Modified Methyl Yellow Indicator For Direct Titration of Sodium Carbonate W A L T E R R. C A R M O D Y , Boston College, Chestnut Hill, Mass.
T
HE peculiar difficulties associated with the titration of sodium carbonate with hydrochloric acid are due in the main to the pronounced buffer effect of the solution on either side of the end point. This buffer effect decreases markedly the response of the indicator to additions of acid, which correspondingly decreases the precision with which one may determine the end point. The end point occurs in the middle of a “transformation interval” throughout which the color change is gradual and, consequently, is not distinguished by a sudden change in color and may not possess a distinctive hue that may be reproduced precisely without comparison with a standard of reference. Although the use of modified indicators apparently has not become common practice, it has long been known (4, 6, 7) that more satisfactory results may often be obtained with such an indicator. The modified indicator possesses two advanthges. I n the first place the modifying colored substance may filter out a portion of those wave lengths that are not appreciably absorbed by either the acid form or the base form of the indicator. Thus, the change occurring a t the end point is made more apparent to the eye. Again, the possibilities of varying the relative amounts of indicator and modifying substance permit production of an indicator that a t the end point may have a distinctive or familiar hue which may be more easily reproducible than that resulting from the indicator alone. Several modified (a, 8) or mixed (3) indicators have been suggested for use with the titration of sodium carbonate. The author’s only justification for suggesting another is that he has found the one herein described more satisfactory for precise work. As indicator for this work methyl yellow was selected rather than the more commonly used methyl ofange because the former 100
80 W
V
Z
$
60
1 METHYL YELLOW
5Z a
(BASE)
2 METHYL ORANGE (east1 3 METHYL ORANGE (ncio)
!= 40
L
400
I
1
I
500
600
700
WAVE L E N G T H IN MILLIMICRONS Figure 1. Absorption Curves for Meth I Orange, Methyl Yellow, and Methylene h u e
was found to give more easily determinable color changes a t the end point. Absorption curves for the two forms of methyl orange and methyl yellow and for methylene blue are shown in Figure 1. Data for the curves were obtained with a Coleman Universal spectrophotometer. Those marked methyl yellow (acid) and methyl orange (acid) were obtained in solutions buffered a t p H 2.2; those marked methyl yellow (base), methyl orange (base), and methylene blue, in solutions buffered a t pH 7.0. These curves compare favorably with those obtained by Mellon and Fortune (6); however, they are presented here only for comparative purposes. A comparison of the curves in Figure 1 indicates the superiority of methyl yellow over methyl orange and the suitability of using methylene blue as a color-modifying substance. Methylene blue has been found satisfactorily t o modify such yellow-red indicators as methyl red (2, 6) and neutral red (5),while Kolthoff (6) has reported that equal parts of methylene blue and methyl yellow produce an excellent indicator with transformation point 3.3. EXPERIMENTAL
Transformation point and sensitivity were investigated on mixtures of alcoholic solutions of methylene blue and methyl yellow of varying proportions and concentrations. These were made using buffer solutions and with carbon dioxide-saturated solutions of 0.25 N and 0.10 N sodium chloride. These mixtures are identical with those obtained when 0.5 N and 0.2 N hydrochloric acid, respectively, constitutes the titrating acid and a n equivalent quantity of sodium carbonate is dissolved in a volume of water equal to that of the acid to be used for the titration. A proportion of 20 parts of methyl yellow to one part of methylene blue was found to yield a n indicator with a sensitive transformation point a t the equivalence point of the titration of sodium carbonate with 0.2 N hydrochloric acid a t 20’ C. This same proportion may be used for titrations involving 0.5 N acid, since the difference in the end point in terms of 0.5 N acid represents less than 0.01~’ of the volume of the acid consumed in the titration. I n alkaline solution the modified indicator yields a yellow-green hue; as the end point is approached this changes to a straw color, then to a sunburned straw, and finally to a pink straw a t the end point. A concentration of 0.8 gram of methyl yellow and 0.04 gram of methylene blue per liter of alcohol was found to be suitable. Approximately 0.1 ml. of this mixture is used per 100 ml. of solution contained in the titration flask a t the end point. The end point is taken aslthe point a t which the first trare of pink is observed in the solution. During the course of this work it was noted that the methyl yellow indicator fades rapidly in alkaline solution. The effect of this upon the end point is appreciable if the indicator is added before the titration is started. This effect may be eliminated by deferring addition of the indicator until the titration is more than half completed. To determine the precision obtainable when the mixed indicator described above is used, several 100-ml. portions of 0.1 A’ solution of sodium chloride were kept saturated with carbon dioxide a t 20” C. and titrated back and forth with standard acid and base from the basic side to the first appearance of pink in the solution. The results obtained indicated that as far as the sensitivity of the indicator is concerned the precision obtainable is 0.04% with 0.2 N acid and 0.02% with 0.5 N acid. This indicates the possibility of obtaining a precision better than that obtained previously by the direct method. To test the indicator adequately, therefore, it was necessary to use methods which would ensure a precision of at least 1 part in 5000. 141
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
142 Table
I. Corrections for Carbon Dioxide Unsaturation and for Temperature Deviations
Unsaturation correction, mi. of HC1 Correction for temperature deviation, HC1 % per a C.
Normality of HCl 0.2 0.6 -0.02 0.00 -0.01 -0,004
Table It. Direct Titration of Sodium Carbonate with Hydrochloric Acid, Using Modified Methyl Yellow Indicator (Hydrochloric acid, prepared concentration, 0.2113) Equivalent of HCl Run Weight of NarCO: Weight of Acid per 1000 G r a m
1 2 3 4 5
Gram
Qrams
0.7004 0.6024 0.6189 0.6621 0.5628
82.53 44.88 55.27 59.12 60.27
0.2113 0.2112 0.2113 0.2113 0.2112
Deviations common to the direct titration of sodium carbonate are those ordinarily associated with the use of volume burets, those caused by the fact that the end-point solution may not be saturated with carbon dioxide a t atmospheric pressure, and those caused by the variation of the end-point color with temperature. To ensure a suitable precision in mechanical operations weight burets were used. The values of “unsaturation correction” and the correction for the deviation of the end-point color with temperature were determined for a particular set of titration conditions. Under the conditions specified and over a wide range of sample size and titration speed, the “unsaturation correction” was nil when 0.5 N hydrochloric acid was used and remarkably constant and reproducible when 0.2 N acid was used. The cor-
Vol. 17, No. 3
rection for temperature deviations from 20” is not large but becomes significant when temperature variations as large as 5” or 6” are encountered. Results are summarized in Table I. The titration vessel was a 250-ml. wide-mouthed titrating flask. The sample was dissolved in a volume of water approximating that of the acid required for the titration. The indicator was added when the titration was more than half completed. The direct titration of samples of sodium carbonate was carried out under the specified conditions. The hydrochloric acid was made up from constant-boiling acid prepared by the method of Foulk and Hollingsworth (1). The sodium carbonate was “volumetric standard” material obtained from a well-known firm. The factor of purity of this salt was determined by comparison with standard hydrochloric acid by the indirect method. Results of the direct titration are given in Table 11. The weights of sodium carbonate shown have been corrected for the factor of purity (0.9998) and the weights of hydrochloric acid have been corrected for carbon dioxide “unsaturation” and for deviations of the temperature of the end-point solution from 20” C. The results indicate that the modified methyl yellow indicator is suitable for precise titration of sodium carbonate with 0.2 N hydrochloric acid. LITERATURE CITED
(1) Foulk and Hollingsworth, J . Am. Chem. Soc., 45, 1220 (1923). (2) Hiokman and Linstead, J . Chem. SOC.,121,2502 (1922). (3) Hoffner, Deut. Zuckerind., 61,361 (1936). (4) Kolthoff, “Acid-Base Indicators”, New York, Macmillan Go., 1937. ( 5 ) Kolthoff and Rosenblum, Biochem. Z.,189,26(1927). (6) Mellon and Fortune, J . Am. Chem. SOC.,60,2607 (1938). (7) Scholta, 2.Elektrochem., 10,549 (1904). (8) Smith and Croad, IND.ENG.CHEM.,ANAL.ED.,9,141 (1937).
Application of Colorimetry to the Analysis of Corrosion-Resistant Steels Photoelectric Determination of Titanium OSCAR MILNER, KENNETH L. PROCTOR,
AND
SIDNEY WEINBERG
Industrial Test Laboratory, United States Navy Yard, Philadelphia, Pa.
A method using hydrogen peroxide for the determination of traces of titanium in corrosion-resistant steel has been developed and adapted to the photoelectric colorimeter. Titanium cannot be completely separated from large amounts of iron, chromium, and nickel by the use of a single cupferron precipitation. A spectro-
photometric study suggests that the effect of these interferences can best be overcome b y a compensating blank reading. Molybdenum and vanadium are removed by the use of sodium peroxide. The procedure is outlined in detail and the validity of the method is established b y the analysis of samples of known titanium content.
DURING
standard titanium solution is added. The method is subject to personal error in the comparison, especially if the titanium content is high, and is limited to solutions containing not more than 0;l mg. of titanium per ml. ( 9 ) . It often requires the addition of impurities to the comparison solution to compensate for interferences remaining in the unknown. Methods for the application of the colorimetric comparison to the determination of titanium in steel have been published previously (3,6, 7,9). It has recently become necessary to determine titanium in steels containing as little as O . O O l ~ oof this element. These steels are frequently of the 19% chromium-9yo nickel and 25% chromium-20% nicke! types. This has made desirable the development of a more accurate and precise method of estimating the titanium color. The photoelectric colorimeter hss been used for the determination of titanium, but application to the analysis of corrosion-
the past year this laboratory has conducted an extensive investigation of methods for the determination of residual elements in corrosion-resistant steels. ks a result, a number of accurate and precise colorimetric methods hhve been developed for the determination of certain elements a t low residual levels. This publication represents the first of these methods. Others will be presented later, as sufficient analytical data become available. PHOTOELECTRIC DETERMINATION OF T I T A N I U M
Small amounts of titanium are commonly determined by the colorimetric procedure using hydrogen peroxide. I n this procedure, the titanium is separated from interfering elements and isolated in a sulfuric acid solution. Hydrogen peroxide is then added, and the resultant color is matched by a blank to which
