titanium, and vanadium in addition to the common metal ions have been issued to the author’s students as unknowns near the end of a semester course in elementary qualitative analysis, with satisfactory results. N ot only do the students appear interested in the direct contact with these metals, which they recognize as of considerable commercial impor
tance, but they are introduced to many o f their properties, including several oxidation and reduction reactions with their several striking color changes. There is also ample opportunity for class discussion of the theory involved, such as the influence of hydrogen-ion concentration in making possible the separation o f chromate and vanadate as the lead salts.
Li t e r a t u r e Ci t e d
(1) Browning, P. E .t Simpson, G . S., and Porter, L, E., A m . J. Sci.t 4 2 ,100 (19 16).
(2) N oyes, A. A ., Bray, W. C., and Spear, E. B., J. Am . Chem. Soc 30,481 (190S).
(3) Porter, L . E., I n d . E n g . C h e m ., Anal. E d., 6 ,1 3 8 (1934).
Re c e i v e d July 12, 1934. Research Paper 355, Journal Series, University of Arkansas.
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Potentiometrie Titration in Nonaqueous Solutions
II. A Source of E rror in Acidimetry
Le l a n d A. Wo o t e n a n d A . E . Ru e i i l e, Bell Telephone Laboratories, N ew Y ork , N . Y .
I
N A C ID IM E T R Y in butyl or am yl alcohol solution, using as reagent an alkali metal hydroxide in the same solvent, several workers have reported the appearance of anoma
lous points of inflection on the titration curves (or maxima on the A E / A V curves) of both used oils and single monobasic acids (3 ,3 ). Recently, in t i t r a t i n g
picric, trichloroacetic, and dichloroacetic acids in butyl alcohol solution the authors have obtained similar titration curves (Figure 1). The appearance of two inflection points on the titration curve of a monobasic acid can be attributed only to the presence, as an impurity, of a considerably weaker or stronger acid than the one being titrated. A systematic search for the source of this impurity has led to conclusions which are of general importance in connection with the use of alcoholic solutions for precise acidimetry.
Ap p a r a t u s
The apparatus used consisted of a thermionic titrometer, an electrode system, a titration cell, and a storage system for alkali solution such as those previously described (/).
The collection of the data in this paper was facilitated by the use of the thermionic titrometer, by which the slope of the titra
tion curve is read directly. This instrument also made possible the elimination of neutral salts such as lithium chloride, which have heretofore been used to reduce the resistance of butyl alcohol solutions (4, 5).
Re a g e n t s
A l k a l i S o l u t i o n . A solution of potassium hydroxide in n-butyl alcohol (0.05 A ), prepared and stored as described in
the former paper ( /) , was used as a standard reagent in most of the work reported. The solvent was purified as described previously, except that the distillations were carried out at reduced pressure (30 to 40 mm. of mercury), and barium oxide was substituted for calcium oxide as a dehydrating agent.
The use, in the latter part of this work, of sodium buloxide as an acidimétrie reagent was found to offer several advantages over po
tassium or sodium h y d r o x id e . This reagent, prepared by the interaction of metallic sodium and pure n-butyl alcohol in a reducing atmosphere, is carbonate-free and is reasonably stable if properly protected from light and the atmosphere. This may be conveniently accomplished by storing under hydrogen in a light-proof bottle.
Deterioration of the alkali solution is indicated by a decrease in neutralizing value in terms of benzoic acid, which is accom
panied as the solution ages by the appearance of a yellow color and a slight turbidity. Â quantitative test for the purity of the alkali reagent is described in the last section of this paper.
Q u i n i i y d r o n e . The quinhydrone used in most of the work was obtained from the Eastman Kodak Company. Recrystalli
zation from butyl alcohol did not perceptibly improve its quality.
Quinhydrone prepared by the method of Valeur (7) was used in a few titrations. The quinhydrone was stored in a dark bottle and was dissolved immediately before use.
S o l v e n t . The solvent employed in most of the work was the practical grade of n-butyl alcohol supplied by the Eastman Kodak Company, used both with and without further puri
fication. The method of purification employed was distillation at reduced pressure from barium oxide.
B l a n k o n R e a g e n t s . The value of the blank titration on 100 cc. of solvent containing 50 mg. of quinhydrone usually was found to be 0.05 cc. or less of 0.05 N alkali solution. If the blank exceeded this value the solvent was redistilled as described above.
In acidimetry in alcoholic solution a uieak acid, resulting from oxidation o f the alkali solution, may be introduced into the system as the alkali salt. A simple quantitative lest fo r the presence o f impurities, in the form o f weak acid salts or weak bases, in the alkali solution may be made by titrating portions o f standard picric acid.
450 A N A L Y T I C A L E D I T I O N Vol. 6, No. 6
A c i d s . Picric acid was obtained from both the Eastman Kodak Company and Merck & Co. Recrystallization from butyl alcohol did not effect any improvement in quality. Solu
tions of picric acid in n-butyl alcohol were found to be very stable, no change in acid value being observed over a period of several months.
The other acids were obtained from the Eastman Kodak Company and were used without further purification.
Ex p e r i m e n t a l
Experiments summarized below were designed to test each of the following possible sources of an acid impurity:
the butyl alcohol solvent, the quinhydrone, the nitrogen used for stirring, the acid being titrated, and the alkali reagent.
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CC. OF R E AG E N T
Fi g u r e 1 . Di f f e r e n t i a l Ti t r a t i o n o f Pi c r i c Ac i d Showing anomalous maximum obtained when using an old solution
of alkali.
It was found that neither increasing the amount of solvent or quinhydrone used, nor purifying these materials, had any effect on the difference between the first and second maxima. Varying the quantity of nitrogen passed through the solution during a titration was without effect, and the same result was obtained when the nitrogen was carefully purified. Purification of the picric acid used did not eliminate the anomalous maximum, nor did it alter the distance be
tween the two maxima. On the other hand, it was observed that the anomalous maximum on the titration curve occurred only when the alkali reagent was comparatively old ; that the distance between the tw o maxima, for a given titer, increased with the age of the alkali solution; and that the distance between the two maxima was directly proportional to the volume of alkali solution used. The data in Table I show the effect of doubling the quantity of acid titrated, thereby doubling the titer to both the first and second end points. The alkali solution used in these titrations was approximately 2 months old.
T a b l e I. E f f e c t o f D o u b l i n g Q u a n t i t y o f A c i d Tit e ro f 10 -c c. Sa m p l e Ti t e ro f 2 0 -c c. Sa m p l e
Ac id Ti t r a t e d 1st 2nd Diff. 1st 2nd Diff.
Cc. Cc. Cc. Cc. Cc. Cc.
Picric 4 .0 4 4 .6 0 0 .5 6 8 .0 5 9 .1 8 1 .1 3
Trichloroacetic 4 .5 9 5 .2 7 0 .6 8 9 .2 0 10.67 1 .4 7
Picric0 2 .7 3 2 .8 6 0 .1 3 5 .5 0 5 .7 4 0 .2 4
° Picric acid from a different source titrated with a different alkali solution.
These experiments eliminated four o f the five possible sources of the unknown acid and pointed to the butyl alcohol solution of alkali used as reagent. Further experiments definitely proved that the source of the unknown acid was
the alkali reagent. Ten cubic centimeters of an approxi
mately 0.3 N butyl alcohol solution of butyric acid were added to 500 cc. o f alkali solution which had been freshly prepared and which showed but one maximum in the titra
tion of picric acid. When picric acid was titrated with the alkali thus contaminated, a second maximum appeared (Figure 3-a). Addition of a second 10-cc. aliquot of acid to the remainder (384 cc.) o f the alkali solution more than doubled the difference between the two maxima when titrat
ing the same quantity of picric acid (Figure 3-6). Good agreement, within the estimated experimental error, was obtained between the calculated and the experimentally found increase in titer between the two maxima. A titration of benzoic acid with the contaminated alkali solution yielded but one maximum on the titration curve. These experi
ments were repeated, employing benzoic and acetic acids as contaminants, with similar results.
Several experiments were performed which indicate how the condition artificially produced above can occur normally.
A technical grade of commercial n-butyl alcohol was used without purification, for the preparation o f an alkali solution.
A titration of picric acid with this newly prepared reagent showed two maxima. The same solvent, after purification, was used for an alkali solution which, when titrating picric acid, did not exhibit a second maximum. An alkali solution, which was prepared from pure solvent and wliich did not give the second maximum when titrating picric acid, was exposed to light and air (protected only from carbon dioxide) for several days. The titration curve of picric acid then revealed two maxima. It was also found that saturating a pure alkali solution with oxygen and then allowing it to stand with no protection from light for 24 hours, would produce a change causing the appearance of a second maxi
mum on the titration curve of picric acid. The fact, re
peatedly observed, that an alkali solution, even when pre
pared from a pure solvent and protected from the atmosphere will, after a period of time, exhibit two maxima when titrating picric acid m ay therefore be attributed to oxidation of the alcohol, resulting either from
leakage of air into the alkali storage s y s t e m o r from the presence of traces of oxygen or other oxidizing agents in the solvent.
The strength of the acid re
sulting from the oxidation o f a n-butyl alcohol s o l u t i o n of alkali appears to be approxi
mately the same as acetic, since when titrating a c id s w e a k e r than dichloroacetic only one maximum o c c u r r e d on the titration curve. This conclu
sion seems justified in view of the results obtained in titrat
ing mixtures o f acetic with each of the c h l o r o a c e t i c acids in n-butyl a l c o h o l , u s in g pure alkali solution. T w o maxima
were obtained in the titration of mixtures of acetic and di
chloroacetic while only one maximum occurred in the titra
tion o f a similar mixture of acetic and monochloroacetic acids.
The magnitude of the errors resulting from the use of an impure solution is limited b y the solubility of the salts of weak acids present. The authors have produced artificially contaminated alkali solutions, employing butyric acid as the contaminant, the use o f which could lead to errors in acid values as high as 30 per cent. The use o f technical alcohol without purification as the alkali solvent, or storage
Fi g u r e 2 . Ti t r a t i o n o f Pi c r i c Ac i d Us i n ga Pu r e
So l u t i o n o f Al k a l i
November 15, 1934 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 451 of the solution w ithout adequate protection from oxygen,
may lead to errors equally as high.
Di s c u s s i o n o f Re s u l t s
The results of the experiments described above show that when titrating a moderately strong acid in n-butyl alcohol using an alcoholic solution of alkali, a weak acid may be introduced into the system as the alkali salt; and
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Fi g u r e 3 . Di f f e r e n t i a l Ti t r a t i o n o f Pi c r i c Ac i d
a. Using alkali solution contaminated with butyric acid; b. Same as a, with increased amount of butyric acid.
that the presence of the weak acid salt in the alkali solution probably is associated with the oxidation of the alcohol, or with the oxidation or condensation of aldehydes present as impurities.
A n alcoholic solution of alkali thus contaminated may be considered to be a mixture of two bases of different strengths, and the neutralization reaction in the presence of an excess of the strong acid H X m ay be represented as follows:
=* + K E-
■ N aX + HA + ROHThus the weak acid H A accumulates until the neutralization o f the stronger acid H X is completed, and is then itself re-neutralized, causing the appearance of a second maximum on the titration curve. Whether or not the second maximum occurs depends upon the relative strengths of the acids, their relative concentrations, and in a special way upon the solvent (6). T h at conditions are more favorable for the separation of the end points o f two acids o f different strengths in the solvent n-butyl alcohol than in water is shown by Figure 4.
The above equation clearly shows that an alkali solution, contaminated by a weak acid salt, will be weaker in neutral
izing power when titrating a weak acid than when titrating a strong one. If benzoic acid is used for standardizing an alkali solution contaminated b y the alkali salt of an acid of the same order of strength as benzoic, and subsequently this alkali solution is used for titrating an oil sample which contains a strong acid, the resulting acid value will be in error in the sense of being too low, assuming the first maxi
mum is selected as the correct end point. If the second
maximum is used in calculating the acid value, however, the correct result will be obtained, since the weak acid titrated must be exactly the equivalent of the salt used in neutralizing the strong acid originally present in the oil sample. Again, if a mixture of strong and weak acids is present in the sample, the second maximum must be selected to give the correct total acidity. W ithout presuming a knowledge of the compositions both of the alkali reagent and of the solution being titrated, however, it is impossible to select in every case the end point leading to the correct acid value, when using an alkali solution contaminated by the salts of weak acids. Moreover, one of the maxima m ay be missed entirely, owing to the limitations o f the indicator electrode; or the proximity of the maxima m ay result in a flat or ill-defined end point. Possibility of error is excluded only if the acid present in the sample is of the same order of strength as benzoic, when only one maximum occurs.
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F i g u r e 4. D i f f e r e n t i a l T i t r a t i o n o f M i x t u r e s o f T r i c h l o r o a c e t i c a n d A c e t i c a c i d s
a. In water; b. In n-butyl alcohol.
It is also clear that no valid conclusions can be drawn as to the presence or absence of mixtures of strong and weak acids in used oils, unless precautions are taken to insure the absence of salts of weak acids from the alkali reagent and to employ a solvent of low blank.
Co n c l u s io n
A simple quantitative test for the presence of impurities, in the form of weak acid salts or weak bases, in the alkali solution m ay be made by titrating portions of standard picric acid solution, carrying the titrations as far as possible into the alkaline region. T h e sensitivity of the test is determined by the quantity of acid titrated, the normality of the reagent, and the size o f the increment of reagent added in the region of the end points. In interpreting the curve obtained it is, of course, essential to take into account the value o f the blank titration on the solvent.
Li t e r a t u r e Ci t e d
( 1 ) Clarke, W ooten , and C om pton, I n d . E n o . C h e m . , Anal. E d . , 3 , 321 (1931).
(2) Evana and D avenport, Ibid., 3, 82 (1931).
(3) Ralston, Fellows, and W ya tt, Ibid., 4, 109 (1932).
(4) Seitz and M cK in ney, I n d . E n o . C h e m . , 2 0 , 542 (1928).
( 5 ) Seitz and Silverman, Ibid., Anal. E d ., 2 , 1 (1930).
(6) T izard and Boeree, J . Chem. Soc., 1 1 9 , 132 (1921).
(7) Valeur, A nn. chim. phys., (7) 2 1 , 547 (1900).
Re c e i v e d June 2 , 1 9 3 4 . *