problems in fast reactors that boron creates in thermal reactors.
be applied to the determination of nitrogen in, for example, Ti, Ta, and W. Work is now in progress, however, to extend the technique to the determination of nitrogen in other materials including those which form refractory nitrides.
CONCLUSIONS Micro-inert gas fusion chromatography has been shown to be a very sensitive, accurate, and relatively precise technique for the determination of hydrogen and‘ nitrogen in small samples. The technique has been used in this laboratory t o determine hydrogen and nitrogen contents ranging from 1 t o 1000 wt ppm. At present the technique is limited t o samples where chemical decomposition is not required for the release of nitrogen or hydrogen, and therefore cannot
ACKNOWLEDGMENT The assistance of C. H . Knox is gratefully acknowledged. RECEIVED for review July 10, 1970. Accepted November 4, 1970. This work was supported in part by the U. S. Atomic Energy Commission.
Novel Method of Determining Weak Bases in Small Amounts Eli0 Scarano, Marco Mascini,’ and Giovanni Gay Institute of Analytical Chemistry, Faculty of Pharmacy, University of Genoa, Italy
CRITCHFIELD AND JOHNSON (1, 2) have found that concentrated (up to saturation or 8 M ) aqueous solutions of strong acid-strong base salts (NaCl, NaI, LiCI, CaC12) can be conveniently used as media for visual and potentiometric titrations of bases with pKb as high as 11-12. This is due to the enhanced potentiometric break at the equivalence point. The high protonation of weak bases is explained in terms of low activity of water and high hydrogen ion activity. Hisashi Kubota and Costanzo (3) have carried out potentiometric titrations of hydrolyzable cations in 10M LiCl. Rosenthal and Dwyer ( 4 ) have studied acid-base equilibria in concentrated salt solutions, particularly 4 and 8M LiCI. These authors also agree in the enhancement of the hydrogen ion activity in these media. Moreover dilute hydrogen chloride and concentrated lithium chloride solutions have high values of the hydrogen chloride gaseous pressure, which are much higher than that of hydrogen chloride aqueous solutions a t the same hydrogen chloride concentration ( 5 ) . So a n aqueous 0.1M HC1 solution can be boiled for 1 hour without appreciable loss of hydrogen chloride if the evaporated water is continuously replaced (6), while a solution -5 X 10-3M of hydrogen chloride in 1 3 M LiCl loses 1 (or 50%) of hydrogen chloride if a nitrogen volume 6 (or 420) times that of the solution is forced to pass through it (5). In this paper a simple and inexpensive method is described for weak base (pKb up to 10) determinations in the pmole range. The method is based o n the high hydrogen ion activity and the high hydrogen chloride pressure of dilute hydrogen chloride-concentrated lithium chloride solutions. A nitrogen flow forced through the hydrogen chloride-lithium
chloride solution strips the hydrogen chloride which is collected and determined. Two hours of stripping are sufficient to collect practically all the hydrogen chloride. The base is dissolved in a known amount of hydrogen chloride and introduced into the lithium chloride saturated solution. Only the excess of hydrogen chloride is stripped, thus allowing the base determination. EXPERIMENTAL C. Erba reagent grade LiCl (with 0.03%, as Li2C03,declared alkalinity) was used as purchased or in the form purified by crystallization. The acid-base substances [with pK, at 25 “C (7)] were: Merck (according to Sorensen) glycine (2.35); Merck reagent grade succinic acid (4.21); Merck reagent grade aniline (4.60); Merck p-toluidine (5.09); Aldrich Chemical picolinic acid (5.32); Merck sodium-5-5 ’-diethylbarbiturate, for buffer (7.98). The apparatus shown in Figure 1 consisted of two 15-ml glass tubes, with ground glass sockets; a glass bridge with caps for the tubes and holes for burets; two microburets (1 ml, 0.01-ml division) with the closure o n the tip (8). Experiments were carried out in a thermostatic bath a t 25 “C. Saturated lithium chloride solution 13 grams (about 10 ml) and 1.2 grams of lithium chloride were weighed in the A tube; 2 ml of water and 0.1 ml of lO-*M methyl red solution were poured into tube B. The tubes were tightly joined together with the glass bridge. A buret filled with a standard hydrogen chloride solution was inserted in tube A ; a buret with 0.01 or 0.02M N a O H in tube B. A stream of nitrogen (120 + 5 ml/min) was then passed through the solutions into the tubes. The hydrogen chloride solution was poured into tube A . The stripped hydrogen chloride passed into tube B where it was titrated a t regular intervals of time with the sodium hydroxide solution. Preliminary investigation proved that hydrogen chloride did not escape from tube B.
Present address, Institute of Analytical Chemistry, University, of Rome, Italy. (1) F. E. Critchfield and J. B. Johnson, ANAL.CHEM., 30, 1247 (1958). ( 2 ) Ibid., 31, 570 (1959). (3) H. Kubota and D. A. Costanzo, ibid., 36, 2454 (1964). (4) D. Rosenthal and J. S . Dwyer, ibid., 35, 161 (1963). (5) D. Glietenberg and M. von Stackelberg, Ber. Bunsenges. Phys. Chem., 72, 565 (1968). (6) H. A. Laitinen, “Chemical Analysis,” McGraw-Hill, New York, N. Y., 1960, p 85.
442
a
RESULTS AND DISCUSSION The influence of lithium chloride purity is shown in Figure 2. The presence of a base in the commercial product was ~
~
(7) V. E. Bower and R. G. Bates, “Handbook of Analytical
Chemistry,” L. Meites, Ed., 1st ed., McGraw-Hill, New York, N. Y., 1963, pp 1-20. (8) E. Scarano and M. Forina, J. Chem. Educ., 47,482 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
, c
La=
I
'd'
2
1
3
4
5
time, h
Figure 3. Influence of lithium chloride concentration solution); (b) 19m; (c) 18m. Solutions prepared with commercial product. 100 pmole hydrogen chloride added for each experiment
( a ) 19.75m (saturated
b
a
Figure 1. Apparatus (a) A tube (stripping cell); (b) B tube (analysis cell); (c) glass bridge; (d) holes for burets
10
5
added
If.
HCI ,
20
pmole
Figure 4. Recovery of hydrogen chloride after 2-hour stripping Lithium chloride saturated solutions prepared with commercial product
1
2
3
4
time, h Figure 2. Influence of lithium chloride purity Saturated solutions prepared with : (a) commercial product; (b) monocrystallized lithium chloride; (c) bicrystallized lithium chloride; (d) tricrystallized lithium chloride; 10 pmole hydrogen chloride added for each experiment
confirmed. However, the declared alkalinity was higher than that found. The amount of base found depended o n the stock of lithium chloride used. The recovery of hydrogen chloride was not complete even after 4 hours with tricrystallized lithium chloride. This should be due to the residual alkalinity and to the very low hydrogen chloride stripping rate a t low values of hydrogen chloride concentration.
The hydrogen chloride volatility was strongly dependent o n the lithium chloride concentration and diminished with it (Figure 3). The curve in Figure 4 shows the relation between the micromole of hydrogen chloride recovered after a 2-hour stripping and the added micromole of hydrogen chloride in the lithium chloride saturated solution. The curve becomes a straight line between 6 and 20 pmole of recovered hydrogen chloride, with a slope of 1 i 0.005. The intercept depended on the lithium chloride stock solution used and its basic impurity. Points above the straight line, at low hydrogen chloride micromole values, can be explained in terms of competition between the two following equilibria: C1-
+ H'
=
HC1 (g)
(1)
where g refers to the gaseous phase.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
443
Table I. Determination of Sodium-J,5’-Diethylbarbiturate. Recovered hydrogen chloride, pmole Experiment Blank Sample 1 8.10 7.00 2 8.00 7.00 3 8.00 6.90 4 7.95 7.05 Av 8.01 Av 6.99 Base given 1.01 pmole. Base found 1.02 pmole. Blank solution: 10-2M HCl. Sample solution: 10-*M HCl and 1.01 X lO-3M C8H1,03N2Na. Eight experiments were carried out; four with the blank solution and four with the sample solution; 1 ml of solution was used for each experiment. Saturated lithium chloride solution prepared with commercial product.
1
2
3
time, h
Figure 5. Behavior of weak acids (a) HCI; (b) glycine hydrochloride; (c) succinic acid. Lithium chloride saturated solutions prepared with commercial product. 10 pmole of acid for each experiment
When the added hydrogen chloride was in strong excess, COS2-was rapidly and completely eliminated as COz, which was not absorbed and detected in tube B . When instead the hydrogen chloride quantity was inferior to the base present, the faster Reaction 1 prevailed and some hydrogen chloride was stripped. However, these points can also be explained with the presence of a nonvolatile partially-protonated weak base (pKb > 10) in the lithium chloride (see below). The effects of nonvolatile acids of different strengths when added in tube A , as hydrogen chloride substitutes, are shown in Figure 5 . Medium strength acids were able to donate protons to chloride ions, thus allowing hydrogen chloride to be stripped by the nitrogen stream. When a n acid of lower strength was used, the rate of hydrogen chloride stripping decreased. The weaker acids, with pK, 7 4, such as aniline hydrochloride, p-toluidine hydrochloride, picolinic acid, 5,5’-diethylbarbituric acid and boric acid, did not give any hydrogen chloride production.
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The above results permitted simple determination in the micromole range of bases with p K b up to 10. Each base was dissolved in a known amount of hydrogen chloride and the excess of hydrogen chloride (6-20 Fmole) was recovered. A blank was carried out with the same amount of hydrogen chloride under the same experimental conditions. The base quantity was given by the difference between the two recovered quantities of hydrogen chloride. The base could be determined as accurately as the measurements of the volumes of the hydrogen chloride and sodium hydroxide standard solutions. F o r example, the results relative to the minimum determined quantity of a base are reported in Table I . The very small required quantities of bases reduced problems caused by the low solubility of bases in the concentrated lithium chloride solutions ( 4 ) . An advantage was offered by the solubility power of the hydrogen chloride solution. The method appears unsuitable for volatile acids and bases. However, it should be useful for base determinations in the presence of colored materials. By improving the hydrogen chloride stripping process and by using a more sensitive and reliable method for stripped hydrogen chloride determination, the amount of base to be analyzed may be reduced and precision may be increased. Principles underlying this work have been developed for studies of solvent systems (9).
RECEIVED for review August 18, 1970. Accepted October 26, 1970. (9) E. Scarano, G. Gay, and M. Forina, ANAL.CHEM., 43,206 (1971).
