Mar.,
T H E JOCRiYAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
1920
C 0 NCLU SI 0 h'S
The modified dichromate method as outlined above makes use of an exceedingly stable oxidizing salt which can be obtained in a very pure condition. The indicator employed is very delicate and affords an easily obtainable end-point. The method is more rapid than the straight dichromate method, and quite accurate. There seems t o be no reason why i t cannot be successfully used in commercial work. -
I N D I C A T O R S A N D THEIR INDUSTRIAL APPLICATION',2 By H. A. Lubs E. I.
DU
PONTDE
NEMOURS & Co., WILMINGTON, DEL.
Perhaps no single operation of t h e various procedures necessary in general chemical manipulation can be considered of greater importance than t h a t of acidimetric or alkalime tric titrations; yet in spite of this fact the subject of indicators is given but scant attention in the majority of our undergraduate colleges and universities. The most recent and useful developments in t h e field of indicators have arisen from the increasing recognition by chemists and biologists of the great influence which the degree of acidity or H+-ion concentration has upon various biological processes. I n order t o select t h e most brilliant and sensitive indicators for the colorimetric determination of H+-ion concentration in certain biological fluids, Sorensen and his co-workers3 at the Carlsberg Laboratory in Denmark investigated over one hundred dyes, determining the zones over which they changed, their brilliancy, and t h e effect of salt, protein, and protein decomposition products on their changing points. At the same time a great improvement was made in the preparation of standard solutions of known H+-ion concentration for colorimetric work. Some years later Rowntreej4 as a result of his physiological studies a t the Johns Hopkins Medical School, suggested t h e use of phenolsulfophthalein, a compound not described by Sorensen, as a useful indicator for the determination ion of t h e "+-ion concentration of blood. This indicator h a d been synthesized a number of years before by of Remsen, and subsequently studied S ~ h o na, student ~ b y Acree and co-workers from the standpoint of th8 quinone-phenolate theory of indicators. After a careful consideration of the work of Sorensen a n d others on indicators and upon investigating a very large number of compounds, it soon became evident t h a t t h e synthesis of new, brilliant, and sensitive indicators would assist materially in a solution of t h e biological problems in which Dr. W. M. Clark, of t h e Bureau of Animal Industry, and the writer were interested. Having in mind the brilliancy of phenolsulfophthalein and knowing from the quinone-phenolate Paper read a t the 58th Meeting of the American Chemical Society, Philadelphia, Pa., September 2 to 6, 1919. * Published by permission of the E. I. du Pont de Nemours & Co.. Wilmington, Del. * Comfit. rend. trau. Lab. Carlsberg, 1 (1909), 1; 9 (1909), 1: B i o c h e n . Z.,21 (19091, 131, 209, 61 (1913), 307; Ergebn. Physiol, 12 (1912), 393. 4 Archis. I n t e r n . Med., 16 (1915), 38. 6 Am. Chem. J., 20 (1898), 25.7. 1
theory of indicators as developed by Acreel and his coworkers, t h a t by suitable variations in the phenolic residue of a sulfophthalein dye there could be produced great variations in the dissociation constants of this group of indicators, it occurred t o us t h a t the synthesis of new compounds of this type should meet the need for sensitive and brilliant indicators covering a wide range of H+-ion concentration. After about two years of work we were successful in obtaining a series of new and brilliant indicators which left little t o be desired for H+-ion concentration between 10-l and 1 0 - l ~ . This series of indicators and the ranges over which they change is graphically shown in the accompanying chart already published by Clark and the writer.* Reference t o this chart shows t h a t t h e new series covers a range from Ph+I to Pb+IO. The useful working ranges are indicated by t h e heavily shaded portions of t h e curves. Ph' is merely a convenient mode
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T H E J O U R N A L OF 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
of expressing H+-ion concentration and is the negative log of the Hf-ion concentration. I n other words, Ph+2, for instance, is equivalent to a Hf-ion concentration of IO-^, or approximately t h a t of 0 . 0 1 N hydrochloric acid. P h + 7 is the neutral point. With the exception of methyl red, propyl red, and cresolphthalein, all of the compounds listed on the chart are of the sulfophthalein series. The uses of this series of indicators in titrations and in general chemical manipulations are numerous but only a few cases applying t o the dye industry will be discussed. Ordinarily, one does not think of any connection between the formation of azo dyes and hydrogen-ion concentration, but in certain cases coupling takes place with great readiness in acid solution, while in other cases a neutral or alkaline reaction is desirable. The speed of coupling can often be increased, and in certain cases the shade of the finished dye influenced by careful control of this fa,ctor. By judicious use of one or more of these indicators the degree of acidity or alkalinity of the reaction medium may be kept within closely defined limits and a more uniform product obtained. I n the isolation of certain intermediates, the intelligent application of indicators will be of great service. For instance, in the precipitation of an amphoteric substance like anthranilic acid from its alkaline solution, insufficient mineral acid incompletely precipitates the organic acid while too much mineral acid redissolves it. One can stop a t exactly the right point by making spot tests from time t o time on paper saturated with the proper indicator. By the addition of mineral acid until an acid reaction is obtained with methyl red, the end-point is approached, and by continuing the addition of acid until a n acid reaction is shown by thymol blue, the point of maximum precipitation is reached. Fractional precipitation is a very old procedure for the separation and purification of organic and inorganic compounds. By use of the proper indicator or indicators in acidifications, tars and other impurities can often be eliminated before precipitating the desired intermediates. It is well known t h a t certain dyes are very sensitive t o acids or alkalies, especially upon drying, thereby causing undesirable effects on the shade of the finished product. I n such cases the use of the proper indicators in neutralization would eliminate the possibility of a n excess of the harmful constituent and facilitate the production of uniform products. One of these new indicators, thymolsulfophthalein, has such interesting and unique properties as t o merit special mention. Since i t shows brilliant and sharp changes over two widely separated working ranges, thymolsulfophthalein serves the same purpose as a combination of two indicators. Hence i t is a n ideal substance for controlling such a n operation as the liming of benzene or naphthalene sulfonation mixtures. So long as ,a test portion gives a red coloration with this indicator there is still free acid present; when the indicator shows a yellow color, the end-point
1’01.
12,
NO. 3;
is approached and the addition of lime can then b e made very cautiously until a blue coloration, showing alkalinity, is produced. I n this one substance we have practically a combination of Congo red a n d phenolphthalein. The acid change, however, occurs a t a slightly higher H+-ion concentration than t h a t of Congo red. The use of this indicator in various acidimetric and alkalimetric titrations has already been described. I n a recent article on indicator test papers, Kolthoff ,z of Utrecht, states t h a t phenolphthalein paper ‘reacts very slowly unless the spot is rubbed with a glass rod, and suggests t h a t this difficulty may be due t o the fact t h a t phenolphthalein crystallizes out on the paper. His conjecture is probably correct. Such trouble would not be encountered in the use of thymolsulfophthalein on account of its greater solubility in water. Although litmus is the most extensively used of all indicator test papers, it is also the most unreliable.8 Being a natural product and a mixture of substances, its purity and consequent sensitiveness varies with the mode of preparation. Upon examining specimens. of litmus paper from a number of different manufacturers, one of the laboratories of the du Pont Company found t h a t the product of only one concern was uniformly sensitive. As a substitute for litmus either of two indicators of the sulfophthalein series can be used, namely, dibromocresolsulfophthalein o r dibromothymolsulfophthalein. The former changes from yellow t o a brilliant purple, and the latter from yellow t o a brilliant blue. Both can be obtained in very pure condition and consequently a uniform product is assured. I n conclusion i t may be stated t h a t for general laboratory use, if only two indicators are t o be selected, methyl red and thymolsulfophthalein will be sufficient for most of the titrations and rough controls which the average chemist needs t o make. The complete series, however, would be desirable when the highest precision and accuracy are t o be attained. A more general and intelligent use of indicators will surely be productive of increased efficiency in many industrial operations. THE ACIDIMETRY OF RED WINES AND FRUIT JUICES By Alex. M. Macmillan and Alfred Tingle ANALYTICAL LABORATORY, DEPARTMENT OF C u s r o . ~ AND s INLAND REVENUE, OTTAWA,CANADA Received September 19, 1919
The method usually adopted for the determination of “total acids’’ in red wine and allied juices depends essentially on titrating 2 5 cc. of the sample with N / I Osodium hydroxide solution, using litmus paper as an indicator. T h a t litmus paper is not a really desirable indicator is generally admitted; i t is employed only for want of a better. In working with white wines phenolphthalein is always substituted, the color of the wine in such cases not obscuring J . A m . C h e n . Soc., 40 (1918), 1443. Pharm. Weekblad, 56 (1919), 175; Chem. Abs., 13 (1919), 1689. 3 J . Bid. Chen., 38 (1919), 5 5 ; J . A m . SOC. Agron., 10 (1918). 180; J . A m . Chem. Soc., 40 (1918), 796; J . Aft‘,Res., 10 (1917), 105. 1
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