FEBRUARY, 1950
PHOTOELECTRIC "COLORIMETRYff WITH INEXPENSIVE EQUIPMENT HERMAN A. LIEBHAFSKY and EARL H. WINSLOW General Electric Research Laboratory, Schenectady, New York
SOME years ago, we had occasion to msemble a simple, rugged, inexpensive apparatus for photoelectric "colorimetry"-more precisely, a photoelectric photometer to measure the absorption of radiant energy in the visible region for the purpose of estimating the iron, the copper, and the chloride contents of water. Even had this purpose not been achieved, publication of the investigation would have been in the interest of chemical education owing t o the ever-increasing importance of such instrumentation in scientific work. Because the inexpensive light meter employed, which contains a selenium photocell, is used also in many exposure meters, it is often possible t o do experiments like the following when costly equipment is not available. The apparatus shown in the figure is largely self-explanatory. I n general, water was placed in the graduated cylinder, and the current through the lamp adjusted until a light-meter reading of 100 was obtained. The indicator (or reagent) solution was then added, the solution (or suspension) stirred, and meter readings were taken. In no case was it difficult to select the Y"U'
'bYU'Y6.
In an ideal apparatus, the cross section of the vessel containing the solution would correspond with that of the light-sensitive area, which means that over half the water is beine wasted in the setuw of the firmre. If it is desirable to keep the volume of sample a t a minimum, then this defect should be corrected.
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Suitable light filters will greatly increase the sensitivity of the methods for iron and copper, as our experiments with copper demonstrate. Any such increase of sensitivity will probably be less pronounced in the chloride test, which depends upon the scattering, not the absorption, of light. Experimental details and results are given below. Chloride in Water. Experimental conditions: 0.175 amperes through 25-watt Mazda lamp. 5 ml. of 1 per cent silver nitrate added for each 200 ml. of water; mixture thoroughly stirred. Readings begun immediately. Not all readings are given below. (Readings change appreciably on standing.) Results: (a)
200 mi. H20;readings after 5 minutes Chloride added (p. p. m.) Meter reading
0 100
2.5 69
5.0 35
(b) 400 ml. H10: readines taken Chloride addeb (p. p. m.) Meterreading
0 100
1.3 60
2.5 36
5.0 31
Copper in Water. Experimental conditions: 0.175 amweres through 25-watt Mazda lamw. 400 ml. of water, 25 drops 6 N ammonium hydrdxide, 10 ml. of aqueous 1 per cent sodium diethyl dithiocarbamate re-
JOURNAL OF CHEMICAL EDUCATION
maximum meter reading with 400 ml. water in cylinder was 82 (not 100); readings begun immediately. (Readings change little on standing.) Results: Copper added (p. p. m.) Meter reading
.
0 68'
0.01 61
0.02 54
* As above, the difference between this reading and the maximum recording (in this case 82) presumably represents copper present as an impurity in the water used. The concentration of this copper wasnot estimated; it is obviously near0.02p. p. m. Iron in Water. Experimental conditions: 0.185 amperes through 25-watt Mazda lamp; 5 ml. of thioglycolic acid reagent (prepared by adding 4 ml. of the acid to 8 ml. concentrated ammonium hydroxide mixed with 50 ml. of water) added for each 200 ml. of water; mixture thoroughly stirred. Readings begun immediately. (Readings change little on standing.) Results: (a) 200 ml. Hz0
Iron added (p. p. m.) Meter reading
0 100
0.13 89
0.25 76
0.50 66
0
0.13 75
0.25 63
0.50 41
(b)
400 ml. HIO
Iron added (p. p. m.) Meter reading
100
Of course, data such as these give only empirical calibration curves. Beer's Law cannot be obeyed so long as the meter integrates all the radiant energy transmitted in the visible region. This integration includes unabsorbed wave lengths in the reading. Consequently, agent mixed; readings taken immediately. (Readings the optical density of the solution is too small by an little affected by standing.) amount that increases with the concentration of the Results: constituent being determined, so that the resulting error 0 0.13 Copper added (p. p. m.) 0.25 0.50 is more pronounced a t the higher concentrations. (The Meter reading 73* 59 49 36 optical density as here defined is log [100/meter read* An amount of copper near 0.23 p. p. m. waa probably present ing] when the reading with water in the cylinder was in the distilled water used in these experiments. A sample of this 100.) water carefully redistilled gave a meter reading of 99 when the r e It is our hope that the engineer in the field, the young agents were added to the water in the cylinder. chemist at home, the teacher in high school or collegeCopper in Water (Light Filter Used). Experimental in short, that everyone interested in doing objective conditions: blue cellophane light-filter a t F, see figure; "colorimetry" with inexpensive equipment-will find 125 volts across 150-watt Mazda projector flood light; some use for the apparatus described above.
