luminol as a Chemiluminescent Indicator in Photometric Titration of Solutions of High Opacity FREDERIC KENNY
AND
R. B. KURTZ, Hunter College, New York, N . Y .
HE new method presented in this paper deals with the use T of a photometer to locate a chemiluminescent end point. The use of luminol as a chemiluminescent indicator has been discussed previously ( 1 ) by the present authors. They have also described the design of a dark chamber Titrimeter for use in chemiluminescent titrations ( 2 ) . I t was pointed out ( 1 ) that the relatively large amounts of luminol, hemoglobin, and hydrogen peroxide, employed as indicator components had an appreciable buffer action and thereby tended to lower the precision of the titrations. In the present work the authors have used vastly smaller amounts of the three indicator materials and have substituted a line-operated Photovolt 520-A multiplier photometer ( 3 ) for visual observation of the end point.
globin and the hydrogen peroxide, and of the constant fresh supply of luminol. The extinguishing of the light could then be attributed to the H30+ of the acid added. To each 30.00-ml. portion of sodium hydroxide solution, 0.5 ml. of 301, hydrogen peroxide and 0.1 ml. of hemoglobin solution were added. The hemoglobin solution was obtained by filtering a freshly prepared 5% aqueous solution. The amount of luminol which was added in the course of the titration was approximately 0.29 mg. I n the photometric titration eight 30.00-ml. portions of sodium hydroxide were titrated with hydrochloric acid. In order to minimize errors due to splashing of the solution, the stirrer was not turned on until about 5 ml. before the end point. Splashing is to be avoided, since luminous drops on the side of the beaker would continue to give light beyond the end point. The first deflections of the photometer needle were observed a t the No. 1 (full scale x 10) sensitivity stage. Toward the end of the titration the No. 2 ( X 100) and KO.3 ( X 1000) stages were
The solution to be titrated is placed in a 100-ml. beaker which rests on a glass plate approximately 0.5 inch above the 1.125inch window of the photomultiplier tube inside the dark chamber Titrimeter. The solution is stirred by an electric motor. The photometer used shows a full scale deflection on the KO.3 sensitivity stage a t a light level of 10-4 foot-candle, which corresponds to a luminous flux of 2 X 10-7 lumen or 0.2 microlumen. The first unit of the 100 unit-photometric scale then indicates 10-6 foot-candle or 0.002 microlumen. In most of the end points observed the last 2 or 3 drops of acid added caused a rapid sweep of the needle from about division 30, where the luminous flux was 0.06 microlumen, down to zero.
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Table I. Milliliters of 0.1 M Hydrochloric Acid-Luminol Solution Required for Photometric Titration of 30.00 M1. of 0.1 M Sodium Hydroxide Solution Clear NaOH Solution Volume of HC1, Deviation ml. from niean 28.75 0.04 28,82 0.03 28.85 0.06 28.75 0.04 28.80 0.01 28.72 0.07 28.80 0.01 28.80 0.01 Mean 28.79 0.034
KaOH Containing 3 Drops of India Ink Volume of HCI, Deviation ml from mean 28.80 28.85 28.80 28.80 28.80 28.80 28.79 28.78 28 80
0.00 0.05
0.00 0.00 0.00 0 00
0.01 0.02 0.01
Solutions of 0.1 Jf hydrochloric acid and 0.1 Jf sodium hydroxide were prepared and 30.00-ml. portions of the sodium hydroxide were titrated potentiometrically with the hydrochloric acid, using a saturated calomel reference electrode, a glass-indicator electrode, and a Leeds and Sorthrup 7660-A vacuum tube potentiometer. The mean of these results indicates that 28.82 ml. of hydrochloric acid are equivalent to 30.00 ml. of sodium hydroxide. This mean is shown graphically by curve A , Figure 1. The chemiluminescent indicator used in the photometric titration of 30.00-ml. portions of the sodium hydroxide contained three components-luminol, 3% hydrogen peroxide to serve as an oxidant for the luminol, and hemoglobin to act as a catalyst for the oxidation. h critical concentration of hydroxyl ion, in the presence of the catalyst, makes possible the production of an observable amount of light and any decrease in this concentration below the critical value causes the light to be extinguished. Preliminary photometric titrations were carried out in which all three components of the indicator were placed in the sodium hydroxide solution. There was considerable doubt, however, whether the final extinction of the light was due to the last bit of hydrochloric acid added or to a depletion of one of the indicator components. Hence, in the photometric titrations described in this paper, hemoglobin and hydrogen peroxide were added to the sodium hydroxide solution and 10 mg. per liter of luminol was dissolved in the hydrochloric acid. I n this way much less light was evolved in the early stages of the titration, oning to the much smaller concentration of luminol present. ?vIore light was therefore available just before reaching the end point as a result of the preservation of most of the hemo-
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VOLUME
H C I ADDED Figure 1. Titration of Sodium Hydroxide with Hydrochloric Acid A.
B. C.
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Mean standardization curve Mean curve in presence of luminol only Mean curve in presence of Iuminol, hydrogen peroxide, and hemoglobin
V O L U M E 2 4 , NO, 7, J U L Y 1 9 5 2 required, successively, as the needle moved toward zero. The falling motion of the needle, as the end point was reached, was swift and decisive. The end point was taken as that point a t which the needle stopped it's motion at, or very close t'o, the zero of the scale. The results obtained are set forth in t,he first two columns of Table I. They yield a precision, expressed as the average deviation of a single observation, of 1.2 parts per 1000, and when compared with the volume of hydrochloric acid required a t the stoichiometric point, we have 28.82 - 28.79 = 0.03 ml., or an accuracy of 1.0 part per 1000. I n order to determine t,he effectiveness of the indicator in solutions of high opacity, eight 30.00-ml. portions of sodium hydroxide were titrated photometrically with hydmchloric acid solution as before, except that prior to t,itration, 3 drops of India ink were added to each portion. The resulting solution was very dark and possessed sufficient opacity so that when held in front of a 200-\vat,t,incandescent, lamp and examined visually, no light was observed passing through the solution. The results of the titrat,ions are shown in the t,hird and fourth columns of Table I, and yield a precision of 0.3 part per 1000, and when compared with the stoichiometric volume of hydrochloric acid, we have 28.82 - 28.80 = 0.02 ml., or an accuracy of 0.7 part per 1000. I n order to study the buffering effect of the indicator used in photometric titrations, some 30.00-ml. portions of sodium hydroxide were titrat,edpotentiometrically in the presence of luminol only, and some in t,he presence of all three indicator components. Curve B of Figure 1 was obtained when only the luminol vias used. Curve C of Figure 1 was obtained when all three indicator components were present. It. appears from these curves that when all three component,s of the indicator are used, most of the buffering act,ion takes place before a pH of 6.3 is reached, and very little beyond t,hat value. If curve C is compared with the curves given in ( I ) , it will be noted that, less buffering action has been encountered in the present work, particularly at pH values less than 6.3. This result should have been expected in view of the fact that the present work uses as much hydrogen peroxide, 1 / 6 as much hemoglobin, and less than 1/138 as much luminol per sample. The possibility of using much less indicator results from the greater light sensitivity of the instrument compared to visual observation. The steeper curve obtained at pH values less than 6.3 13-ould tend to yield better precision beyond that point. I n order to compare visual observation of the end point with
1219 photometric observation, several samples of sodium hydroxide were tit'rat'ed photometrically to within a few drops of the end point where the needle indicated about 20 to 30 scale divisions and were then transferred to the dark room. KOlight was observed by one experimenter, whereas the faintest discernible glow was observed by the other, indicating that visual observation would be unreliable with the very small quantit'ies of indicator components used in these titrations. The photometric method was possible with such lot\- light intensity at the end point, owing largely to the use of a photomultiplier tube. An instrument a.it,h one t,enth the sensitivity would not have regist,ered a t all just prior to the end point. The increased sensitivity of the photometric method tends to give a lower value of extinction pH on a steeper port,ion of the titrat,ion curve where it more nearly coincides with the standardization curve and where a better precision and accuracy can be expected. From an inspect,ion of A and C of Figure 1 it would appear that a pH range between 5.5 and 6.0 would be the best from the point of view of both precision and accuracy, since this pH range corresponds very closely to the stoichiometric point of 9. Curve B shows that the luminol alone has only a slight buffering action a t values of pH greater than 7.2 and practically no buffering action at pH values below 7.2. The photometric titration of eight samples containing India ink required the use of only sensit,ivit,y stages No. 2 and No. 3. I n these titrations the final motion of the needle was not EO extensive as in the titration of the clear solution, but was ample to indicate the end point with an even better precision of 0.3 part per 1000. The over-all precision for the 16 titrations with and wit,hout, India ink $vas 0.74 part per 1000. ACKNOWLEDGMENT
The authors wish t,o express their appreciation to the Photovolt
Co. for their cooperation in selecting a line-operated Photovolt multiplier photometer of suitable sensitivity and stability for use in this research. LITERATURE CITED
(1) Kenny, F., and Kurtz, R. B., ISD. ENG.CHEM.,Ax.4~.ED.,23,
339 (1951). (2) Ihid.,p. 382. (3) Photovolt Corp., Sew l'ork. S .
T.,Descriptive Material on
Multiplier Photometer 520-.1. RECEIVED for review December 21, 1951. .Iccepted April 17, 1952.
Determination of Arsenic in Organic Nitrogen Compounds of the Guanidino Type WILLI.4M C . STICKLER' Department of Chemistry, Columbia University, ]Yew York 27, .V. Y
F T H E many methods for the determination of arsenic re-
0
ported in the literature, the procedure described by Schulek and Villecz ( 3 ) appears most convenient, since it is rapid and reputedly applicable even in the presence of halogens and heavy metals without prior separation. It consists of the destruction of the organic substance by digestion with concentrated sulfuric acid and hydrogen peroxide; a t the same time trivalent arsenic is oxidizpd t o the pentavalent state; arsenic ( V ) is then reduced again with hydrazine sulfate; any excess of this reagent is subsequently decomposed into sulfur dioxide and nitrogen b?- boiling the reaction mixture; the arsenic (111) is determined bromo-
1 Present address, Department of Chemistry, University of Denver, Denver 10. Colo.
metrically with or without indicator according to the method by Gyory ( 2 ) . I n testing the procedure M ith guanidino arsenicals (1 ), complete destruction of organic matter proved difficult. Runs on S-guanylarsanilic acid, p-arsanilic acid, and even arsenic trioxide gave varying and nonreproducible results. High values can be attributed to incomplete removal of sulfur dioxide during the second heating period, a hile low values may be due to incomplete reduction of the pentavalent arsenic by hydrazine sulfate under the conditions given by Schulek and \'iIlecz (3). There the reagent is added in solid form to the sulfuric acid solution, in which it is insoluble; the mixture is immediately brought to boiling, a t which temperature the hydrazine sulfate is decomposed. Thus, it has very little chance of performing its task as reducing agent. A modified procedure doubles the amount of concentrated sulfuric acid, but uses less hydrogen peroxide; in this way, the
