromous solution was standardized against, standard cupric solut'ion in 6*V hydrochloric acid as recommended by Lingane and Pecsok (1). The standard cupric solution was prepared from copper sulfate pentahydrate and was standardized electrogravimetrically as described b!Killard and Furman (7'). Baker and Adamson's "reagent quality" zinc was amalgamated x i t h :il)out 2y0 of mercury by shaking it for about 10 minutes in a mercuric chloride solution in dilute hydrochloric acid. -%tfirst, a solution of mercuric nitrate in dilute nitric acid was used to amalgamate the zinc; however, the chromous solution obtained by using this amalgamated zinc alxays had a normality about 2 to 3 7 , too low, although the amalgam was washed thoroughly before use. An approximately 0.1S ferric alum solution, acidified with was used in the back-titration of excess sulfuric acid (to lS), chromous chloride solution. The solution was freed of oxygen by passing nitrogen through it for about 15 minutes. The nitrogen was freed of oxygen in the usual manner. The solution was standardized either by titrating with standard permanganate (f)portions of the solution which were reduced by amalgamated zinc in a Jones reductor or by titrating aliquots with standard chromous solution. Stmdard solutions of the organic compounds investigated were generally prepared by dissolving a known weight, of the purest commercially avai1:ible material in water, or in glacial acetic acid, if the substance \vas insoluble in water. The sample of nitroguanidine was recrystallized from water two times and air dried. One of the solutions of the monopotassium salt of acetylene dicarboxylic acid was prepared from material synthesized according to the method of Moureu and Bongrand ( 2 ) . Procedure. Except where otherwise specified the follon-ing procedure was employed. Suitable aliquots of the solution to be analyzed were pipetted into t,he titration vessel a-hirh cont,ained about 15 ml. of water. Ten milliliters of concentrated hydrochloric acid were added, and the initial volume of the solution was adjusted to about 50 ml. by adding water. Carhon dioxide, freed from traces of oxygen by passage through acidified chromous solution in contact, with amalgamated zinc, was bubbled through the solution for about 10 minutes. At the end of this period the carbon dioxide n-as passed over the surface of the solution. An excess of 0.1000.\- chromous chloride solution was added and generally the solution was allowed t o stand about 1 t o 2 minutes, depending on the rate of reaction, before backtitrating n-ith standard ferric alum solutions. The titrations were performed at room temperature. Since platinum is a catalyst for the decomposition of chromous ion by hydrogen ion, the platinum electrode was kept out of the solution until it was time to back-titrate at, which time it n-as lowered into the solution. -1 similar procedure ivas emplo>-ed n-ith the saturated calomel electrode. I n the determination of p-nitrobenzeneazoresorcinol, 25 nil. of concentrated hydrochloric acid had to be used in place of the usual 10 ml., otherwise a precipitate would form. The samples of nitroguanidine and of the monopotassium salt of acetylene dicarboxylic acid were not prepared in acid medium. The aliquots were added t o enough water t o give an initial volume of about 50 ml. I n the presence of hydrochloric or sul-
Table I.
Analysis of Organic Compounds Using Chromous Chloride
hleq. Minimum No. of Taken Excess, % hleq. Found Detn. 4 200 0 . 6 3 5 f 0.004 o-Nitrobenzoic acid 0,6345 3 0.903 200 0.905 f 0.003 4 1.264 f 0 . 0 0 4 1.269 200 3 1,806 200 1.808 f 0.008 2.4.6-Trinitrobenzoic acid 0.7965 300 0.795 f 0.007 4 1.593 1.596 f 0 , 0 1 9 200 2.4.6-Trinitroresorcinol 0 , 5 1 1 300 0.511 f 0.003 6 0.635 300 0.630 f 0.007 5 1.022 250 1.011 f 0.005 8 1.270 4 1 . 2 5 9 f 0.005 250 2,4-,Dinitrophenyl hydra- 0 . 5 2 8 5 450 0 . 4 2 5 i 0.004 zine 0.857 250 0.861 f 0 . 0 0 7 Nitrogiianidine 0,462 200 0.448 f 0.003 3 4 0.495 f 0.002 0.501 250 0,904 200 0 890 f 0 . 0 0 2 3 1.002 0 . 9 9 0 i 0 008 250 10 p-Xltrobenzeneazoresor0 369 250 0.371 f 0.002 4 rinol 0 738 250 0.731 f 0 . 0 0 3 6 250 0.500 3 0 . 4 9 3 i 0.002 250 4 1 000 0 995 f 0 . 0 0 1 Kitroso R salt 0.425 300 4 0 423 f 0 004 0 537 300 0 536 i 0 001 3 0.850 300 0 850 & 0 004 5 4 1.074 260 1 074 f 0 002 Nonopotassium salt of O.517a 400 0 514 f 0 004 6 400 0 528 f 0 005 5 acetylene dicarboxylic 0 5275 acid 0 661; 0 662 & 0 002 400 3 1.034~ 250 1 025 f 0 003 4 1 . 0 5 5 f 0 001 250 3 1.055 3 1,323 230 1 325 f 0 002 .Inthraquinone-2,7-di1.000 1 006 f 0.001 3 sodium sulfonate 4 1,187 1.194 f 0.000 2.000 4 2.015 f 0.002 2.374 2 396 f 0.002 4 a Material synthesized according to proredure of AIoureu and Bongrand
s
furic acid, the results were verj- low. In the case of nitroguanidine, the use of a citrate butrered solution did not improve the results appreciably, and in addition, the end-point response was sluggish as compared with a nonbuffered solut,ion. In these determinations it n-as not necessary to allow the solutio11 to stand for several minutes before back-titrating the excess chromous solution. The determination of 2,4,6-trinitrobenzoic acid had to be performed a t an elevated temperature. At room temperature in either hydrochloric or sulfuric acid solution only about 707' reduction n-as obtained, whereas in a citrate buffered solution. the reduction was increased t o about SO'%. The procedure that was finally adopted involved heating a hj-drochloric arid solution of the sample to 85' C. and allowing it to cool to 5 5 " C. before back-titrating the excess chromous chloride. A blank using the same solvent conditions as used for the eamples was run on the chromous chloride for each set of reductions. The anthraquinone 2,i-disodium sulfonate as well as an impure sample of 1-nitro anthraquinone-5-sodium sulfonate was t i t r a t d directly in a hydrochloric acid medium. The reaction at the ; a &minute wait was allowed before end point n-as s l o ~ therefore making the final reading. Heating did not improve the encfpoint response. RESULTS A i i D DISCUSSIOS
Table I cont,ains the results t,hat were obtained for the conipounds investigated in this study, which indicates that satisfactory results can be obtained. The minimum excess bf chromous chloride t o lie added to :I given sample, so as to obtain satisfactory results, varied with the nature of the compound and generally depended on ita concentration in the solution. Therefore, a systematic investigation was necessary to carry out the per cent reduction for a given added excess at a particular Concentration of the sample. I n general. for 10 nil. of an approximately O.1S solution an excess of 200 to 250y0 n-as sufficient. K i t h smaller samples the required excess may be the same as or slightly greater than with samplesof a higher concentration, or it may be considerably greater as ill the cade of the monopotassium salt of acetylene dicarhaxylic acid or 2,4dinitrophenylhydrazine.
1184
ANALYTICAL CHEMISTRY
Tht: indirect determination of anthraquinone 2,7-disodium sulfonate using the general procedure invariably produced low results. This was due to the very easy oxidation of the anthrahydroquinone by ferric ion. However, the indirect determination of 1-nitroanthraquinone 5-sodium sulfonate gave the same results as the direct titration. Since the sample was impure, these data are not presented in Table I. The first appreciable voltage change (about 200 mv.) was taken to be the end point for the back-titration. After this break, there was a gradual voltage change with added ferric solution, and finally another substantial break corresponding to the oxidation of the anthrahydroquinone. This 1% as observed qualitatively in that the reddish-brown color of the anthrahydroquinone eventually gave way to the characteristic green color of the chromic ion. This study of the use of chromous chloride for the deterniination of reducible organic materials is being continued. A more thorough study of nitroso and acetylenic compounds is now in
progress. An investigation Rill be made of the possibility of determining hydrazo compounds, diazonium salt9, and certain types of carbonyl compounds, such as a-diketones, by chromous chloride reduction. LITERATURE CITED
Lingane, J. J., and Pecsok, R. L., .&SAL. CHEM., 2 0 , 4 2 6 (1948). Moureu, C., and Bongrand, J. C., -4nn. chim.,1 4 , 4 7 (1920). Reilley, C. N., Cooke, W. D., and Furman, K.H., A N A L .CHEY., 23, 1223 (1951).
Someya, K., 2. anorg. u. allgem. Chenz., 169, 293 (1928). Terent’ev, A. P., and Goryacheva, G. S., Uchenuie Zapiski (Wiss. Ber. Moskau Staats-Univ.), 3 , 277 (1934). Willard, H. H., and Furman, N. H., “Elementary Quantitative Analysis,” 3rd ed., pp. 230-1, Van Kostrand, New York, 1940.
Ibid., p. 444.
RECEIVED for review December 4 , 1954. Accepted February 8, 1955.
Precise Determination of Chloride in Plasma by Differential Potentiometric Titration VINCENT P. DOLE and NlELS A. THORN The Hospital of The Rockefeller Institute for M e d i c a l Research, N e w York 21,
Chloride in blood plasma can be titrated directly with a differential potentiometric method. ~h~ precision of the measurement (0.1% coefficient of variation between replicates) has permitted study of small but systematic fluctuations in concentration of electrolyte, The sharp end point of the titration it possible to determine chloride in highly dilute solutions. The rapidity and ease of the method, once the apparatus is assembled, suggest that it might be of general value for clinical work.
N. Y.
mixing of the whole solution when the sleeve is raised. When the sleeve is lowered, the rubber ring a t the foot presses onto the bottom of the beaker with the weight of the lead collar and prevents appreciable mixing of the inner and outer solutions, although the solutions remain in electrical contact through a film of electrolyte. The electrodes are made in the usual way: Silver and silver chloride are deposited electrolytically on a platinum wire sealed into the end of a glass tube (1). The glass shield can be used interchangeably with any electrode-an advantage over the original design of MacInnes and others. A coat of sili-
D
IFFERENTL4L potentiometric titration gives a more exact measure of chloride than any other method now availablr. MacInnes and associates (7, 8) reported an average difference between replicate analyses of only 0.003% on titration of large volumes of inorganic solution with two silver chloride electrodes, one of which dipped into the main body of solution and the other was enclosed in a segregated portion. A small amount of silver nitrate added t o the outer solution under these conditions reduces the external concentration of chloride but leaves the segregated portion unaffected. The resultant potential between the t\vo similar electrodes measures the rate of change of titration potential with change in concentration of chloride and has a sharp maximum a t the end point. The method, although simple and rapid, has been neglected, possibly because of the special equipment required and because it was not known that i t could be applied to biological mixtures R ithout preliminary ashing Twelve years later Cunningham, Kirk, and Brooks ( 2 ) demonstrated that acidification of blood plasma suppressed the formation of silver proteinates and permitted direct potentiometric titration of chloride. Their method, however, employed bimetallic electrodes and thus registered cumulative rather than differential changes of potential on titration. Kirk ( 5 ) noted the possible advantages of a direct differential method for the microdetermination of halides, but left the idea for future development.
Plcistic holder
2
Figure 1. Titration assembly PROCEDURE
The apparatus shown in Figure 1 was designed to isolat,e one electrode when the glass sleeve is lowered, and to permit rapid
Left. Details of electrode pair and glass sleeve Right. Complete unit including weighing bottle, buret, and capillary for gas stirring
