Nonaqueous Titration of Dilute Acids and Bases in Acrylonitrile M. L. OWENS,
JR., and
ROBERT L. MAUTE
Monsanto Chemical Co., Texas City, T e x .
Precise methods developed for the determination of small amounts of weak and strong acids and bases in acrylonitrile are rapid and free of interference from atmospheric carbon dioxide. The application of nonaqueous techniques has made possible the titration of low (0.002 weight %) concentrations of choline hydroxide, as well as acetic and acrylic acids with a precision of rtO.001 weight %. The procedures do not require blank corrections or special equipment, and thus results are more rapidly obtained and are more reliable than those obtained by the usual aqueous procedures.
C
EItT.4IS investigations made it necessary to develop rapid, reproducible methods for the routine determination of small amounts of organic acids and bases in acrylonitrile. For example, solutions of acrylonitrile containing 0.0005 to 0.0050 J\ eight % of the strong organic base, choline [(2-hydrouyethyl)trimethylammonium hydroxide], were required to be analyzed with a high accuracy and precision. Acrylonitrile solutions containing similar concentrations of acrylic and acetic acids \yere also under study. The colorimetric procedure of Dyer ( 1 ) is applicable for vholine and possibly choline salt, but the intensity of the color \vas found to be sensitive to small amounts of water. Acrylonitrile contains small, but varying, amounts of n-ater which precluded the use of this method. The determination of acids in organic solvents and monomers is usually carried out by extraction or dissolution ( 2 ) with water followed by titration of the extract employing standard aqueous acidimetric procedures. For precise determinations the water must be carbon dioxide-free and the titration must be carried out in a carbon dioxide-free atmosphere. Alternatively, blanks may be run to correct for carbon dioxide interference. When the values expected are 0.02% acid or higher these procedures usually suffice. However, some of the mixtures in this study contained less than 0.002% acetic or acrylic acids and preliniinary experiments indicated that the blank corrections frequently mere several times larger than the known acid concentrations. A more direct approach to both problems, acids and bases, seemed available in the recently emphasized methods of nonaqueous titration. Choline, as a strong base, and even the more n-eakly basic choline salts seemed amenable to titration, provided suitable titrants, solvents, and indicators could be discovered. Shortly after the laboratory phase of this work vias completed, the publication by Markunas and Riddick ( 4 ) of nonaqueous determination of choline salts of carboxylic acids pointed up the etfectiveness of this approach. Their nork involved the assay of certain pure choline salts; however, the present discussion is voncerned with trace analyses. Another recent reference of interest is that of Keen and Fritz ( S ) , who describe the nonaqueous titration of small amount. of amines. The literature abounds in applications of nonaqueous techniques to assay problems and macrotitrations, but the nonaqueous determination of trace components in high-purity commercial products seems to be a fruitful field, and these approaches are expected to become increasingly important. Indeed, the methods described here are recommended for consideration in routine application t o the determination of traces of acids or bases in the specification analyses of commercial acrylonitrile
samples. obvious.
Application to other monomers and solvents should be METHODS
The procedures used in this paper apply both potentiometric and indicator methods of detecting the end points. Seaman and Allen ( 7 ) suggested application of a potentiometric determination of the end point for each new acid or base considered. I n this manner it is possible to screen indicators and to determine the precise indicator shade at which an equivalence point has been reached. Bromothymol blue gives a sharp color change from yellow to blue when titrating as little as 0.001 weight % of acrylic or acetic acid in acrylonitrile. In titrating choline with standardized acid the reverse color change, from blue to yellow, occurs. The transition through a green shade is rapid. Although in routine practice, the indicator titration. are carried out directly in acrylonitrile without the addition of a solvent, potentiometric titrations involving acrylonitrile require the addition of solvents, especially, when commercially available glass and calomel electrodes are used. The G-H (glycol-hydrocarbon) solvent system described by Palit ( 6 ) proved satisfactory for the titrations of acids and the strong bases, such as choline or benzyltrimethylammonium hydroxide. For weaker bases, such as choline salts, the acetic acid solvent of Pifer and Wollish (6) gave good potentiometric end points. In this latter system, the visual indicator found most useful was crystal violet. REAGER-TS AND APPARATUS
Indicator Solutions. Dissolve 100 mg. of Harleco bromothymol blue in 100 ml. of C.P. methanol and adjust to the neutral (green) with dilute (0.01S) sodium hydroxide. Crystal violet indicator is made up to be O.lyO in acetic acid. Standardized Perchloric Acid Titrant, 0.02N. Dissolve 1.70 ml. of 72% perchloric acid (G. Frederick Smith Chemical Go.) in 1 liter of C.P. dioxane. Standardize potentiometrically against potassium acid phthalate dissolved in C.P. dioxane. Standardized Methanolic Sodium Hydroxide, 0.02N. Dis. hydroxide in 1 liter of c . ~meth. solve 0.80 gram of c . ~sodium anol. Protect from carbon dioxide absorption with an Ascarite tube on the buret. Glycol-Hydrocarbon Solvent. Equal volumes of ethgIene glycol (c.P.) and isopropyl alcohol (c.P.) are mixed. If technical grade solvents are used, corrections for traces of acids must be determined. pH Meter. Beckman Model H2, with standard glass and calomel electrodes, with magnetic stirrer was used. PROCEDURE
Organic Acids. To a 25-nil. sample of acrylonitrile containing organic acids add 6 drops of alcoholic bromothymol blue indicator. Titrate with 0.02S methanolic sodium hydroxide using a microburet until the color changes from yellow to blue. The color change is sharp. Organic Bases. To a 23-ml. sample of acrylonitrile containing c,holine or other strong base add 6 drops of bromothymol blue indicator. Titrate to the green end point using standardized (0.02N) perchloric acid in dioxane. Weak Organic Bases (basic salts of choline). To a 25-ml. bample of acrylonitrile containing organic bases add 23 ml of glacial acetic acid and 4 drops of c r y t a l violet. Titrate nith 0 02X perchloric acid in dioxane or acetic acid using crystal violet indicator. The indicator changes from blue to green a t the equivalence point. Potentiometric Titrations. In the determination of acids and strong bases 50 ml. of glycol-hydrogen solvent is added to a beaker and the apparent pH recorded. Acrylonitrile ( 2 5 ml.) is then added, with stirring. The deflection in apparent p H is observed and acid or base titrant is added as required until 1177
ANALYTICAL CHEMISTRY
1178 Table I.
Nonaqueous versus Aqueous Methods Known
W t . 70
0 0020 0 0035
Acetic acid in purified acrylonitrile
C oniniercial acrylonitrile I
6
Found W t . % Nonaqueous” Aqueousb 0.0020 0 OD60 0.0040 0.0070 0.0030
0.0110 0.0120 0.0140
DATA AND DISCUSSION
Indiestor titration, bromotl.ytii~d :.:,I?.. l:iLiirator titrstion. phenol~.l.rhalcin.
Table 11. Indicatora litrations
Acetic acid in acrylonitrile
.-1 rrylic arid in acrylonitrile
Clloline in acrylonitrile
Basic choline salt in acrylonitrile
Known
V-eight % Found
0.0020 0 0032 0.0071 0 0120
0.0020 0.0041 0.0069 0.0114
0.0013 0.0028 0.0066 0.0085 o on05 0.0008 0.0010 0,0020 0,0040 0 . no80
0.0015 0 . on35 0.0062 0.0080 0.0003 0.0008 0.0011 0.0016 0.0031 0.0072
0.0047
0.0040 0.0036 0.0083 0.0083 0.0078 0,0119 0 0203
0.0089 0.009i 0.0132 0.020,i a
Abs. error
0.0000 0.0009
0.0002 0.0006 0.0002 0.0007 0.0004 0.0005 0.0002
0.0000 0.0001 0.0004 0.0009 0.0008 0.0007 0.0011 0.0006 0.0006 0.0017 0.0013 0.0002
Bromothymol blur indicator.
Table 111.
LITERATURE CITED
KO.of Detn.
Std. Del..?, Wt. %
Commercial acrylonitrile
0 0004-0 0009b
8
0.00014
Acrylonitri!e plus 0.0044% of acetic acid
0 0047-0 0030b
3
0.00015
0 0094-0 0099; 0 0079-0 0083
7 7
0.00020 0 00019
acid
The usual aqueous methods of titrimetry were applied to the first samples studied. This was done by diluting acrylonitrile with 15 volumes of distilled water, followed by blank correct,ions for carbon dioxide present in the water. Although carbon dioxidefree water was used, and care was t,aken to minimize carbon dioxide absorption during the titrations, the blanks were high compared to the acid and base concentrations expected. The simpler nonaqueous procedures gave more reasonable values when applied t o purified (redistilled) acrylonitrile to which small amount,s of choline or acrylic acid had been added. Table I indicates the recoveries of acetic acid by both aqueous and nonaqueous methods. The procedures described gave very satisfactory recoveries. Table I1 relates experimentally determined values to calculated amounts of acetic and acrylic acids and choline as well as choline salts. The variations found were generally less than 0.001 n-eight yo and exhibit’ed no const,ant bias in the cases of acids and strong bases. However, in the case of weak bases, the results were frequently low., occasionally by as much as 0.0015 weight %, which was within the precision required for routine determinations during this study. The precision, defined as the standard deviation a t the 1-u level, of several samples is shown in Table 111. These show the precision to be 0.0002 weight yofor this series of samples.
Statistical Data Range Found, Wt. % Acetic Acid
Acrylonitrile plus 0.0088% acetic
the p H is returned to the original value. The titrant equivalent required for this operation is then calculated. I n the titration of weaker bases than choline, such as carboxylic acid salts of choline, the preferred potentiometric solvent is C.P. acetic acid with standard perchloric acid in acetic acid or dioxane as the titrant. For weak bases, plot the readings from the electromotive force scale, rather than pH, against titrant volume and determine the end point as the increment of titrant causing the largest potential change.
Standard deviation a t 1-u level. b Indicator (bromothymol blue) titrations.
a
Potentiometer (G-H solvent) titrations
Dyer, W. J., J . Fisheries Research Board of Can., 6, 351 (1945). (2) Jacobs, M. B., and Scheflan, L., “Chemical Analysis of Industrial Solvents,” pp. 209, 299, Interscience, New York, 1953. (3) Keen, R. T., and Fritz, J. S., ANAL.CHEM., 24, 564 (1952). (4) AIarkunas, P. C . , and Riddick, J. -4., Ibid., 24,312 (1952). (5) Palit, S. R., ISD. ENG.CHEY.,ANAL.ED.,18, 246 (1946). (6) Pifer, C. TV., and Wollish, E. G., ANAL.CHEM.,24, 300 (1952). (7) Seaman, W., and Allen, E., Ibid., 23, 592 (1951). (1)
RECEIVED for review December 11, 1934. Accepted January 31. 1935. Presented a t the Southwestern Regional AIeeting of the SOCIETY. Fort Worth, T e r . , Decemher 2 , 19.j4.
. ~ M E R I C ~C SHE~IICAL
Circular Paper Chromatographic Method for Estimation of Thiamine and Riboflavin in Multivitamin Preparations K. V. GlRl and S . BALAKRISHNAN Department o f Biochemistry, lndian lnstitote of Science, Bangalore 3, lndia
A simple circular paper chromatographic method has been developed for the separation and simultaneous estimation of vitamins Bi and B2 in niultivitamin preparations.
T
HE separation, identification, and quantitative estimation of B-complex vitamins bv paper chromatography has been drawing the attention of inany workers in recent years. Beran and Sicho ( 2 ) reported the conversion of thiamine into thiochrome on paper by treatment with alkaline ferricyanide and detected i t by its blue fluorescence under ultraviolet light. Miyaki and
others ( 1 2 ) described a procedure by which the thiamine in 95% ethyl alcohol containing logo sodium hydroxide was chromatographed and identified by spraying with diazotized p-aminoacetophenone and alkali, and applied this procedure for the quantitative estimation of thiamine by an area method. Fried ( 5 ) suggested a paper electrophoretic separation of thiamine from other fluorescent substances. ,4paper partition chromatographic method was described by Crammer ( 4 ) for the eeparation and identification of riboflavin and other flavine compounds. Hais and Pecakova (9) used this method for following the photolpsis products of riboflavin and applied it for the analysis of commercial injection solutions.
