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,ed potentiometrically 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
1220
ANALYTICAL CHEMISTRY
oxidizing power of the acid is not impaired as much by dilution with the aqueous peroxide as in the original method. The hydrazine sulfate, in turn, is added in aqueous solution instead of in the solid form, presumably enabling the reagent to perform the reduction before the water boila off and the hydrazine decomposes. The following general procedure gave acceptable and reproduoible results for a variety of inorganic and nitrogen-containing organic arsenic compounds. The method is adaptable to a Bemimacro SD&IC.
PROCEDURE
Ten millilitertrs of concentrated sulfuric acid am added to the sample (100 to 200 mg.) in a Rjcldahl flask, folloired by 3 ml. of fresh 30% h diogen peroxide and then by another 10 ml. of sulfuric acid. $he solution is usually completely colorless a t this point; i t is then warmed gently, with occasional cautious addition of a few drops of hydrogen peroxide t o destroy any ye)low coloration. (The t,otal amount of hydrogen peroxide used ranged from 3 to 10 ml., while 25 to 35 ml. were used in following the original procedure.) The sohkion is heated strongly for as long a8 1 hour until heavy white fumes of sulfur trionidc are evolved. Tho flask is allowed t,o cool t o about 50" nnd then an
aqueous solution (about 10 ml.) of I50 mg. of hvdraaine sulfate is added. The mixture is heated to boiling and kept there for
0.75 t o 1hour t o ensure complete removal of sulfur dioxide from the long-necked flask. After cooling, the solution is dilutod with 20 ml. of distilled water and after the addition of 0.1 gram of potassium bromide is titrated with standard potassium bromate solut,ion. First appearance of yellow color duo to free bromine was takon BE the end point.
Results of Arsenio Determination in Various Compounds Compound Arsenious aoid Arsanilio aoid N-Gnan~l~lasnilie acid
POrln"l*
Calod., liound. % -4s (la As
75.74 76.63 34.56 34.78 27.08 27.10 27.12 CIH~oO~NaAs.CarIiOiN,15.17 15.21 15.15 .iS*08.
CaHeOIhhr
CiHmOaYa4s.ILO
A'-Gusnylarsanilia aoid picrate
LITERATURE
crrm
(1) Bogert, M. T.,and Stickler, W.C . , Science, 100, 526 (1944). (2) Gyiiry, S., 2. anal. Chern.. 32, 415 (1893). (3) Schulek, E., and Villecs. P.V., Zb
