V O L U M E 26, NO. 4, A P R I L 1 9 5 4
179
DISCUSSION OF DEVELOPMENTS
No loss of nitrogen need he feared from heating the contcnts of the flask in the first step of the method when no organic matter is present. The nitrates react a8 Soon as they are in solution, and nitrosalicylic acid is very stable to heat. The solution may he heated to the boiling paint before adding the mdium thiosulfate and the loss of nitrogen will be only about 0.10%. The method suffers from the same interfering substances as thc elassiea1 Kjddahl method. Chlorides present with the nitrate will cause a, slight loss of nitrogen as NOCL. Starch and tobacco and most other organic compounds present with the nitrate develop reducing substances on heating in sulfuric acid before the nitrate is in solution, and these reducing substmces apparently take oxygen from the NO, group in its transition stage from the nitrate to the salicylic acid, with some lass of nitrogen. If these organic compounds arc d d e d after tbc nitrate is in soh-
tion, thc nitrogen found on completing Lhe Kjeldahl cyde will be the sum of thatin the nitrate plus that in the organic compound. The results reported here Yeprosent work on a reagent grade of sodium nitrate which passed a 14-mesh sieva but w&8retained on a 3O-mesh, and wa8 dried a t 110' C. This greatly facilitated manipulation and minimized errors due to absorption of moisture, adherence to brushes, etc., and made a very acceptable standard in Iijeldahl work. Treating 0.7 gram of this material by the proposed method gave results ranging from 16.40 to 16.48% nitrogen, and left little to be desired as to precision and accuracy. LITERATURE CITED
(1) Assoc. Offiic. h g r . Chemists, "Official and Tentative ,Methods of Analysis," 6th ed.. BP. 27-8. 1945. (2) Asaoc. Offio. Agr. Chemists, "Official Methods of Analysis,'. 7th d
Differential Photometric Detection in Couloi EDWARD N. WISE', PAUL W. GILLES, and CHARLES A. REYNOLDS, JR. Department of Chemistry, University of Kansas, Lawrence, Kan.
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Xl'I?IlIME,NTS in the coulometric titration of acid and base with photometric detection of the end point were complicated by the formation of bubbles during titration ( d ) . These bubbles were formed b y the generating electrode and produced a density in the solution x-hich the photometric detection section of the automnt,ie titration apparatus \,-as unable to distinguish from a change in dcnsity due to a ehenge in the color of the indicator employed. It was found possible to titrate automatically those systems in which the end point was characterized by a change in the indicator which produced an increase in the absorbance of t.he solution with several false terminations of the titration to allow the bubbles to leave the solution. Howover, if an indicator rere used which caused a decrease in absorbance at the end point, the absorbance due to the formation of bubbles compensated for the I
1
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Present address, University of Arizona. Tucson. A
decrease in density due to the color chango of thi a variable overtitration inevitably occurred.
To overcome the adverse effect of the absorbah, 1.10Ay8 the formation of bubbles, a differential 07 ratio-detecting photometxic unit u m designed and constructed. The principle on vhich this unit opcrates is as follows: Light from a tungsten lamp
IISV.
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Figure 2. Cireuil Diagram for Differential Photorn, Deteotion Unit G . 0.01 sf., mica 6. 0.25 pf.. 400 volts
G. 0.1 "1%400 *olta C,. 4 pf.. 200 ro1ts 4 pf., 2w volts , ma., 6.3volts, 6. 115 ~ o l t s30 R,. m,wo ohms, 2 w a t t s Rz. 22 megohms, 0.5 w a t t R I , 14. 0.25 megohm, 2 watts Xi. 56 Ow ohms, 0.5 w a t t RI. 68'000 ohma, 0.5 w a t t R,. 10~000ohms, 1watt Ra. 5000 ohms, 10 w a f t s R.. 20,000 ohms, 4 watts SI. Double-pole, double-throw switch TI, Tz. No. 929 TI. No.12AX7 Td. No. 6AS5
e..
Figure 1. Differential Phototube Assembly
passes through the solution being titxated and is separated by a heam splitter into two beams of equal intensities a t right anglcs to each other. Each beam passes through a tube, which may contain a colored solution t o serve as an optical filter, and terminates on the photooathode of a photoelectric tube.
ANALYTICAL CHEMISTRY
780
Considering the caae of a two-color indicator, such a8 thymol blue, there are two colored forms which the indicator,may adopt, and the ratio of their concentrations in a solution is a function of the hydrogen ion concentration of that solution. If an indicator which is predominantly in its aeidie form is placed in front of one phototube and the same indicator predominantly in ita hasic form is placed in front of the other phototube and that same indicator is used in the titration, then the light coming through the solution when i t is acidic will psas through the filter containing the acidic form of the indicator with little absorption, but will be attenuated by passing through the filter containing the indicator in its basic form. Such an arrangement produces a ratio of light intensities whioh will not be affected by an over-dl diminution of the light transmitted through the solution being titrated, such a.8 is caused by the formation of bubbles. When the indicator changes from its acidic t o its basic form in the course of the titration, the ratio of light intensities definitely changes, for now the phototube behind the filter composed of the acidic form of the indicator receives less of the light being transmitted by the solution than does the other phototube. A photograph of the interior of the phototube housing is shown in Figure 1. Light comes into this unit from the left after passing through the solution being titrated. The beam splitter is a t the extreme left of the figure, and the tnhes containing a solution of the indicator in its acidic and basic forms are placed between the prism and their respective phototubes. The electrical circuit diagram of the unit which converts the output of the phototubes into a direct current potential proportional to the ratio of ,the light inoident on the photot.ubes is shown in Figure 2. This circuit is a modification of a c.ircuit
published by RCA in 1948 for the measurement of light-intknsity ratios (1). The absorption characteristics of two-color indicators differ appreciably, and optimum operation of the differential photometric detection device wm obtained by experimentally varying the ooncentration of the extreme colored forms of the indicator in the filter tubes. The concentration was adjusted 80 that minimum response in electrical output was schieved when the intensity of light incident on the titration vessel was attenuated to simulate the formation of huhhles, preferably by use of a diaphragm in the light heam, and so that maximum response was obtained when the indicator in the titration vessel was varied from its acidic to its basic colored form. The operation of the device was studied using the indicators thymolsulfone-phthalein (thymol blue), m-cresol purple, bromothymol blue, and bromocresol purple. It wm found possible to detect photometrically the equivalence point in a coulometric titration of either an acid or a base using the above two-color indicators, even when the absorbance decreased a t the end point. The authors express appreciation to the Oflice of Ordnance Research of the U. S. Army far partial support of this research. LITERATURE CITED
(1) Radio Corp. of America, Tube Department, Harrison, N. J., "Phototubes." 1948. (2) Wise. E. N. Gilles. P. W., and Reynolds, C. A,, Jr., ANAL. CnEM., 25,1344 (1953). RECEIVED ior review Ootober 14. 1953. Accepted January 22. 1954. Abstrhoted irom the PI~.D..thesisof Edward N. wise, University of Kansas, Allgust 1953.
0. 2-Naphthoic Acid ( HlNlEH, JR.,
Armour Research Foundation of
Structural Formula for 2-Naphthoic Acid HREE
modifications of 2-naphthoic acid have been observed in
TExcellent these studies. crystals of modification I (melting point 185'
C.)
and of modification I1 (melting point 177.5" C.) can be obtained from sublimation (Figures 1 and 2). In general, the temperature of the candmsing surface determines which of these two madifications is obtained by sublimation. A surface temperature of 75" C. gives crystals of 11; a temperature above 110' C. favors crystals of I. Intermediate temperatures give varying proportions of these two modifications. Modification 111 (melting paint 176" C.) was not obtained from sublimation but only from the melt (Figure 3). Goad orydals of 2-uaphthoic acid I can be obtained by macrorecrystallization from alcohol, ether, chloroform, and benzene. Modifications I1 and 111were not obtained from solution. 2-Naphtboic acid, as used in these studies, was purified by the following procedure: The compound was dissolved in water by the addition of a minimum amount of 10% sodium hydroxide. Charcoal was added and the solution was filtered. The oompound rr,as fractionally precipibated with hydrochloric acid. The center cut
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ACKNOWLEDGMENT
C:RYSTALLOGRAPHIC DATA
Contributed by JOHN KRC, JR., and RALPH Illinois Institute of Tochnolosy Chicago 16, 111.
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Figure 1.
Crystals of 2-Naphthoie Aoid I fip o r n Sublimation Cpossed nicdol.
