A LABORATORY EXPERIMENT I N PHOTOMETRY JAMES B. HICKMAN West Virginia University, Morgantown, West Virginia
Tms article describes a
laboratory experiment suitable for use in undergraduate and beginning graduate physical chemistry courses. The experiment involves the measurement of the density of the image on photographic film, and requires as special equipment only an amateur photographic exposure meter, which is used without change or damage. The advantages of the experiment are these: (1) it serves an an illustration to the text chapters on light, which are not. ordinarily represented in the laboratory experiments; (2) it gives students an opportunity to process photographic materials; in schools not having a required course' in instrumental or spectrographic analysis, most undergraduates and many graduates complete their work without ever having practiced this technique; (3) it gives students an opportunity to work up and interpret simple numerical data. The experiment here described can be done by a student working alone within a period of three hours, preferably distributed over two days' meetings of the laboratory section, in order to allow the film to dry before measurement. I n this experiment, photographic film is uniformly exposed to light. Sections of the exposed film are developed a t different temperatures, or a t different
concentrations of the developer, and the amount of light transmitted by the exposed and developed film is compared to that transmitted by unexposed but processed film. The numerical data obtained can be used to examine in a general way t,he effect of temperature or of concentration of developer on the rate of photographic development. I t is the purpose of the experiment to give the student actual experience with the simplest principles of photometry, wit,h the processing of photosensitive materials, and with the interpretation of numerical data, rather than to examine the nature of the photographic process. Such examination would require apparatus and techniques not appropriate to the beginning physical chemistry course. The following apparatus is required: a photoelectric exposure meter, for example, the General Electric DW-08 meter; the base of a laboratory microscope (or an improvised apparatus of wood or metal-the only object of this apparat,us is to provide a means of holding the film firmly and exposing it to a lighted area of uniform size); a 35-mm. camera, for example the Kodak "Signet"; three 400-ml. breakers; three 1000ml. beakers; scissors; and ticket punch. The materials needed are these: Eastman Kodak Co. "Fine Grain Positive" film; 35-mm. Kodak "Dektol" developer-a stock solution prepared according to the directions on the can; acetic acid; Kodak Acid Fixer solution. EXPERIMENTAL PROCEDURE
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In a darkroom lighted by a bright red (Series 1) safelight, the camera is loaded with a strip of the "Fine Grain Positive" film about six feet in length (sufficient for the work of two students). The camera is taken out of doors and 30 pictures of a piece of white poster board are taken. The camera should be held a t such a distance that the poster board fills most of the field of view. Care should be taken to see that the board is uniformly lighted, and that no shadow falls on it. If the board is lighted by diffuse sunlight, an exposure of second a t f/5.G will suffice. The film is rewound into the magazine, unloaded, and the magazine taken to the dark room. There, under the bright red safelight the magazine is sprung open, the leading end of the film started out, and the magazine closed again (this procedure permits one t o turn on regular illumination between sessions of developing). The film is processed according to standard procedure except for the following modifications: (1) theL'Dektol" solution prepared according to the directions on the can 560
VOLUME 33, NO. 11, NOVEMBER, 1956
is diluted, one part solution to 19 parts water, this mixture being referred to below asL'diluted'Dektol"'; (2) 400-ml. beakers containing 300 ml. of solution are used rather than trays; (3) contrary to the best photographic procedure, the developed pieces of film are left in the stop bath until all are ready for transfer to fixer; (4) temperatures of development are not those specified by the manufacturer of film and developer. I n order to examine the effect of temperature on the chemical reaction of development, the film is developed a t O", loo, and 20". In preparing the solution for use a t Oo, it will be found convenient to substitute ice for some of the water used in diluting the "Dektol." The temperature of the developer may be held essentially constant by suspending the beaker containing it in a 1000-ml. beaker containing ice, or water a t the appropriate temperature. Under the safelight, about eight inches of film is pulled from the magazine and transferred to the diluted "Dektol" a t 0'. After exactly one minute, one inch of the film is cut off and placed in the stop bath. At the end of two minutes, another inch is cut off, placed in the stop bath, and identified by a single hole made by the ticket punch; the piece cut off a t the end of the third minute is identified by two holes, and so on. All the pieces of film are fixed, washed, and dried according to standard nrocedures. in a envelope ~h~ dr; Dieces should be labeled ~ & hthe student's name and the temperature of development. A similar series of pieces is prepared for the temperatures of 10' and 20". In the same fashion, two or more series of pieces of film can he prepared, using developer at room temperature, hut varying the concentration-for example, one series with 300 ml. of diluted "Dektol," another series with 150 ml. diluted "Dektol" to 150 ml. of water, and ~ e r h a a~third s series with 100 ml. diluted "Dektol" to 200 mi. of water. The last six inches of the film (unexposed) should be processed through all stages, with about four minutes in the develoner. This will serve as a "blank." a measure of the light absorution of the film itself ifter processing. The light absorption of the pieces of film is measured on an improvised "densitometer" such as that shown in Fieure 1. A. section of the unex~osed.orocessed film is :aid over the hole in the micr&ope stage, and the light meter inverted over it. The light source and mirror are adjusted until the meter indicates a value close to but not a t its maximum. This reading is recorded as po, and no change is made in light source or mirror until the series of readings .is complete. If at any time the light source or mirror should be moved, it would be necessary to return the blank to the stage, and readjust light source and mirror until the pa value was duplicated. Each piece of film processed is measured in turn, the meter reading when it is in place over the hole in the stage being recorded as the p for that piece. Properly,
Time, minutes
the image of the poster board on each piece of film should be larger than the hole in the microscope stage. If it is not, it may be necessary to prepare a microscope slide, masked with ink or black tape, smaller than the smallest dark image, centering the pieces of film over this when determining po and p. Care should be exercised not to mix pieces from the various envelopes;
Time, minutes Fi-
3. E f f d of conuntrmtion on Rat. of D.velopm.nt
A , diluted "Dektol:" hnd diluted "Dektol."
B , 1 : l mixture of water
562
after measurement, each series of negatives should be returned to its envelope, which is preserved until the experiment is completed. CALCULATIONS AND DISCUSSION
The quotient po/p is calculated for each measurement, and plotted on semilog paper (the pa/p values being plotted on the logarithmic s a l e against time in minutes on the arithmetic scale). Figure 2 shows typical results a t O", lo", and 20'. Figure 3 shows some results obtained a t room temperature with two different dilutions of "Dektol." The following questions indicate the type of analysis to which the results might be subjected: (1) Although the plots of log (pol?) against time are essentially linear, they are not entirely so. To what extent is this deviation the result of uncertainty in measuring values of po/p? (Remeasure this value for several pieces of the film to determine reproducibility.)
JOURNAL OF CHEMICAL EDUCATION
(2) Assuming that the opacity of the film (log po/p) is proportional to the extent to which the chemical reaction of development has proceeded, and comparing results for the different dilut,ions of "Dektol," what is the order of the reaction? (3) How does the time to reach the same degree of development compare for the temperatures O", lo", 20°? Can the rule of thumb-"a ten-degree rise in temperature doubles the rate of a chemical reaction1'be applied here? If this rule is inapplicable, suggest reasons, or state limitations of the rule. (4) For those points in the plots that deviate more from linearity than the low reproducibility indicates can you suggest reasons for the deviation'? The reasons may either be possible errors in the procedure (failure to measure effective development times accurately, nonuniform processing, etc.) or they may concern factors involved in the chemical reaction (depletion of developer as reaction proceeds, for example).
