Titration of Acids in Nonaqueous Solutions An Improved Quaternary Ammonium Hydroxide Titrant for Strong Acids ROBERT H. CUNDIFF and PETER C. MARKUNAS R. J. Reynolds Tobacco Co., Winston-Salem, N. C.
b Although tetrabutylammonium hydroxide, prepared by the reaction of tetrabutylammonium iodide and silver oxide, is an excellent titrant for weak, very weak, and mixtures of weak and very weak acids, its use for resolution of acid mixtures containing a strong acid leads to small but significant errors. The source of these errors, an impurity in the titrant, may be eliminated by passing the titrant through a short section of a strongly basic anion exchange column. This yields a titrant suitable for determining all types of acids and acid mixtures. HE use of quaternary ammonium T h ydroxide as a titrant for acids in nonaqueous solutions has been reported by several investigators ( I , 2. 4-6'). Deal and Wyld (4) used a 0.2Atetrabutylammonium hydroxide titrant in ivater-isopropyl alcohol solution. Harlow, Noble, and Wyld (6) and Bruss and Wyld (1) used 0.2N tetrabutylammonium hydroxide in isopropyl alcohol as a titrant. The authors ( 2 ) used 0 . l K tetrabutylammonium hydroxide in benzene-methanol, while Fritz and Yamamura (6) used 0.1N triethyl-n-butylammonium hydroxide in benzene-methanol as a titrant, Tetrabutylammonium hydroxide prepared by the reaction of silver oxide with tetrabutylammonium iodide is an excellent titrant for the determination of weak, very weak, or mixtures of weak and very weak acids. However, thi? titrant is not entirely satisfactory for differentiating titrations of acid mistures containing a strong mineral acid. During work in measuring exactly the two equivalents of sulfuric acid by titration with tetrabutylammonium hydroxide, the two equivalents were almost always unequal. The total acidity values were always correct, but the second equivalent was consistently higher than the first. The magnitude by which the second equivalent was high corresponded very closely to the value by 11-hich the first equivalent \%-as low. All acid mixtures containing a strong acid yielded results which were not exactly in accord with the expected
1450
ANALYTICAL CHEMISTRY
values. Generally the magnitude of the error was in the range from 1 to 3% although in some instances it was as high as 6 to 7%. These discrepancies were due to impurities in the titrant and to solvent-solute reactions. No difficulty of either type was experienced in the determination of acids or acid mixtures when strong acids were not present in the solution. Passing the titrant through a small anion exchange column effectively removed the impurities, making it satisfactory for the titration of all types of acids. Pyridine is the only solvent reported in this study. The authors found other solvents recommended for use with quaternary ammonium hydroxide titrants react in varying degrees with strong acids ( 3 ) . I n this work as in the previous investigation ( d ) , acids similar in strength to the mineral acids are designated strong acids, those similar in strength t o the unsubstituted monocarboxylic acids are designated weak acids, and those similar in strength to phenol are designated very weak acid.. . REAGENTS AND APPARATUS
Tetrabutylammonium iodide, obtained from Rymark Laboratories, Terre Haute, Ind. Tetrabutylammonium hydroxide, 0.1N, in 10 to 1 benzene-methanol Dissolve 80 grams of tetrabutylamnionium iodide in 180 ml. of reagent grade absolute methanol. Place in a n ice bath, add 40 grams of finely ground silver oxide, stopper the flask, and agitate intermittently for 1 hour. Filter through a sintered-glass funnel of fine porosity, rinse the flask, and precipitate with three 50-ml. portions of cold benzene and add to the filtrate. Dilute the filtrate to 2 liters with dry benzene. An equimolar portion of tetrabutylammonium bromide may be substituted for the tetrabutylammonium iodide. Use of the bromide permits the agitation time to be cut to 15 minutes. To prepare an anion exchange column, fill a 25 X 400 mm. chromatographic tube about half full with dmberlite resin IRA-400 (OH-), analytical grade. Pass 2N sodium hydroxide solution through the column until the eflu-
ent gives a negative halide test. Rinse with distilled water until the effluent is neutral t o Alkacid test paper (Fisher Scientific Co., Pittsburgh, Pa.). Pass 500 ml. of absolute methanol through the column, followed by 500 ml. of 10 to 1 benzene-methanol. Pass the tetrabutylamnionium hydroxide solution through the column, collecting the effluent when basic to Alkacid paper. Allow the solution to pass through the colunin at the rate of 15 to 20 ml. per minute. Collect the tefrabutylammonium hydroxide in a flask protected from carbon dioxide and moisture, and store in a reservoir protected from these. This solution is stable for a t least 60 days; longer storage periods have not been tried, Pyridine Allow technical grade pyridine to stand overnight over sodium hydroxide pellets, then flash distill. This material has a very low blank due. Beckman general-purpose glass electrode, S o . 4990-80. Becknian sleel-e-type calomel elecsrode Yo. 1170-71, modified by replac:ng the saturated aqueous potassium chloride solution in the outer jacket with A saturated solution of potassium chloride in methanol ( 2 ) . Xicroburet, 10 ml. (80G1155, FischerPorter Co., Hatboro, Pa.), or equivalent, equipped with a Teflon stopcock. PROCEDURE
For all acids or acid mixtures, 0.7 to 0.9 meq. of acid were dissolved in 50 ml. of pvridine and titrated potentiometx a l l y under a nitrogen blanket. The end points were determined from a plot oi potential us. volume of titrant. The solvent blank was subtracted from the final end point. The titrant n.as standardized against benzoic acid by the same procedure. .kids which were not readily soluble in pvridine were first dissolved in 1.5 inl, ,of n-ater, 50 ml. of pyridine were d d e r i . :and the solution was titrated. EXPERIMENTAL
I n the titration of sulfuric acid with tetrabutylanimonium hydroxide, two inflections were obtained in the potentiometric curve, the volume to the first end point representing the first equivah i t . and the difference in volumes between the second and first end points
representing the second equivalent of the acid. T h e n using the nonanion exchanged titrant, the volumes of titrant for the two equivalents were not exactly equal although the volume of titrant for total acidity was stoichiometric. This is evidenced by the data in Table I, where tetrabutylammonium hydroxide solutions from t n o different preparations 1% ere used. The results based on either the first or second equivalents indicated serious discrepancies. These errors also varied n i t h the batch of titrant and would be more significant in differentiating titrations of mixture. containing sulfuric acid. The anomaliei observed in the titration of sulfuric acid were noted also in titrating any mixture containing a strong acid. This iq illustrated in Table 11, nhich lists results obtained with nonanion exchanged tetrabutylammonium hydroxide in determining hydrochloric acid, benzoic acid, and phenol: a mixture of typical strong, neak. and w r y neak acids. Three inflections were obtained in the potentiometric curve (Figure l), the first representing hydrochloric acid. the second benzoic acid, and the third phenol. Good results were obtained for the phenol portion of the mixture and the total values, although slightly high, have good precision. The hydrochloric acid values were all low, while the benzoic acid values xere correspondingly high. This compares to the results for the first and second equivalents of sulfuric acid (Table I). These values indicated that a n impuritj may be present in the titrant which reacted with a strong acid, such as hydrochloric acid or the firqt equivalent of sulfuric acid, to form a compound that subsequently titrated as a weak acid. The total acidity remained unchanged, but in determining sulfuric acid a portion of the first equivalent would titrate n ith the second equivalent, or in a h j drochloric acid-benzoic acid mixture a portion of the hydrochloric acid mould titrate as benzoic acid. There n as no interference in titration of weak and very weak acids, as a strong acid must be present for this side reaction to occur. The impurity niight have been carried over from the tetrabutylammonium halide used in preparing the hydroxide; however, preparations from the highest purity quaternary ammonium halides failed t o produce a titrant completely free of the impurity. Thereafter, tributylamine {vas considered the most logical contaminant, -4s addition of tributylamine to the strong acid solution. did not alter the results on titration n-ith the anion exchanged titrant, it does not contribute t o the obserrrd tliscrepanciw. Another possibility ia
bath yielded a titrant containing less impurity than one prepared at room temperature, which lends credence to this bossibility. The contaminants in the titrant were effectively removed by passage t,hrough an anion exchange column as described (Tables I11 and IV). Sulfuric acid solutions were titrated in the same manner as before, substituting t,lie anion exchanged tetrabutylamnioniuni hydroxide. Table I11 gives the results obtained x i t h two separate batches of tetrabutylammonium hydroxide. Table 11’ lists results obtained in titration of mixtures of hydrochloric acid, benzoic acid, and phenol with anion exchanged tet,rabutylammoaium hydroxide.
Figure 1. Titration of hydrochloric acid, benzoic acid, and phenol in pyridine with 0.1 N tetrabutylamrnonium hydroxide
DISCUSSION
The titrant used by Harlolv, Soble, and Vyld (6) n-as prepared entirely by anion exchange of the tetrabutylainnioniuin iodide, and should not be subject to the discrepancies of the titrant originally proposed by the authors (2).
that the impurity was a by-product of the reaction of the quaternary ammonium halide with the silver oxide. Preparation of tetrabutylammonium hydroxidp in a flask held in a n ice
Table I.
Determination of Sulfuric Acid by Titration with Nonanion Exchanged Tetrabutylarnrnonium Hydroxide
Titrant
Added, Found, Recovery, Found, Mg. 23 64 47 47 70 91 47.84 46.18 68.55
A
B a c
70
h1g.a 22 35 44 51 66.76 47.09 45.41 67.81
1lg.b 25 21 49 83 75 03 48.56 46.99 69.28
94.6 94.2 94.2 98.4 98.4 98.9
Recovery,
7c
106 6 105 4 105 8 101.5 101.8 101.1
Found,
Recovery,
Mg.c
7c
23 78 47 17 70 90 47.83 46.20 68.55
100 6 99 8 100 0 100 0 100.0 100.0
Based on volume of titrant to first end point. Based on difference in volumes between second and first end points. Based on total volume of titrant.
~~
Table 11.
Analysis of Acid Mixtures by Titration with Nonanion Exchanged Tetrabutylammoniurn Hydroxide Bcid Added, Mg. Acid Found, M y . yo Recovery
HC1
Benzoic Phenol
2- 81 -~ 5.03 10.07 5.03 5.03 15.10
10 44 15.63 15.63 31.26 15.63 46.89 ~.
Table 111.
~~
8 64
12.89 12.89 12.89 25.79 38.68
HC1
Benzoic
Phenol
2 61 4.85 9.70 4.74 4.85 14.77
10 80 16.12 16.73 32.12 16.12 47.75
8 67 12.89 12.80 12.89 25.89 38.68
103.1 107.0 102.7 103 1 101.8
100.0 99.3 100.0 100.3 100.0
Total 100 9 100.9 101.7 101.2 100.9 100.5
Determination of Sulfuric Acid by Titration with Anion Exchanged Tetrabutylarnmoniurn Hydroxide
hdded, Found,
Recovery,
Found,
Recovery,
JIg. l\Xg.5 e’ /c Mg,* A 13 90 13 89 13 98 100 0 21 94 21 94 21 94 100 0 29 41 29 30 99 ‘7 20 41 37 34 37 38 100 1 37 25 B 23 64 23 68 23 78 100 2 32 51 32 47 99 9 32 47 47 27 47 16 99 8 47 36 5 Based on volume of titrant to first end point. b Based on difference in volumes hetn-een serond and Based on total volume of titrant.
Titrant
HC1 Benzoic Phenol 103 4 100 3
92 9 96.4 96.3 94.3 96.4 97.8
70
100 100 100 99 100 99 100
6 0 0 8 6 9
2
Found, JIg c 13 93 21 94 29 37 37 34 23 73 32 47 47 2G
Recovery,
70
100.2 100.0 99.9 100,o 100,4 99.9 100.0
first end points.
VOL. 30, NO. 9, SEPTEMBER 1958
1451
ACKNOWLEDGMENT
Table IV.
Analysis of Mixtures in Pyridine Solution b y Titration with Anion Exchanged 0.1 N Tetrabutylammonium Hydroxide
Acid Added, Mg. HCl Benzoic Phenol 7.04 14.07 7.04 14.07 '14.07 7.04
26.13 26.13 52.27 52.27 26.13 52.27
21.55 21.55 21.55 43.10 43.10 43.10
Acid Recovered, Mg. HC1 Benzoic Phenol
yo Recovery HC1 Benzoic Phenol
Total
6.97 14.00 6.97 13.93 14.00 6.97
98.9
100.0
99.4
io0.o
99.8 99.8 99.7 99.8
25.88 26.13 52.i5 52.15 26.13 52.15
The modification described, although adding to the preparation time, still affords the most expeditious means of preparing the titrant in nonaqueous media. The amount of impurity in the titrant prepared by reaction of tetraalkylammonium halide and silver oxide is small, particularly if high purity halide is used as a starting material and the reaction is allowed to
21.55 21.55 2i.55 43.10 42.91 43.10
99.5
98.9
99.0 99.5 98.9
99.1 100.0 .. ~
99.8 99.8 100.0 99.8
1on.n
100.0 99.6 100.0
99.9 ..._
proceed at low temperature. This small impurity may be removed by use of a comparatively small exchange column. The nonanion exchanged titrant may be used directly in the determination of weak and very weak acids. Use of the anion exchanged titrant is mandatory only in analysis of solutions containing strong acids.
The authors express their appreciation to Eleanor G. Rollins and Tony J. Miller who performed many of the determinations, and to A. J. Sensabaugh for preparation of the figure. LITERATURE CITED
(1) Bmm, D. B., Wyld, G. E. A, ANAL. CHEM.29, 232 (1957). (2) Cundiff, R. H., Markunm, P. C., Zbid., 28, 792 (1956). (3) Zbid., 30, 1447 (1958). (4) Deal, V. Z., Wyld, G. E. A., Zbid., 27, 47 (1955). (5) Fritz, J. S., Yamsmura, S. S., Zbid., 29, 1079 (1957). (6) Harlow, G. A., Noble, C. M.,Wyld, G. E. A., Zbid., 28, 787 (1956).
RECEIVED for review November 29, 1957 Accepted March 19, 1958.
Indirect Complexometric Ana lysis with Aid of Liquid Amalgams WILLIAM G. SCRIBNER and CHARLES N. REILLEY Department o f Chemistry, University of North Carolina, Chapel Hill,
L
N. C.
R ?vi +n zHg (1) where 0 is a soluble species in a higher oxidation state or an organic compound with a reducible functional group, M(Hg),. is a liquid amalgam of metal M, R is the reduced form of 0, and M+" is the metal ion liberated from the amalgam by reaction with 0. On the basis of Equation 1, there are sev-
era1 modes for quantitatively determining 0. The change in concentration of M in the amalgam may be measured before and after reduction, but this is hardly practical. A second method consists in analyzing the resulting reduced product, R. For example, iron(111), tin(I1) , titanium(1V), molybdenum(VI), metavanadic acid, tungstate, uranyl, chromate, and many more have been estimated in this manner. Various Japanese workers have been leaders in this field; most notable are Nakazano, Kikuchi, and Someya,
,The application of liquid amalgams lends a new perspective to the field of complexometric titrations. The principle involves reduction of one or more components of the sample by a metal amalgam with liberation of an equivalent quantity of metal ion from the amalgam. Thus, species which are not readily amenable to direct EDTA titration can be reduced with zinc amalgam, the zinc ions liberated being easily titrated. Another application i s to exchange an easily masked metal for one that i s not masked, or vice versa. In addition, advantage can be taken of the difference between equivalent weights for reduction and for complexometric titra-
tion-e.g., analysis of a bismuth(ll1)lead(l1) mixture by titration of aliqbots before and after reduction with zinc amalgam. Certain multicomponent mixtures as well as reducible organic compounds can be analyzed b y these techniques. The method i s related to control!ed-potential coulometry and similar equations apply; the titration yields a rapid, convenient integration of the current-time curve. An important advantage of the amalgam method i s the greatly decreased time required for quantitative reduction. A complete operation from introduction of sample through final titration requires only 15 to 20 minutes.
have found extensive application as reducing agents in analytical chemistry. The various modes of operation can be summarized by the reaction: IQUID AMALGAMS
0
+ M(Hg)=
1452
-+
0
+
+
ANALYTICAL CHEMISTRY
whose work has been reviewed by Brenneke (4) and by Kolthoff and Furman (12). Lobunetz and Per'e have extended the amalgam method to the determination of organic nitro compounds ( I S , 19, 23-26). An aromatic nitro compound is reduced with a liquid amalgam and the solution of the amine is titrated bromometrically. This method is limited by the fact that the product may not easily be titrated by this means. A third method consists in estimating the quantity of metal ion, M+", liberated. Tananaev and Davitashvili (31, 38) utilized liquid tin amalgam for the reduction of iron(III), tin(IV), mercury(11) and (I), antimony(V) and (111), arsenic(V) and (111),lead(II), silver(I), and others. I n cases where the metal ion to be determined is reduced t o the zero valence state, the equivalent amount of tin(I1) ion liberated from the amalgam is titrated with dichromate. I n other cases the metal ion is reduced to a lower valence state and consequently both resulting ions are titrated oxidimetrically. Tin amalgam is unique in this behavior. This paper reports an investigation of the third method, taking advantage of the fact that species such as zinc(II), cadmium(II), lead(II), and bismuth(11) can be determined rapidly and con-
