V O L U M E 2 3 , NO. 2, F E B R U A R Y 1 9 5 1
339
Table 11. Analysis of Organic Bases by Direct Titration Determined Purity, % Indicator Potentiometric method method Diethylaniline" 99.17 99.36 8-Hydroxyquinoline a 99.45 99.78 Diphenylguanidinea .... 99.91 Aniline b 99.41 .... n-Butylaniline" 99.32 Tris(hydroxpmethyl)aminoinethane C 99.94 Benzocaineb .... 99.20 2-Nonyl-4,4-bis(hydroxymethyl)-2-oxazoline~ 99.85 100.00 Eastinan Kodak white label chemicals. b Purchased C.P.chemicals. C Purified b y recrystallization from methanol, m.p. 171.1' C . d Purified b y recrystallization from benzene, m.p. 91.2 t o 91.6' C.
compounds they predicted could be analyzed do not react stoichiometrically; the reason for this is not yet apparent. ACKNOWLEDGMEYT
Chemicals
The authors wish to express their appreciation to Joseph B. C'reedon for performing many of the titrations during the survey. LITERATURE CITED
(1) Blumrich, K. G., and Bandel, G., Angew. Chem., 54,374 (1941). (2) Conant, J. B., and Hall, N. F., J . Am. C h e m Soc., 49, 3047 (1 927). (3) Hall, N. F., Ibid., 52, 5115 (1930). (4) Hall, N. F., and Werner, T. H., Ibid.,50, 2367 (1928). (5) Kolthoff, I. RI., and Willman, A , Ibid., 56, 1014 (1934). (6) Nadeau, G. F., and Branchen, L. E., Ibid., 57, 1363 (1935).
C. J., and Rolfson, F. B., IND.ENG.CHEM.,ANAL.ED., 15,337 (1943). (8) Tomicek, O., Collection Czechoslov. Chem. Communs., 13, 116 (1948). (9) Wagner, C. D., Bran-n, R. H., and Peters, E. D., J . Am. Chem. Soc., 69, 2609 (1947). (10) Wilson, H. N., J . SOC.Chem. I d . (London),67,237 (1948). (11) Wlttmann, G., -4ngezo. Chem., A60,330 (1948). (i Penther, )
termination. In fact, it is possible to analyze aqueous solutions of this compound by a slight modification of the method given herein. (A report on this application is being prepared,) A study of the results of this survey shows that the method is applicable to the analysis of salts of strong bases and weak acids, salts of wrak bases and Teak acids, and weak organic bases. The major limitation of the method is the insolubility of some of the compounds in acetic acid. The authors have found that some
RECEIVEDMarch 4, 1950. Presented before the Division of Analytical SOcIErY, Chemistry a t t h e 117th hleeting of t h e .4\rE~1caaCHEMICAL Houston, Tex.
luminol as a Chemiluminescent Indicator In Acid-Base Titrations with a Dark-Chamber Titrimeter FREDERIC KENXY
AND
R . B. KURTZ, Hunter College, .Yew York, N. Y .
Luminol indicator is a new type of acid-base indicator which emits light at the end point. It was developed to make possible acid-base titrations in solutions having sufficient color to prevent the use of ordinary indicators, and at the same time avoid the use of a potentiometric titration. The indicator, consisting of luminol, hemoglobin, and hydrogen peroxide, emitted light at a pH very close to the stoichiometric point in the titration of 0.1 M hydrochloric acid with 0.1 M sodium hydroxide. The presence of a highly colored component such as gentian violet did not interfere with the end point. No indicator error was observed in the titration of either colorless or colored solutions. This new indicator should have wide application in titrating a variety of colored solutions.
A
F T E R careful consideration of various chemiluminescent materials that might be used as indicators in acid-base titrations, it was decided t o use luminol (5-amino-2,3-dihydro1,4-phthalazinedione), hydrogen peroxide, and hemoglobin, because this combination is sensitive t o small variations in pH. In each titration approximately 40 mg. of luminol, 6 ml. of 3% hydrogen peroxide, and 30 mg. of hemoglobin were used.
+ 'I
Luminol
0 -4nion
Oxidation products
+
Light
This indicator emits light above a given pH, as contraskd with ordinary indicators, which absorb light differently above a given pH. Ordinary indicators function alone, whereas luminol, at
room temperature and in aqueous solution, functions only when associated with hydrogen peroxide and a catalyst. Ordinary indicators are not destroyed during the color change, whereas luminol is consumed a t the pH a t which light and oxidation occur. Thus the indicator reaction is irreversible. It lends itself t o titration of acid with base. If, however, a base is t o be titrated with acid, a measured excess of acid must be added, followed by backtitration with base. The behavior of the indicator may be summarized by the formula in column 1. It is necessary for the hydroxyl ions t o remove protons from the luminol t o form the anion before the reaction producing light can proceed (20, 21). The oxidation processes which emit light do not start until the solution pH is increased t o a point close t o the neutral point. This critical p H value is the basis for the accompanying method of acidimetry. The maximum chemiluminescence intensity as a function of the molarity of the sodium hydroxide present ( 2 1 , 2 2 ) shows a sharp maximum a t 0.01 to 0.05 M , corresponding to p H values between 12 and 12.7. .I perceptible light appears, however, when sufficient sodium hydroxide has been added t o give a p H in the neighborhood of 7.
ANALYTICAL CHEMISTRY
340 Table I. Volume of Sodium Hydroxide Solution Equivalent to 30.00 RI1. of Hydrochloric Acid
in the presence of the colored component and without the use of electrical instruments. EXPERIMENTAL
Phenolphthalein a8 indicator and luminol as a chemiluminescent indicator) Phenolphthalein Sodium Deviation hydroxide, from mean, ml. ml 31.80 0.02 0.00 31.82 0.06 31.88 0.02 31.80 0.04 31.86 0.01 31.81 0.02 31.80
.
Luminol Sodium Deviation hydroxide, from mean, ml. ml. 31.82 0.00 0.07 31.75 0.04 31.78 0.08 31.90 0.02 31.80 0.02 31.80 0.08 31.90 3 1 . 8.. 0
n.nz . .
31.82
0 04
~~
Mean
31.82
0.02
Table 11. Volume of Sodium Hydroxide Solution Equivalent to 30.00 RII. of Hydrochloric Acid
Approximately 0.1 JI solutions of hydrochloric acid and sodium hydroxide were prepared. Swen 30.00-ml. portions of the hydrochloric acid were t,itrated with the sodium hydroxide, using phenolpht,halein as the indicator. The end point was taken as the faintest otmervahle pink color. Eight 30.00-ml. portions of the hydrochloric acid were then titrated with the sodium h\-droxide, using the luminol indicator, and carrying out the t,itlationr in the dark-chamber Titrimeter (13). The results of both set,s of titrat,ions are shown in Table I. When phenolphthalein was used as the indicator, the average deviation of a single observation obtained was 0.63 part per 1000. The luminol, however, yielded a precision of 1.25 parts per 1000.
(Luminol indicator in presence of 0.003% gentian violet) Sodium Deviation Hydroxide, from Mean, Titration MI. 1'1I. 1 31.90 0.08 2 0.02 31.80 3 0.07 31.89 n 07 31.80 4 .~ 5 31.75 0.07 6 31.89 0.07 0.12 7 31.70 8 31.77 0 05 0.14 9 31.68 10 0.02 31.80 11 31,80 0.02 12 31.90 0.08 o 07 13 31.75 14 31 90 0 0s Mean 31 82 0 07 ~~
In the present research this light, has been used to mark the end point of the titration of acid xvith base. The catalyst used must be stable in both acid and alkaline solution. Soluble complex salts of transition metals such as cobalt and especially iron in the form of hemin, hemoglobin, or ferricyanide are examples. Substances like permanganate and hypochlorite, while bringing about t,he evolut,ion of light, are rapidly destroyed, whereas hemoglobin can function for a much longer time (21), Theorirs for the behavior of hemoglobin have Iieen formulated by a variety of investigators, some of whom regard it as a peroxidase react,ion ( 1 , 5 , 11, 14; 17, pp. 268, 295; 18, pp, 249,252,268,274; $1). The function of hydrogen peroxide is to bring about the oxidation of the anion derived from the luminol. The manner in which it does this has been the subject of much speculat,ion (2, 5, P I ) . According to recent ideas, t,he hydrogen peroxide may furnish free radicals to the solution (3-5, 9, 10, 16-19), which when react,ing with thr anion of the luminol in the presence of hemoglobin, produce a series of transient inkrmediate free radicals (2, 5-7, 12, 21) and the evolut,ion of light from the final diradical ( 1 5 ) . These changes may involve the transient formation and immediate dissociat,ion of transannular peroxides ( 8 ) . The concentrations of the three components of the indicator may vary over rather wide limits. For Iuminol the limits include 0.01 to 1%, for hemoglobin 0.001 t o 1%, and for hydrogen peroxide 0.03 to 1%. In this research these three components were, respectively, 0.1, 0.1, and 0.3% at the beginning of the titration. Use and Advantages of Luminol Indicator. There are many situations in which an acid-base titration cannot be carried out with ordinary acid-base indicators, because the solution cont,ains a highly colored component which makes it impossible to observe the color change of the indicator. Such situations have been dealt with heretofore either by the use of an instrumental method or, if feasible, by the prior removal of the colored component. The use of luminol makes it possible to carry out the titration
I
I
I
0
I
29
30
31
32
33
0
34
I
35
VOLUME OF SODIUM HYDROXIDE SOLUTION ADDED Figure 1. Titration of Hydrochloric Acid with Sodium Hydroxide A . Using luminol indicator B. Using phenolphthalein indicator C. Using luminol indicator with hydrogen peroxide o m i t t e d
V O L U M E 2 3 , NO. 2, F E B R U A R Y 1 9 5 1
34 1
h t h indicators yielded a mean of 31.82 ml. of sodium hydroxide cquivalent t o 30.00 mi. of hydrochloric acid. Thus the indicator vtmr was negligible. In order t o study the behavior of the luminol indicator in the pwence of a highly colorcd component, 14 titrations of the hytltvchloric acid with thc sodium hydroxide were carried out, using thct luminal indicator in the presence of gentian violet. The conthe solution a n cc~ntrationof gentian violrt uscd--0.003%-gave intense color which in:idc it utterly impossible t o observe t,he end point of any of the ordinary acid-base indicators. The results of thwc titrat,ions, shown in Table 11, yield a mean value for the volume of sodium hydroxide required which is the same as t h a t required in both titrations shown in Table Inamely, 31.82 ml. The deviat,ion from the mean, however, is somewhat larger. I t s value-0.07 ml.-corresponds t o a precision of 2.13 parts pcr 1000. I n order to study more carefully titrations i n which luminol indicator is used, and to account, if possible, for the lower precision obtained with luminol as compared with phenolphthalein, five titrations of hydrochloric acid with sodium hydroxide were carried out potentiometrically in the presence of the luminol indicator. A glass indicator electrode, a saturated calomel reference electrode, and a Leeds and Northrup i66O-ii vacuum tube potentiometer were employcd. The temperature of the laboratory was 25" * 2" C. The average curvc obtained is plotted in Figure 1, A . The potentiometric titration of the hydrochloric acid with the sodium hydroxide in the abxnce of luminol indicator is plotted as B . The two curves cross very close t o the luminol end point. The inore gradual s l o p of the luminol curve accounts for the lower precision obtained with this indicator. Work is in progress t o modify the composition of the luminol indicator, so as to give a curve with a steeper slopc, t,hereby improving the possibility of oljtaining a higher degree of precision. If the titrations employing luminol in both Tables I and I1 are considered, the over-all precision obtained is 1.85 parts per 1000. Indicat,ions a t the present time are that this can be improved upon. The more gradual slope of A as compared with B is t o be expected, in view of the fact that each component of the indicator functions a s a weak electrolyte. Because both luminol and hemoglobin combine with hydrogen ion, the p H values preceding the stoirhiomet#ric point will be higher than otherwise, and because
these same components supply protons t o hydroxyl ions after the stoichiometric point has been passed, the p H values obtained will be IoR-er than in the absence of these components. This effect is further accentuated by the presence of the weak acid hydrogen peroxide, which supplies protons t o hydroxyl ions in the neighborhood of the stoichiometric point. As an aid t o further study of the luminol indicator a potentiometric titration was carried out using luminol indicator from which the hydrogen peroxide had been omitted. The results are plotted as C in Figure 1. Part but not all of the buffering action of the indicator appears to be caused by the hydrogen peroxide. LITERATURE CITED
Abel, E., Molonatsh., 79, 457 (1948). Anderson, R. S., Ann. N. E'. Acad. Sci., 49, 337 (1948). Bacon, R. G. R., Trans. Furuduy Soc., 42, 140 (1946). Baxendale, J. H., Evans, 11. G., and Park, G. S., Ibid., 42, 155 (1946).
Bernanose. -&., Bremer, T., and Goldfinger, P., B I L Lsoc. chim. Relges, 56, 269 (1947). Drew, H. D. K., and Cross, €3. E., .I. Chem. SOC.(London),1949, 639.
Drew, H. D. K., and Pearman, F. H., Ibid., 1937, 588. Etienne, A , and Bichet, G., Compt. rend., 228, 1136 (1949). Haber, F., and Weiss, J . Proc. Rou. Soc., 147A, 332 (1934). Haber, F., and Willstaetter, R., Ber., 64, 2844 (1931). Hauroivitz, Felix, "Progress in Biochemistry," pp. 172, 281, New York, Interscience Publishers, 1950. Kautsky, H., and Kaiser, K. H., Natur~issenschufte?L,30, 148 (1942).
Kenny, F., and Kurts, R. B., IND.ENG.CHEM.,ANAL.ED.,23, 382 (1951).
Legge, J . IT., and Lemberg, R., "Hematin Compounds and Bile Pigments," p. 387, New York, Interscience Publishers, 1949. Lewis, G. K.,and Kasha, &I,, J . Am. Chem. Soc.. 66, 2107 (1944).
Mornan. L. B . . Truns. Faradau Soc.. 42. 169 11946). Thetrell, H., "Advances in Enzymology," Vol. 7 ; Kew York, Interscience Publishers, 1947. Waters, W. A,, "Chemistry of Free Kadicals," Oxford, England, Clarendon Press, 1946. Waters, W.A , , and Mera, J. H., J . Chem. Soc. (London), 1949, "
I
515,2427.
Weber, K., Ber., 75B, 568 (1942). Weber, K., Reaek, A , . and Vouk, V., Ibid., 75B, 1141 (1942). Zellner, C. K.,and Dougherty, G., J . A m . Chem. Soc., 59, 2581 (1937). RECEIVED June 19, 19.50
Polarographic Behavior of Organic Compounds Analysis of Mixtures of Dichloroacetic and Trichloroacetic Acids PHILIP J. ELVING
AND
CHING-SIANG TANG
T h e Pennsylvania State College, State College, Pa.
E
LVING and Tang ( 1 ) reported that in the pH range of 6.8
t o 10.4 and the potential range of 0.4 t o - 1.9 volts, acetic :tiid monochloroacetic acids give no polarographic wave, while trichloroacetic acid gives two waves and dichloroacetic acid gives one wave; the latter wave is identical in characteristics with the more negative wave of trichloroacetic acid. The diffusion curicTnts of each of the two waves of trichloroacetic acid and of the o w wave of dichloroacetic acid, when corrected for the effect of the electrocapillary curve, are identical; the diffusion currents :LI'CL directlv proportional t o the concentration of the acids. The
waves are due to the successive removal of halogen whereby trichloroacetate is converted t o dichloroacetate, which can then be reduced t o monochloroacetate. I n the procedure described for analyzing mixtures of dichloroacetic and trichloroacetic acids, the latter is determined from the diffusion current of its first wave, while the dichloroacetic acid can be measured by deducting the adjusted diffusion current of the first wave of the trichloroacetic acid from the total diffusion current of the second wave. The standard series method of calibration can be used.
