Novemher 15,1934
IN D U ST 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
449
and the zinc is detected in the filtrate by the DETECTION OF VANADIUMIN THE ALUMINUM DIVISIONhydroxide, hydrogen sulfide. The filtrate from the treatment with sodium hydroxide and sodium peroxide may contain sodium chromate, vanadate, aluminate, and zincate. If it is just acidified with dilute nitric acid (1 to 3) and an excess of 3 cc. of acid are added for each 50 cc. of solution, the chromate may be removed as the lead salt upon the addition of an excess of lead nitrate. After filtration, the presence of chromium is confirmed by the use of nitric acid and hydrogen peroxide. If to the filtrate 1 or 2 grams of ammonium acetate are added, the pH will be sufficiently increased to cause complete precipitation of the vanadium as Pb,(VO&. As little as 0.5 mg. of vanadium may be detected in this way after the removal of the equivalent of 50 mg. of potassium chromate as the lead salt. The lead vanadate is filtered off and washed thoroughly to remove soluble lead salts, which would otherwise interfere with the end test for vanadium. Three or four drops of dilute ammonium hydroxide are poured over the lead vanadate on the paper and washed through with 5 cc. of water. This solution is then saturat,ed with hydrogen sulfide to yield a red solution of ammonium sulfovanadate, (NH4)aVSd,if vanadium is present. The filtrate from the lead vanadate is treated with 3 or 4 cc. of 3 N hydrochloric acid, and the excess of lead is precipitated by the use of hydrogen sulfide. The lead sulfide is filtered off and washed with 5 or 10 cc. of N hydrochloric acid to dissolve any zinc sulfide that may have been precipitated because of the presence of the ammonium acetate. After the removal of the sdfide by boiling, the aluminum is separated by ammonium
us8
of
Solutions which may contain tungsten, molybdenum, titanium, and vanadium in addition to the common metal ions have been issued t o the author's students as unknowns near the end of a semester course in elementary qualitative analysis, with satisfactory results. Not only do the students appear interested in the direct contact with these metals, which they recognize as of considerable commercial importance, but they are introduced to many of 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 of chromate and vanadate as the lead salts. LITERATURECITED (1) Browning, P. E., Simpson, G. S., and Porter, L. E., Am. J . Sei., 42,106 (1916). (2) Noses, A. A., Bray, W. C., and 30,481 (1908).
Spear, E. B., J . Am. Chem. Sot..
(3) Porter, L.E., IND.ENG.CHEW,Anal. Ed., 6 , 138 (1934). RECEIWDJuly 12, 1934. Research Paper 355, Journal Series, University of Arkansas.
Potentiometric Titration in Nonaqueous Solutions 11. A Source of Error in Acidimetry LELPNDA. WOOTENAND A. E. RUEHLE,Bell Telephone Laboratories, New York, N. Y.
I
N ACIDIMETRY in butyl
I n acidimetry in alcoholic solution a weak
khitg'';
~ ~ w,a,Ss~s,"f",~~ ~ ~ ~ or amyl alcohol solution, acid, resulting from oxidation of the alkali work reported. The solvent was using as reagent an alkali solution, may be introduced into the system as P $ ~ ~ ~ t ~f ~~t ~ ! ' ' ~ l l ~ ? ~ ~ ; metal hydroxide in the same the alkali salt. A simple quantitative tesf for carried out at reduced pressure (30 solvent, several workers have rethe presence of impurities, in the form of weak ported the appearance of anomalous points of inflection on the acid salts or weak bases, in the alkali solution oxide as a dehydrating agent. titration curves (or maxima on may be made by titrating portions of standard w ~ ~ ~ ~ ~ ; $ ~ ~ ~ l f $ o ~ the AEIAV curves) of both used picric acid. acidimetric reagent was found to oils and single monobasic acids offer several advantages over po(b,3 ) . Recently, in t i t r a t i n g tassium or sodium h y d r o x i d e . picric, trichloroacetic, and dichloroacetic acids in butyl alcohol This reagent, prepared by the interaction of metallic sodium and solut,ion the authors have obtained similar titration curves pure n-butyl alcohol in a reducing atmosphere, is carbonate-free is reasonably stable if properly protected from light and the (Figure 1). The appearance of two inflection points on the and atmosphere. This may be conveniently accomplished by storing titration curve of a monobasic acid can be attributed only under hydrogen in a light-proof bottle. t o the presence, as a n impurity, of a considerably weaker or Deterioration of t8healkali solution is indicated by a decrease stronger acid than the one being titrated. A systematic in neutralizing value in terms of benzoic acid, which is Itccomsearch for the source of this impurity has led to conclusions panied as the solution ages by the appearance of a yellow color and a slight turbidity. A quantitative test for the purity of the which are of general importance in connection with the use alkali reagent is described in the last section of this paper. of alcoholic solutions for precise acidimetry. The quinhydrone used in most of the work QUINHYDRONE. was obtained from the Eastman Kodak Company. RecrystalliAPPARATUS zation from butyl alcohol did not perceptibly improve its quality. The apparatus used consisted of a thermionic titrometer, Quinhydrone prepared by the method of Valeur ( 7 ) was used an electrode system, a thation cell, and a storage system for in a few titrations. The quinhydrone was stored in a dark bottle and was dissolved immediately before use. alkali solution such as those previously described (1). The collection of the data in this paper was facilitated by the SOLVENT.The solvent employed in most of the work was the use of the thermionic tit,rometer, by which the slope of the titra- practical grade of n-butyl alcohol supplied by the Eastman tion curve is read directly. This instrument also made possible Kodak Company, used both with and without further purithe elimination of neutral salts such as lithium chloride, which fication. The method of purification employed was distillation have heretofore been used to reduce the resistance of butyl at reduced pressure from barium oxide. alcohol solutions (4, 5 ) . BLANKON REAGENTS.The value of the blank titration on 100 cc. of solvent containing 50 mg. of quinhydrone usually was REAGENTS found to be 0.05 cc. or less of 0.05 N alkali solution. If the ALEALI SOLUTION.A solution of potassium hydroxide in blank exceeded this value the solvent was redistilled as described n-butyl alcohol (0.05 N ) , prepared and stored as described in above.
~
~
~
~
~
~
~
ANALYTICAL EDITION
450
0
500
I 1
CC
.
OF REAGENl
FIGURE1. DIFFERENTIAL TITRATION OF PICRICACID 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 masima. 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 between 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 two 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.' TABLEI. EFFECTOF DOUBLING QUANTITYOF ACID ACIDTITRATED
TITB~R OF IO-cc. SAMPLB T I T ~ OF R 20-oc. S A M P L ~ J 1st 2nd Diff. 1st 2nd Diff.
cc.
cc.
cc.
cc.
cc.
cc.
Pioric 4.04 4.60 0.56 8.05 9.18 1.13 Triohloroacetic 4.59 5.27 0.68 9.20 10.67 1.47 Picric" 2.73 2.86 0.13 6.50 5.74 0.24 a Picric acid from a different source titrated with a different alkali solution.
These experiments eliminated four of 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
Vol. 6, No. 6
the alkali reagent. Ten cubic centimeters of an approximately 0.3 N butyl alcohol solution of butyric acid were added to 500 cc. of alkali solution which had been freshly prepared and which showed but one maximum in the titration of picric acid. When picric acid was titrated with the alkali thus contaminated, a second maximum appeared (Figure 3 4 ) . Addition of a second 10-cc. aliquot of acid to the remainder (384 cc.) of the alkali solution more than doubled the difference between the two maxima when titrating the same quantity of picric acid (Figure 3 4 ) . 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 experiments 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 of 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 which 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 maximum on the titration curve of picric acid. The fact, repeatedly observed, that an alkali solution, even when prepared from a pure solvent and protected from the atmosphere will, after a period of time, exhibit two maxima when titrating picric acid may therefore be attributed to oxidation of the alcohol, resulting either from leakage of air into the alkali 700 storage s y s t e m or from the 600 presence of traces of oxygen or other oxidizing agents in the solvent. 500 The strength of the acid resulting from the oxidation of a s 4oo n-butyl alcohol s o l u t i o n of o alkali appears to be approxi- 2 3oo mately the same as acetic, since when titrating a c i d s w e a k e r than dichloroacetic only one ,oo maximum o c c u r r e d on the titration curve. This conclusion seems justified in view of 0 3 4 5 6 7 the results obtained in titratCC OFREAGENT ing mixtures of acetic with each F~~~~~ 2. T~~~~~~~~OF of the c h l o r o a c e t i c acids in PICRIC ACIDUSINGA PURE n-butyl alcohol, u s i n g pure SOLUTION OF ALKALI alkali solution. Two maxima were obtained in the titration of mixtures of acetic and dichloroacetic while only one maximum occurred in the titration of a similar mixture of acetic and monochloroacetic acids. The magnitude of the errors resulting from the use of an impure solution is limited by 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 of which could lead to errors in acid values as high as 30 per cent. The use of technical alcohol without purification as the alkali solvent, or storage
INDUSTRIAL AN D ENGINEERING CHEMISTRY
November 15,1934
of the solution without adequate protection from oxygen, may lead to errors equally as high. DISCUSSION OF RESULTS The results of the experiments that when titrating a moderately alcohol using an alcoholic solution may be introduced into the system
described above show strong acid in n-butyl of alkali, a weak acid as the alkali salt; and
451
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. Without 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 may be missed entirely, owing to the limitations of the indicator electrode; or the proximity of the maxima may 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.
1000
800
y
600
;i
\
$
400
200
200
6
'4
8
10
CC
12 14 OFRLAGENT
16
I0
FIGURE3. DIFFERENTIAL TITRATION OF PICRIC ACID Using alkali solution contaminated with butyric acid; b Same as a, with increased amount of butyric acid. a.
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. An 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 HX may be represented as follows:
HX
'
+ R:{+-:
+
N ~ X HA
B
g
a
I50 100
50 0
0
I
2
3
5 6 7 CC OF REAGENT
8
9
10
11
12
FIGURE4. DIFFERENTIAL TITRATION OF MIXTURES OF TRICHLOROACETIC AND ACETIC ACIDS a,
I n water;
b. I n 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.
+ ROH
Thus the weak acid HA accumulates until the neutralization of the stronger acid HX 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). That conditions are more favorable for the separation of the end points of two acids of 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 neutralizing power when titrating a weak acid than when titrating a strong one. If benzoic acid is used for standardizing an alkali solution contaminated by 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 maximum is selected as the correct end point. If the second
4
CONCLUSION
A simple quantitative test for the presence of impurities, in the form of weak acid salts or weak bases, in the alkali solution may be made by titrating portions of standard picric acid solution, carrying the titrations as far as possible into the alkaline region. The sensitivity of the test is determined by the quantity of acid titrated, the normality of the reagent, and the size of 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 of the blank titration on the solvent. LITERATURE CITED (1) Clarke, Wooten, and Compton, IND.ENO.C H ~ MAnal. ., Ed., 3, 321 (1931). (2) Evans and Davenport, Ibid., 3, 82 (1931). (3) Ralston, Fellows, and Wyatt, Ibid., 4, 109 (1932). (4) Seltz and McKinney, IND. ENQ.CHEM.,20,. 542 (1928), (5) Seltz and Silverman, Ibid., Anal. Ed., 2, 1 (1930). (6) Tizard and Boeree, J. Chem. SOC.,119, 132 (1921). (7) Valeur, Ann. chim. phys., (7) 21, 547 (1900).
RECIOIYED June 2, 1934.
