Ferenc Szabadvary Technical University Budapest, Hungary Translated by Ralph E. Oespar University of Cincinnati Cincinnati, Ohio
Indicators
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A historical perspective
O n e of the most common operations of the practicing chemist is to dip a piece of indicator paper into a solution to learn its reaction. This is an old technique; it goes hack more than three centuries. Accordimg to our present knowledge, Robert Boyle (1627-91) was the first to employ various natural plant juices as indicators, both in the form of solutions and as indicator papers. The Notuml Acid-Base Indicators
The ability of certain plant juices to act as coloring agents was known even in the sixteenth century, perhaps earlier. Such juices were employed a t that period in France for the dyeing of silk. Probably, it was observed that many of these juices changed their color under the action of certain substances. However the fact that acids made them red and alkalis made them green or blue had little significance a t that time since the concepts "acid" and "base" had not yet been defined. The elucidation of chemical concepts really began in the seventeenth century: the division of compounds into such categories as acids, bases, salts started with attempts to define these classes (an effort that has not yet reached its conclnsion). It was ohserved that certain compounds showed similar behavior toward certain materials, an observation that was especially strik'mg with regard to the change in color produced in plant juices. The earliest and clearest definition of "acids" is the familiar definition given by Boyle, who stated among other characteristics that acids turned plant juices red. Boyle wrote frequently about the uses of plant juices as indicators, especially in his hook "Experiments upon colours" (1663). He employed the jnice of violets, cornflowers, roses, snowdrops, brazilwood, primroses, cochineal, and litmus. He described the action of indicator paper as follows:
these indicators did not show precisely the same color changes. For example, T. Bergman wrote (1775): Blue plant juices are sensitive to acids to a varying degree. Thus nitric acid makes sugar paper turn red, whereas vinegar does not possess this property. Litmus, but not syrup of violets, is made red by air-acid etc. When in this way dl blue juices are examined with regard to their sensitivity, a suitable progression is obtained to measure the comparative strength of acids (3).
From the industrial standpoint, Guyton de Morvean in 1782 was the first to use indicators; he needed a means for establishing the neutralization of nitric acid in his manufacture of saltpeter. He was seeking a method "that could be employed by the most unintelligent workers." He used paper impregnated with turmeric or brazilwood. The h t use of vegetable indicators in titrations came a t about this time although titration procedures had been employed previously. Endpoint Indications in Acid-Base Titrations
The titration technique was an offshoot of the neutralization reactions. In 1658, J. R. Glanber directed: This liquore nitri h i [K~COI]should be slowly added drop by drop to the spiritum nitri that has diatilled over [HNOsl until the effervescence occasioned by the addition ceases and both disagreeable natures, namely the spiritus acidus and liquor h s have slain each other (4).
Take good syrup of violets, impregnated with the tincture of the flowers, drop a little of it upon a whits paper (for by that means the chenge of colour will be more conspicuous, and the experiment may be practised in smaller quantities) and on this liquor let fall two or three drops of spirit, either of salt or vinegar or almost any other eminently acid liquor, and upon the mixture of these you shall find the syrup immediately turned red (I).
In 1729, C. L. Geoffroy studied the strength of vinegar by treating a weighed sample with solid potassium carbonate until no effervescence ensued. From the amount of potassium carbonate consumed, he calculated the relative strengths (concentrations) of the different vinegars in terms of each other. Ranke Madsen (5)believes this to be the first purely titrimetric method of analysis described in chemical literature; hut here once more effervescence served as the indicator. The method of titrations was further developed in the succeeding years but it was not until 1767 that William Lewis (6) proposed that the endpoint be signaled by means of plant jnices. In his book "Experiments and Observations on American Potashes: With an easy method of determining their respective qualities" he wrote:
From Boyle on there were frequent reports of the use of plant juices as indicators. For instance, H. Boerhaave (1668-1738) states that the alkaline compounds can be characterized as such through indicators which give a red color with acids: "Cum succo heliotropi, rosarum, violarum et similinm viridescit, qni cum acidic rudebat" (8). Violet jnice and litmus were used most often. During the eighteenth century it was noted in fact that
The quantity of acid necessary for the saturation [neutralization] of the lyesbould be determined not by drops or tea-spoonfuls but by weight; and the point of saturation not by the ceasing of the effervescence, which it is extremely difticult, if not impracticable, to hit with tolerable exactness, but by same effect less ambiguous and more strongly marked, such as the change of colour produced in certain vegetable juices, or on paper stained by them. What I have chiefly made use of, and found very convenient is a thick writing paper stained blue on one side with an infusion of lacmus or blue archil, and red by a mixture of the same infusion Volume 4 1 , Number 5, May 1964
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with so much dilute spirit of s d t [hydrochloric acid] ae is sufficient iust to redden it. . . .If either the acid or slcali considerablv sudden. Pour gradually some of the acid from the vial into the solution of salt of W a r [potassium carbonate] so long as it continues to raise a. strong effervescence; then pour or drop in the acid very cautiously, m d after every small addition, stir the mixture well with a 91888 cane and examine it with the stained papers. So long as it turns the red side of the paper blue, more acid is wanted; if it turns the blue side of the paper red, the acid hrtrr been overdosed.
For the next hundred years, chemists used the natural plant juices in acid-base titrations, particularly violet or litmus solutions; they complained that the color change was not sharp enough and that the indicator solutions did not keep well. Many suggestions were advanced to improve the situation. For instance, C. F. Mohr (1806-79) recommended a solution of silver chloride in ammonium hydroxide, arguing that silver chloride would come down as soon as the ammonia was neutralized. However, the literature contains no record that Mohr himself ever used this indicator. H. Weiske (7) proposed that iron(I11) chloride-salicylic acid be employed as indicator. The violet color of the iron(II1) salicylate is present in neutral solution, but is discharged in the presence of inorganic acids. However, this suggestion, like many others, came to nought; it is mentioned here only because it involved the use of a synthetic material. Synthetic Indicators
Preparative organic chemistry developed a t a tremendous rate during the second half of the nineteenth century. This growth included the rise of the great synthetic dyestuff industry, and among its products were many that functioned as indicators. The first really successful and usable synthetic indicator was phenolphthalein; it was suggested by E. Luck in 1877 (8). The next year methyl orange was proposed by G. Lunge (9). By 1893, a paper contained mention of 14 synthetic indicators. The difference in behavior of the new indicators became more obvious as their number increased, and these observations led to the development of the theory of indicators. A new and important field of application of indicators was provided in the colorimetric determination of pH. The pioneers in this technique (19034) were P. Szily and H. Friedenthal (lo), whose work was discussed recently by the present writer (11). Their method led to numerous other new procedures, that differed from each other mostly in the use of newer indicators and buffer solutions. Methyl red which is widely used was introduced in 1908 by E. Rupp and R. Loose (18); the sulfonephthaleins, which are especially suited to pH determinations because of their numerous color changes, were introduced by H. A. Lubs and W. M. Clark (13) in 1915. In addition to color indicators for acid-base titrations, a considerable number of fluoresceing indicators are also employed, particularly with dark solutions. The first fluorescence indicator was fluorescein, which as the first synthetic indicator had been recommended by F. Kriiger (14) as early as 1876. In alkaline solution, the illuminated indicator emits green light which disappears in acidic solution. The chemiluminescent indicators act in a similar fashion hut they emit light 286
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even without illumination. The first of these, luminol, was recommended by F. Icenny and R. B. Kurtz (15) in 1951. Redox Indicators
The first redox titration was made by F. A. H. Descroizilles around 1788; he titrated hypochlorite solution with indigo solution. The endpoint was indicated by the colored titrant solution itself: as soon as the hlue color no longer was destroyed the titration was over (16). During the nineteenth century redox titrations sufferedmostly from the drawback that no proper means of indicating their completion were available; consequently only the two methods in which the colored titrant solution itself signaled the endpoint, perrnanganimetry and iodimetry, were employed to any notable extent since they required no special indicator. Of course there had been efforts to 6nd suitable indicators for such reactants. The first redox indicator was used in 1835 by L. J. Gay-Lussac (17). He determined hypochlorite with arsenious acid in the presence of a few drops of indigo whose color was discharged a t the endpoint. The indicator action was irreversible. Later the use of spot reactions was proposed; they involved the removal of a drop of the solution being titrated and testing it on a suitable reagent paper. Obviously this procedure was tedious and inaccurate but since nothing better was available this use of external indicators was tolerated for quite a long time. The first indicator spot test reaction was reported in 1846 by W. Crum (18). He titrated hypochlorite with iron(I1) solution and from time to time tested a drop of the solution with hexacyanoferrate(II1) paper to learn whether the iron(I1) solution was still being consumed. A most remarkable proposal was to look a t a Bunsen flame through a flask containing the iron(II1) solution that was being titrated with tin(1I) chloride. It was claimed that the flame looked hlue until the endpoint was reached and then appeared green (19). There were many such complicated or outlandish proposals which were advanced primarily because no suitable reversible indicators were available. The truly reversible redox indicators were not introduced until the present century; the first was diphenylamine that was used by J. Knop (80) in 1915 for titratious with chromium(V1) solutions. Other Indicators
Lacking some better means, the endpoint in precipitation titrations was taken for many years as the point at which no further precipitation could be observed, a point that obviously is difficult to detect. This procedure was employed for example by F. Home as early as 1756 in the first precipitation titration when he determined the hardness of water by means of alkali carbonate solution, which was to he added. "If no whiteness arises in the water, it is then soft; if there does, go on drop by drop until no more white clouds arise" (21). The use as indicator of a second precipitate produced in the solution was first employed by Saint Venant in 1846; when determining chloride argentometrically, he added a little lime to the solution so that brown silver oxide would appear a t the endpoint (22), J. Liebig (83) used this same principle in the first complexometric determination. He titrated chloride
with mercury(I1) nitrate solution in the presence of urea; a colorless mercury-urea compound began to precipitate a t the endpoint. At present, the usual practice is to use adsorption indicaton in precipitation titrations. The first of these was the argentometric determination of chloride in the presence of fluorescein as described in 1923 by K. Fajans and 0. Hassel (24). The important so-called complexometric method employs the metallochrome indicaton; the first was murexide introduced in 1946 by G . Schwarzenbach (26). Theory of Indicators
Chemists seemingly used indicators for centuries without raising the question as to why the color changes occurred. W i e l m Ostwald was the first (1894) to attempt an explanation. In addition he made efforts to provide theoretical foundations for analytical chemistry, a branch which up till then had been a purely empirical discipline. He postulated that the indicators are weak acids or bases whose undissociated molecule differs decidedly in color from the ion. The dissociation equilibrium determines whether the molecular or the ionic species predominate in the solution. This hypothesis made it possible also to develop a qualitative picture of the divergent acidic or basic sensitivity of the indicators (26). In 1907, A. Hantzsch announced the so-called chromophore theory (27) in which the question is viewed rather from the organic chemical standpoint. The two modes of thought were brought together by I. M. Kolthoff, who also wrote the first monograph on the acid-base indicators (28). New theories regarding acids and bases are being brought forward from time to time, and the ideas regarding indicator action are changed accordingly. However, none of the theories has as yet gained general acceptance. L. Erdey (29) advanced a theory covering the chemilumiuescence indicators in 1953. The extended treatment of the theoretical aspects of redox indicators studied during the 1920's by W. M. Clark and his associates mark a real milestone in this field (50).
Literature Cited (1) BOYLE,R., "WorkB,"London, 1744,Vol. 2,p. 53. H., "Elements Chemise," Leyden, 1732, (2) BOERHAAVE, Vol. 2, p. 57. T., in Preface to SCEEPPER,H. T., "Chemische (3) BERGMAN, ForelSmingar," Uppsda, 1775 a9 cited in R A N C ~ E MADSDEN, E., "The Development of Titrimetric Andysis till 1806," Copenhagen, 1958, p. 68. 3. R., "Opera Chymica," Frankfurt, 1658, Vol. (4) GLAUBER, 1, p. 524. (5) GEOPFROY, C. L., "Mem. Acad. Paris," 1729, p. 68; cf. Rmcke Mdsen, p. 25. (6) LEWIS, J. L., "Experiments and Observations on Arne& can Potashes: With 8s easy method of determining their respective qualities," London, 1767, p. 28; cf. RANMADSEN, p. 51. (7) WEISKE,H., J . Prakt. Chem., 120,157 (1875). (8) LUCK,E.,Z. anal., C k . , 16,332 (1877). (9) LUNGE,G., Chem. Bw., 11, 1944 (1878). H., (10) SZILY,P., ON. Hetilap, 1903, 509; FR~EDENTHAL, Z. Electrochem., 10,341 (1904). F.. (OESPER. R. E.. translator). TKBI (11) SZABAD~~RY. ~
(12) RUPP,E., AND LOOSE,R., Chem. Ber., 41,3905 (1908). (13) L w s , H. A,, AND CLARK,W. M., J. Wash. Acad. Sci., 5,
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(14) Kniio~n,F., C h a . Ber., 9, 1572 (1876). (15) KENNY, F., AND KURTZ, R. B..Anal. C h a . , 23,339 (1951). .
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(17) (18) (19) (20) (21)
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(1795). GAY-Luss~c, L. J.,Ann. Chim. Phys., 60,225 (1835). CRUM,W., Polytechnk. J., 96,40 (1846). MORGAN, F. H., J. A n d . Chem., 2,169 (1888). KNOP,J., Z. anal. Chem., 63.81 (1923). HOME,F., "Experiments an Bleaching," Edinburgh, 1756, p. 299. SAINTVENANT, Cmnpt. rend., 23,522 (1846). LIEBIG,J., Ann. C h a . ,85,289 (1835). FAJANS, K., AND HASSEL,o., Z. E l e d ~ o c h a . ,29, 495 (1923). SCHWARZENBACH, G., Helv, Chim. Acta, 29, 1338 (1946). OSTWALD,WILEELM, "Wissenschaften Grundlagen der analytisches Chemie," Leipsic, 1894, p. 103. HANTZSCH, A., Chem. Ber., 40, 3017 (1907). KOLTHOPF, I. M., "Der Gebrauch von Farbenindikatoren," 2nd ed., Berlin, 1923, p. 106. EEDEY,L., Acta Chim. Hung., 3,81(1953). CLARE,W. M., U.S. Public Health Reporte, Washington, D.C., 1922-8.
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