characteristics of the recorder with those of the niicroammetel . Performance. T h e input voltages required t o produce different absorbanCe scale readings are sholT.ll in
I' The "we supplied b y a T y p e 8662 Leeds 8: Xorthrup Potentiometer. For Perfect response, the absorbance scale reading
should equal t h e logarithm of the ratio of t h e voltage producing zero scale deflection, EO,to t h a t Producing the observed reading, E. The errorb in the observed readings at different positions on t h e scale. shown in the last column of the table, are not greater than 2%. The reproducibility of the distance of pen t r a w l TWS found to he h 0 003 ab-
sorbarice unit a t an absorbance of 0,100, k0.003 unit a t a n absorbance of 0.900, and 10.013 unit at a n absorbance of l.iO0. The above levels of accuracy and of reproducibility will be adequate for practically all work for which the use of the present type of indrument i q indi ca t ed .
Circulation Apparatus for Photometric Titrations Carl Rehm,' Jerome Irwin Bodin,2 Kenneth A. Connors, and Takeru Higuchi, School of Pharmacy, University of Wisconsin, Madison, Wis.
detection of titrimetric points possesses advantages over more conventional approaches. For example, st,oichiomctric end points can be determined Ti-ith good precision by linear extrapolation of indicator absorption data, even for systems t h a t give uncert'ain results by other methods [Higuchi, T., Rehm, C., Bariistein, C., . ~ x . A L . CHEX 28, 1506 (1956); Rehm, C., Higuchi, T., Ibicl., 29, 367 (1957) 1. Photometric titrations have been little used, because titration units I-I~T-Pheen t,xpensive or too specialized. A simple, inexpensive titration unit, used in these laborat'ories for 3 years with conipletely satisfactory rrsulhs, cwnsists of only t>n-op a r k HOTOUETIZlC
p end
The titration vessel is a sinal1 Erlentneyer flask equipped with a n outlet tube sealed tangentially at the lowest part of t h e side and an inlet tube which enters the bottom of the flask at its center. These two tubes are bent' to be parallel (Figure 1,A). The absorption cell varies with the photometer used. A modified 1-cm. cylindrical cell (Pyrocell Manufacturing Co., 207-11 East 83th St., Xew York 28, F.Y.) used with the Cary recording spectrophotometer, having inlet and outlet tubes instead of the usu:il single opening, is shown in Figure 2,B. This cell is also suitable with the Beckman DU and B instruments, if the cell holder is modified to hold the cylindrical cuvette. The test tube cells supplied n-ith the Bausch 8: Lomb Spectronic 20 colorimeter may be adapted for use in this inexpensive instrument. A rubber stopper receives the inlet and outlet tubes, which terminate above the level of the light pat'h (Figure 2,A). The surface of the solution remains a t the level of the outlet tube. The titration flask and absorption cell are connected with short lengths of flexible tubing: rubber, Tygon, or Vinyl (available through medical supply houses as blood transfusion tubing). For titrations in very acidic solvents, Present address, Analytical Research Department, Ciba Pharmaceutical Products, Inc., Summit, N. J. Present address, Chas. Pfizer R- Co., Brooklyn, N. Y.
such as acetic acid, the last is preferred. The slightly basic properties of rubber and Tygon prevent their use in tfhetitration of very weak bases. The titration stirring bar is placed in the flask, which is supported over a magnetic stirrer. The solution (25 to 35 ml. in a 50-ml. titration flask) is circulated through the tubing to the cell and back to the flask by the action of the stirring bar (Figure 1,B). Complete mixing is achieved in 20 to 25 seconds with both the cylindrical and the tubetype cell. Figure 3 shows the rate of mixing TT ith the cylindrical cell. A &ml. aliquot of an acetic acid solution of llalachite green was pipetted into the titration flask, which contained 25 ml. of acetic acid. The recorder of the Cary spectrophotometer and the magnetic stirrer were turned on simultaneously and absorbance was recorded a t 620 mp.
buret tip through a stopper. A positive pressure may be maintained b y feeding dry air or nitrogen into the flask. The assembly constitutes a very modc.st investment if the Speetronic 20 is used as the measuring instrument; the total cost is comparable ith that of ronimnn potmtiomctric titr:ttion [quipincnt.
- - - - R U B B E R STOPPEP
SPECTRCNIC 2C CELL HCLDEP
1Iodificatioii of the spectrophotometer is usually unnecessary or slight. A small hole drilled in the cell compartment cover of the Cary, for example, allon s the flexible tubing to pass into the compartment. A black cloth thronn over thc tubing excludes stray light, R S with other instruments. The titration apparatus is particularl> convenient if a closed system is necessary. Atmospheric moisture and carhon diovide arc ewluded b r passing the
Figure 2. Bausch & Lomb Spectronic 20 absorption tube modified for photometric titrations (A), and cylindrical cell for titrations with Cary and Beckman spectrophotometers (B)
MAGNETIC
R
I
TOP VIEW TIME (SEC) ARROWS SHOW DIRECTION OF LIQUID FLOW
Figure 1 .
Titration flask
Figure 3. Time required to attain homogeneity of titration solution using, cylindrical cell
VOL. 31, NO. 3, MARCH 1959
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