Photoelectric Colorimetry in Microanalvsis J
Photoelectric Methods in Macro- and Microanalysis RALPH H. MULLER, New York University, New York, N.'Y.
0
€'TICAL methods have enjoyed widespread use
in
Photoelectric methods afford precise for investigating c o ~ o ~ m e t r i c tions. Most of the methods are in the range Of microan&'siS* The highest Precision is undoubtedly affordedby the vacuum photoelectric cell-amplifier method. Barrier-layer cells are precise but considerably simpler. Either method can be made to yield data of fundamental importance.
in t h e p a s t a n d n o d e t a i l e d e x p l a n a t i o n of t h e importance of the m i c r o s c o p e , refractometer, p o l a r i m e t e r , o r spectrograph as an tool is necessary. Colorimetric analysis h a s b e e n e m p l o y e d either for its convenience and rapidity, or for the estimation of traces of substances for which the ordinary gravimetric or volumetric methods are inadequate. There has been a feeling in some circles that colorimetric methods are necessarily crude and approximate and are likely to lack specificity to the extent that their employment is resorted to with reluctance and only because other methods fail. The adaptation of photoelectric photometry to the needs of colorimetry has introduced a new point of view and promises to revolutionize the field; progress in physical photometry has been so great that we are already in possession of instruments yielding measurements far more precise than the most exacting re-
I
C
FIGURE1.
Io
4
I
I
such depths of liquid tl and t2 that the emergent light intensities I , and IZ a r e e q u a l , we have I O = const.
-
Cltl
I1 = I2
Therefore, or
= czta
Photoelectric methods can be adapted here with little change; the cell merely replaces the eye and decides when the fields are matched. If we are concerned with the more usual photoelectric procedure of keeping the thickness constant and measuring the intensities I1 and 1 2 , then the same law is used in a different manner (Figure 1). Since I1 = Ioe-kol and ZZ = Iae-kc2 I Therefore log -!= K(c2 I2
I
c
With i n i t i a l i n t e n s i t y of light lo incident on both cups, containing tration c1 and solutions cz, adjusted of concento
- CI)
If we simplify matters by using pure solvent in one compartment and designating the light which emerges from this cell as lo, then the intensity I which emerges from the compartment filled with colored solution of concentration c, will be given by I = Iae+
c,
1
or log- = -kc Io Since I/Io is the transmittancy, we can write log T = -kc. The various forms which the expression takes can be summarized as follows
MEASUREMENT OF INTENSITY
PI~OTOELECTRIC
quiremtmts of' the analyst. If we accept the thesis that photoelectric photometry is already in the class of precise physical measurements and that the near future will offer even more refined and convenient means, it then becomes necessary to reexamine colorimetry and see in what direction further research is desirable. This paper shows definite results which have been obtained in the
