2 74
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
2
Mar.. 1920
T H E J O U R N A L OF I N D C S T R I A L A N D ENGINEERING CHEMISTRY
t h e end-point. Thus i t happens t h a t titrations of red and white wines are not strictly comparable, nor are t h e results obtained with red wines either very accurate or always strictly comparable among themselves, for different samples of litmus paper vary in sensibility. We have found t h a t 0 . 4 cc. of N / I O alkali must be added t o z j cc. of distilled water before the resulting mixture gives a n alkaline reaction with some of t h e red litmus paper used in this laboratory. Being convinced t h a t t h e large experimental error and the troublesome manipulation of test drops were alike avoidable, we resolved t o use phenolphthalein, detecting t h e end point by the spectroscopic method which has already been described’ as generally applicable t o t h e acidimetry of colored solutions. One of us had already received a private communication t o the effect t h a t this method had failed when applied t o wine lees. I t appeared, however, t h a t this failure was due, a t least in part, t o the fact t h a t t h e analyst had not sufficiently adapted t h e working details t o meet his individual case. Perhaps the principles on which modifications should be based were not enunciated with sufficient clearness in the original communication. , We have determined t h e conditions under which red wines and fruit juices can be spectroscopically -titrated, and believe t h e method is both more rapid and accurate t h a n t h a t commonly used. DETAILS AND RESULTS O F T H E TITRATIONS
The wine or juice t o be examined ( 2 5 cc.) is introduced into t h e titration vessel, followed by (approximately) 7 5 cc. of water. If desirable the wine may be first measured into an ordinary flask in which i t can be heated, after which i t is washed into t h e titration vessel with t h e water necessary for dilution. N/IOsodium hydroxide is then rapidly added from a burette until t h e end-point is near. This may be known either by the change in t h e natural coloring Phenolmatter or by a previous rough titration. phthalein ( 2 cc. of one per cent solution) is next added, and t h e titration is continued with ordinary caution, the liquid being examined before a small spectroscope after each addition of alkali. The end-point is shown by t h e sudden appearance of t h e absorption band characteristic of alkaline phenolphthalein. I t lies in the green region of t h e spectrum, very close t o t h e yellow. I n t h e absence of bright sunlight, any steady good batswing gas burner will be a satisfactory source of illumination. The thickness of liquid suitable for observations a t the dilution specified is 30 t o 3 5 mm. Vessels of the necessary capacity, b u t of such shape as t o give so thin a layer of liquid, are not usually made for chemical work. We found i t convenient t o perform our titrations in 6 oz. bottles shaped like pocket flasks and originally used for retailing small quantities of whisky or rum. The sides of such bottles are not parallel and are made of quite inferior glass, but these 1
J A m Chem SOC.,40 (1918), 873
2
75
facts do not interfere in any way with t h e necessary observations. The results we have obtained on a variety of samples, the conditions of successful titration, and t h e agreement inter se, of different titrations made on‘the same material, are tabulated below : NaOH Solution
NATUREAND CONDITION
( N / 1 0 Required)
O F S.4MPLE
Port Wine-A 25 cc. wine
+ 75 cc
water
+ 2 cc. phenolphthalein.. . . . . .
Grape Juice-A 25 cc. jllice
+ 75 cc. water + 2 cc. phenolphthalein., . . . .
Port Wine-B 25 cc. wine
+ 75 cc. water + 2 cc. phenolphthalein.. . . . . .
,
cc. (20.7 { 20.8 120.8
{ 1::
32.8 16.6
16.6
1:;:; 16.7
Grape Juice-B 25 cc. juice
+ 75 cc.-water + 2 cc. phenolphthalein., . . . , ,
“Medicated Red Wine” 2.5 cc. wine 75 cc. water
+ 2 cc. phenolphthalein.. . . . .
“Raspberry Vinegar” 25 cc. juice 50 cc. water
+ 2 cc. phenolphthalein,.
+
+
In each case, thickness of liquid examined
= 30
f
41.6 41.8 14.8
1 :::: . . .. { :::: ,
,
55.6
t o 35 mm.
I t will be observed t h a t less water was added befo,re titration t o t h e “raspberry vinegar” than t o t h e other samples. This was not only because t h e juice was less deeply colored, b u t also because i t was much more acid. The larger volume of standard alkali solution required brought the dilution a t t h e end-point to t h e desired amount. Some wines which we have examined required much greater dilution t h a n those mentioned above. I n one case in our experience, 300 cc. of water were added. REUARKS
ON
SCOPIC
THE
PRINCIPLES UNDERLYING
TITRATIONS
AND
THEIR
SPECTRO-
PRACTICAL
APPLICATION
Experience in this laboratory has shown t h a t the spectroscopic method of detecting t h e end-point in acidimetry is often useful. It is capable of giving exact results in many cases where a very wide limit of error would otherwise be unavoidable. S o apparatus is required but such as should be found in any analytical laboratory and is used for many other purposes. But the successful application cf the method depends on observing t h e conditions demanded by each individual case. N o set of working rules can be laid down which would meet every analysis t o be performed. Success can be obtained only by remembering the general principles on which the method is based, and deducing therefrom t h e experimental details which will meet individual requirements. These general principles do not seem t o have been made quite plain in t h e original description of the method,l but i t seems reasonably apropos t o consider them here. The fundamental principle is t h a t for many indicators there is an easily detected difference in absorption spectrum, according to whether t h e solution is
’
LOC.
cit.
2
1
76
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
acid or alkaline, and t h a t the transition point can be readily recognized spectroscopically. B u t this difference, and therefore the neutral point, can be seen onlx when t h a t part of the spectrum in which i t occurs is not obscured by absorption bands due t o the coloring matter originally present in the liquid t o be titrated, and when the “effective concentration”’ of the indicator itself is great enough t o show the characteristic change distinctly. A liquid too highly colored for titration b y ordinary methods can almost always be rendered amenable t o spectroscopic titration by reducing the effective concentration of its coloring matter. This may be done either by diluting with a solvent or by decreasing t h e thickness of the layer t o be spectroscopically observed. A combination of these two methods is most generally useful. But if the amount of indicator added remains constant, its effective concentration is reduced a t the same time and a point may thus be reached where i t is so small t h a t the spectroscopic change is obscured. More indicator must then be added. On the other hand, the effective concentration of such an indicator as cochineal must not exceed a certain amount. I n fact, there is a connection between the quantity of indicator which should be used, the thickness of liquid observed, and the total volume of liquid present a t the end-point. This is not a novel principle, since i t holds good for titrations conducted in t h e ordinary way, and is commonly, but rather vaguely, recognized. I t s observance and application are essential when the spectroscope is used. As regards the spectroscopic sensitiveness of phenolphthalein, we have noted t h a t the end-point was strongly shown when 0 . 1 5 cc. of N / I O alkali was added t o I O O cc. of distilled water containing z cc of one per cent phenolphthalein solution, the thickness of liquids observed being 30 t o 3 5 mm. We have also found t h a t when for any reason the effective concentration of the original coloring matter cannot be so reduced a s t o give an entirely bright and unimpeded light in t h a t region of the spectrum where the absorption band of the indicator is situated, t h e latter band can often still be observed, provided the effective concentration of the indicator is increased, i. e., a dark band may be superposed on a relatively light one. Since the spectroscopic method of titration was first described, additional experience has been gained in the use of direct vision spectroscopes of different designs. An instrument may have too high, as well ,as too low, dispersive power for convenience. The “Beck-Thorp Patent Reading Pocket Diffraction” instrument seems best designed in this respect, and has the additional advantage t h a t its “luminous pointer” measuring device enables the necessary 1 The term “effective concentration’’ has been found a convenient one in this and similar connections where optical methods of analysis are under discussion. It is dependent not only on the relative amounts of solvent and active solute, but also on the thickness of the layer of solution observed, and sometimes on other factors. As applied to polarimetric observations, two sugar solutions of the same concentration, examined under otherwise identical conditions but in tubes of different lengths, would vary in “effective concentration” directly as the length of the tubes.
Vol.
12,
No. 3
readings t o be taken more quickly and accurately t h a n with an illuminated scale or with cross wires. We have also conducted certain experiments on methyl red as an indicator for use in connection with the spectroscope, and find i t well adapted t o the purpose if in sufficient concentration. When the total volume of titrated solution is about I O O cc. and the thickness observed is 45 t o 50 mm., 3 cc. of 0.05 per cent methyl red solution give a strong absorption band in the green region while the solution is acid. This band disappears entirely and sharply a t the point which marks neutrality for this indicator. The titration readings in N / I O solution are the same as those obtained by t h e naked eye when a smaller quantity of methyl red is employed in a n otherwise colorless solution. THE DEROODE-PERCHLORIC ACID METHOD FOR DETERMINING POTASH’ By T. E. Keitt LABORATORY OF THE GEORGIA EXPERIMENT STATION, EXPERIMENT, GA.
Increasing industrial needs and the limited supply of platinum make a substitute for t h a t element in the determination of potash more and more imperative. Of all the reagents suggested as substitutes perchloric acid seems the most promising. A study of available literature on this subject indicates t h a t there are three substances that must be removed t o secure concordant results: sulfates, ammonia, and organic matter. The sulfates may be removed by precipitating with BaClz solution before making t o volume, and the DeRoode moist combustion removes ammonia and organic matter. Using the DeRoode method of combustion the writer has done a small amount of work with perchloric acid as the reagent and makes this as a preliminary report, hoping t h a t others will t r y i t out and give him the benefit of their experience with the method. PROCEDURE
Place 2 . 5 g. of the sample on a 1 2 . 5 cm. filter paper and wash successively with portions of boiling water into a 250 cc. flask until the washings amount t o about 2 0 0 cc. Acidify the solution with 5 cc. of conc. HC1, while hot, precipitate t h e sulfates by adding drop b y drop, in slight excess, normal BaClz solution acidified with HC1 ( I O cc.*usuallysuffice). Cool, make t o mark, and shake. Allow t h e precipitate t o settle. Transfer a 50 cc. aliquot part, corresponding to 0.5 g., t o a I 7 5 cc. porcelain evaporating dish, add 30 cc. of aqua regia and evaporate t o dryness on hot plate; add a second portion of 30 cc. of aqua regia and evaporate t o dryness; then add about I O cc. of conc. HC1 a n d 2 0 GC. of distilled water and evaporate t o dryness. Dissolve in 20 cc. of hot water and add 5 cc. of perchloric acid, 1.12 sp. gr.; evaporate on hot plate o r steam bath until copious fumes come off, remove and run the liquid around the bottom of the dish, and if i t does not solidify on cooling, evaporate more and cool again. It can be rapidly cooled by floating the 1 Presented at the 58th Meeting of the American Chemical Society, Philadelphia, Pa., September 4, 1Y19.
