30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 49 



oxalate actinometer 40 is still the most widely used and is described in detail 

 by Masson et al. 38 For most purposes, however, the simple procedure out- 

 lined by Bowen 41 is entirely adequate. If desired, the inconvenient titration 

 of undecomposed oxalate may be replaced by difference spectrophotometry 

 of the amount of oxidant consumed in the control and irradiated samples, 

 using eerie sulphate as the oxidizing agent; this method is also claimed to 

 have the advantage of making possible measurements of tenfold lower in- 

 tensities. 42 



For much lower intensities (not exceeding 10 13 quanta/sec.) the malachite 

 green leucocyanide acinometer is perhaps the most suitable, and involves 

 only the measurement of the optical density of dye formation resulting 

 from exposure to radiation; 43 but this has not been widely used. 



The potassium ferrioxalate actinometer proposed by Parker 44 has under- 

 gone further development and has now been described in very great detail. 45 

 It is simple to use, since only spectrophotometric measurements are in- 

 volved, and it is said to be more sensitive than the uranyl oxalate actinome- 

 ter besides being applicable over a wider range of light intensities. 



A thermopile or photocell may be useful for checking the constancy of 

 output of a given source, but the accuracy required in most photobiological 

 experiments is seldom such that this is necessary. 



For measuring light intensities of resonance lamps we have found it 

 convenient in our laboratory to follow the rate of decrease in optical den- 

 sity at 262 m/x of a solution of 10~ 4 M uridine phosphate (Up) in a 10-mm. 

 cuvette at neutral pH. The rate of decrease of optical density may be cal- 

 culated from the quantum yield (see Table II) or calibrated against a 

 uranyl oxalate or other actinometer. 



It is desirable to carry out actinometry in the irradiation cell so as to 

 eliminate as much as possible errors due to the geometry of the system. 



4. Conversion Factors for Light Intensity 



Because of the fact that results are frequently reported in the literature 

 with different units for light intensity, the following conversion factors 

 may be found useful : 



1 calorie = 4.185 joules 



1 joule = 10 7 ergs 



40 W. G. Leighton and G. S. Forbes, J. Am. Chem. Soc. 52, 3139 (1930). 



41 E. Bowen, "The Chemical Aspects of Light." Oxford Univ. Press, London and 

 New York, 1946. 



42 J. N. Pitts, J. D. Margerum, R. P. Taylor, and W. Brim, J. Am. Chem. Soc. 77, 

 5499 (1955). 



43 J. G. Calvert and H. J. L. Rechen, J. Am. Chem. Soc. 74, 2102 (1952). 



44 C. A. Parker, Proc. Roy. Soc. A220, 104 (1953). 



46 C. G. Hatchard and C. A. Parker, Proc. Roy. Soc. A235, 518 (1956). 



