INDUSTRIAL A N D ENGINEERING CHEMISTRY
April 15, 1931
169
and not with sodium citrate with a pH of 7.63, a difference of less than one pH unit. From this it would seem that it is the function of the buffer to hold the hydrogen-ion concentration within certain rather wide limits, and more particularly t o act in the nature of a catalyst t o promote a rapid reaction between the reducing solution and the dye. A specific effect of a similar nature was also noted by Calcott and English ( 2 ) . The buffer must also be of such a nature that it will not react with the titanium trichloride solution.
tion, but also to promote the reduction of the dye. It has been found that in the case of sunset yellow FCF, sodium citrate gives more nearly correct results than sodium bitartrate. There are several buffers given that may be used with each dye. This is of value in titrating mixtures of dyes, as it makes possible the choice, in most cases, of a buffer that is satisfactory for them all.
Conclusion
(1) Ambler, J. A,, Clarke, W.F., Evenson, 0. L., and Wales, H., U. S. Department of Agriculture, Dept. Bull. 1390 (revised 1927). and Suppl. 1 (March, 1930). (2) Calcott, W. S., and English, F. L., IND. ENG.CHEM.,15, 1042 (1923). (3) Evenson. 0. L., and McCutchen, D. T., I b i d , 20, 860 (1928). (4) Knecht, E., and Hibbert, E., “New Reduction Methods in Volumetric Analysis,” Longmans, 1918.
I n the evaluation of color in a dye by means of titanium trichloride, it is essential that the correct buffer be chosen for the individual dye. The function of the buffer (or buffer catalyst) is not only to regulate the hydrogen-ion concentra-
Literature Cited
Determination of Hydrogen-Ion Concentration with Photoelectric Colorimeter’ Ralph H. Muller and Herman M . Partridge WASHINGTON SQUARE COLLEGE, NEWYORKUNIVERSITY, NEWYORKN.Y .
By means of a simple photoelectric colorimeter A previous communiThe amplified photoelectric greater precision has been obtained than by the orcation ( 2 ) the authors current is read on a milliamdinary procedure of visual estimation. have described the use of meter. With linear amplifiOperation of the colorimeter depends upon the the photoelectric cell in the cation the r e a d i n g of the variation in photoelectric current with the transmitperformance of a u t o m a t i c milliammeter d e p e n d s only tancy of the indicator solution with changing pH value. t i t r a t i o n s . It was shown upon the transmittancy of The feeble photoelectric current is amplified by a threethat the change of color octhe solution for the particuelectrode vacuum tube of suitable mutual conductance, curring during a t i t r a t i o n lar spectral region admitted and pH values are read in terms of plate current of the can be followed by the photoby the filter, F. For each triode. electric cell and, f u r t h e r , p a r t i c u l a r system investithat the amplified output of gated. a filter was chosen a photoelectric cell can be made to interrupt the addition of which transmitted light of waveiength corresponding to one the titrating agent at any desired end point. It was pointed absorption maximum of the solution. out that with certain modifications this arrangement of photoExperimental Details electric cell and thermionic amplifier can be used for the accurate colorimetric determination of hydrogen-ion concentraI n describing the method in detail, it will be necessary tion. to discuss very briefly the characteristics of the amplifier n circuit, because certain optimum conditions have been found S, S F in the development of the method. Figure 2 shows the cirI cuit employed, when using batteries as the source of current. The normal plate current of the vacuum tube, indicated by the milliammeter, MA., depends only upon the plate voltage supplied by B, and the filament voltage, A . The grid
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Figure 1--Scheme of Principal Features of Method
It is the object of this paper to describe the photoelectric method in detail and to show its general applicability to colorimetric procedure. Figure 1 shows schematically the principal features of the method. Light from the lamp L passes through a slit, S, and filter, F, after which it traverses the solution to be investigated, X. The light transmitted by the latter passes through the slit S , and falls on the photoelectric cell, P.C. 1 Received January 22, 1931. Presented before the Division of Physical and Inorganic Chemistry at the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16. 1927.
I “ A a Figure 2-Amplifier
Circuit Employed
potential is constant, and during operation the voltages are held constant. Any change of potential of the grid, G, with respect to the filament, F, will cause a change in the plate current. The change in plate current due to a change in grid potential-i. e., the mutual conductance of the tube-is the
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tube constant in which we are most interested in this case. It will be seen from the diagram that changes in the grid potential are caused by the photoelectric cellin the folGwing manner: When light falls on the active surface of the photoelectric cell, electrons are emitted, proportional in number to the intensity of the incident light, which flow through the high resistance, R, owing to the applied potential, V . The Ri drop across R is applied directly to the grid and filament of the vacuum tube. For uniform amplification it is essential that the mutual conductance of the tube be constant over the working range. That is, the relation between I p and Eg must be linear over that range which is employed in the measurement. A further requirement is that this linear relation must obtain with negative grid potentials. Figure 3 illustrates characteristics for several types of tubes. Only the 199- and 120-type tubes are satisfactory, because the linear portion of the characteristic is on the positive side for the other types for plate potentials of less than 100 volts. The second requirement is important because positive values for the grid potential cause a flow of current between filament and grid so that the resistance of the grid circuit becomes
7
Vol. 3, No. 2 finite and hence is less sensitive to changes due to the photoelectric cell. If a 120-type tube is employed, B should be 90 volts, A , 3 volts, V , 135 volts, and the resistance, R, should be 5 megohms. The measurements given below were made with a sensitive millivoltmeter in the plate circuit, properly shunted so as to read milliamperes. By changing the value of the shunt the range of the instrument could be increased. This is a useful arrangement since it may happen that the system to be investigated absorbs light in a spectral region where the sensitivity of the photoelectric cell is low (in the red or orange), and therefore it becomes imperative to measure slight changes in the plate current. Theoretical Considerations Considerable work has been done on the absorption spectra of indicators and the dependence of absorption. on the hydrogen-ion concentration. Most important in this respect is the work of Brode ( I ) , in which it was shown that, contrary to the findings of many, there is no shift in wave length of the absorption maximum with change of hydrogen-ion concentration, but merely a change in the height of absorption band-i. e., change in transmittancy. I n a two-color indicator there are two absorption bands, one for each color of the indicator. I n the transition from one color to another, with a change of pH, the one band decreases in height as the other band increases, Brode gives accurate spectrophotometric data for most of the Clark and Lubs indicators a t values covering the entire pH range. He shows that a knowledge of the transmittancy for a given wave length enables one to find immediately the pH value by referring to the pHtransmittancy curve for that indicator. The authors have based. their work on these findings and shown that if light corresponding to the maximum of absorption of the indicator is passed through a solution of the indicator and allowed to fall on the photoelectric cell, the amplified photoelectric current will depend only upon the pH of the indicator solution, other things being held cdnstant. Results
The data for the curves of Figure 4 were obtained in the following manner: For each indicator studied, the absorption maximum was obtained from the data of Brode. A filter was then selected which transmitted light in this region. The Wratten filters were found to be very convenient for this puppose. Each filter was checked roughly with a small spectroscope and compared with one absorption band of the indicator. It should be pointed out that it is quite impossible to obtain a filter exactly suited to each case, but since the pH-plate current values are all purely empirical relations, this makes little difference. With the filter in place, tubes containing the same amount of indicator but buffered to various pH values were placed in front of the photoelectric cell and the plate current read. A curve was then plotted with the arbitrary plate-current values as ordinates and t h e corresponding pH values as abscissas. The plate-current values are reduced to comparable values for the reason that. the current obtained depends upon the spectral region which! is being used, as mentioned above.
April 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
The curves so obtained represent a calibration curve for each indicator. By treating an unknown solution with the same amount of indicator, keeping all the other factors the same, the pH value can be obtained by comparing the plate current with the calibration curve. It should be emphasized that this procedure may be dispensed with by simply adjusting the intensity of the lamp, vacuum-tube filament, etc., to the same value at each measurement, and multiplying the observed plate current by the factor representing the ratio between pH and current for that indicator. This is permissible because the curves are all linear (over the useful range of the indicator). By the use of simple shunts and switches it is possible to use one millivoltmeter to check all voltages before a determination. While reasonable reproducibility can be expected from the amplifier and photoelectric cell, a chemist would probably prefer to check an unknown reading in most cases by reference to a system of known pH. To do this, it is simply necessary to introduce buffered solutions containing the same indicator of several pH values above and below the unknown value, and by interpolation assure himself that the value for the unknown is not in error because of a change in tube characteristic or a variation in sensitivity of the photoelectric cell. It should be emphasized, however, that such checks need only be made occasionally, for great constancy was obtained even over long periods of operation. It is obvious that with slight modifications in procedure, i t is possible to extend the method t o most colorimetric problems.
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Fieure 4-Calibration Curves for Various Indicators M.C.P.--m-Cresol purple B.T.B.-Bromothymol blue B.P.B.-Bromophenol blue P.R.-Phenol red T.B.-Thymol blue B.C.G.-Bromocresol green B.C.P.-Bromocresol purple
It has been employed in this laboratory to follow color changes occurring during chemical reactions, and in this way substitute accurate values for the heretofore qualitative estimation. Literature Cited (1) Brode, J . A m . Chem. Soc., 46, 581 (1924). (2) Muller and Partridge, IND ENG.CHEM.,20, 423 (1928).
Measurement of Rate of Formation of Oxidative Decomposition Products in Fats and Oils' D. P. Grettie and R. C. Newton SWIFT& Co., UNIONSTOCKYARDS,CHICAGO, ILL.
HE intense research which is being applied to shortening material by both the producer and the user has emphasized the need of better methods for testing its various functional qualities. The susceptibility of fat to oxidative rancidity is one of the most important factors in evaluating a shortening for many uses. There is, therefore, need of a quick reliable method for determining the relative susceptibility of various fats to rancidity. Such a method should give quickly an accurate indication of the stability of a fat under ordinary conditions. Any method in which rancidity is developed rapidly requires the application of rigorous conditions, hence it is to be expected that any such method will not give absolute comparisons of different kinds of fats for all conditions. It is possible, however, that such a method may give reliable comparisons of samples of fat from the same general source, and some indication of the relative keeping qualities of samples from different sources, if the method provides an accurate means of measuring the rancidity developed. Several accelerated rancidity tests have been proposed. The best known of these are: (1) the oxygen absorption method, (2) the Bailey method, and (3) incubation followed by organoleptic inspection. The oxygen absorption test (2) has received consideralhe attention, and much weight has been attached to the induction period of a fat under this test as a criterion of its keeping qualities. There is no reason to assume that the induction period for oxygen absorption is always the same as the in-
T
1
Received January 2, 1931.
duction period for the formation of volatile oxidative decomposition products. Since it id the volatile decomposition product which renders a fat unusable, a proper test for keeping qualities of the fat should be based upon a direct measurement of these products. For example, semi-drying oils consume large amounts of oxygen in the formation of gums, and condensation products with very little formation of the undesirable odor of rancidity. The Bailey method (1) makes use of Schiff's reagent to measure the amount of volatile aldehydes produced under controlled conditions. The objection to this method lies in the fact that the Schiff's reagent, in which the volatile products of rancidity are condensed, is very unstable and difficult to maintain in a colorless condition. There is some difficulty also in measuring accurately the amount. of color developed. Probably the simplest and most reliable method heretofore developed is the incubation test in which the fat is kept in an oven at an elevated temperature and inspected for odor and flavor at regular intervals. I n this test rancidity usually develops a t some time between 2 and 40 days, depending upon the stability of the particular sample under test. There are, however, two objections to this method for ordinary use: the time required to get results is entirely too long for many practical purposes; and the criterion is that of odor and taste, and therefore subject to a very decided personal element of possible error in judging the point of inception of rancidity. The Kreis test (4) may be used as an auxiliary indication of
