The Photoelectric Calorimeter JAMES B. SUMNER Cornell University, Zthaca, New York
BACK in 1910 Folin, a t the Harvard Medical School, introduced the DuBoscq colorimeter for analytical work in biochemistry. This instrument was of great value for the quantitative determination of total nitrogen, ammonia, urea nitrogen, sugar, matinine, uric acid, phosphorus, etc., when great accuracy was not essential, when only small quantities of material were available for analysis, when a speedy analysis was desirable, and when many analyses had to be run. However, most analytical chemists did not consider these colorimetric methods to be the equal of titrimetric methods when accuracy and reliability were of prime importance. Colorimeters usually were in good condition when first purchased, but after several months of use by students were almost certain to have degenerated because of having had corrosive solutions spilled on the opal glass reilector, or on the gears. The balsam cement on the prisms sometimes turned yellow and this made one side of the field darker than the other side. With such a colorirneter i t was nearly always impossible to get both fields to match, even when one rotated the instrument to illuminate one field more strongly than the other. Recently photoelectric calorimeters have become available and a t a cost not much greater than that of one of the more expensive DuBoscq colorimeters. The
vital parts of these photoelectric colorimeters are usually protected against corrosive liquids. I used to long for the day when our laboratory could afford to buy enough student colorimeters to issue one to every four or fivestudeuts. We used to try,although often without success, to make the students read their solutions rapidly and to prevent them from lingering for many minutes, whereby they developed eye fatigue and held up the line of those awaiting a turn. Now, using the photoelectric colorirneter, I find that each student makes a rapid reading and departs. This speed is probably due to the fact that the inexperienced analyst can have no doubt in his mind as to the result indicated by the galvanometer needle. The accuracy obtainable with the photoelectric colorimeter is, in my opinion, fully as great as that obtainable by titration. This accuracy, coupled with convenience, speed, and the lack of need of keeping a standard solution (this having been replaced by a standard graph) is beginning to cause a revolution in the biochemical laboratory. Henceforth, we can expect our biochemistry students to hand in more analytical results than they formerly did and to obtain more accurate values. Then too, we shall worry less about damage caused by spilling solution.
Some men haw thousonds of reasons why they can't do something they w n t to, when all they need is one rwson why they eon.-Willis R. Whitney
