T o load the machine F must be in the top position. This is achieved by disengaging clutch B and using handle E to crank up F. The loaded syringes are put in place and all plungers touch F (the clipssupplied with the syringes must he in place; otherwise the plunger may fall by its own weight and the solution be spilled on the chromatogram). Then the tip of the syringe is lowered so as to touch the paper. Clutch B is locked on the motor shaft. The fan shown a t the lower left of Figure 2 and motor A are started. If the motor turns a t 10 r.p.m. it will cause bar F to travel downward 5 cm. in 1 hour, the syringe delivering up to 1 ml. during that time. For a room temperature air blast drying a water solution this is too high a speed; consequently a rheostat, H , is inserted between the power source and A. If the motor speed is cut down to 3 r.p.m.,
a room temperature air blast will dry a water solution fast enough to keep the spot a t a diameter of 1 to 1.5cm. whether Schleicher & Schuell or Whatman No. 1 paper is used; 700 pl. will be deposited in 2 hours. When F has traveled all the way down, a switch, D,cuts off motor A. If the stability of the analyzed samples allows it, hot air may be circulated by the fan shown a t left of Figure 2; no hot air may be used with the dryer shown at the right of the photograph, as this would cause an expansion of the solution in the syringes, and the flow would be faster than the drying power of the fans. If a solvent more volatile than water is used with a hot air draft, the speed of deposition will be greater. This instrument can be built for a p proximately $150; except for the gear
boxes, which are custom-made, all the other parts are available on the market. Detailed full scale blueprints will be supplied upon request. The author is very grateful to J. P. Greenstein for encouraging this project and to Charles Byrne, Instrument Shop, National Institutes of Health, for technical assistance. LITERATURE CITED
(1) Logotbetis, J., Neurology 5,
767
(1955).
(2) Nordmann,
J., Gauchery, O., DuRuiasesu, J.-P., Thomas, Y., Nordmann, R., Bull. sac. chim. Bioi. 36, 1461 (1954). (3) Urbach, K. F., Sciace 109, 259 (1949). (4) Wiegand, 0. F., Shrank, A. R., ANAL. CHEM.28, 259 (1956).
Apparatus for Spectrotitration of Subrnilligmrn Samples H. E. Boar and J. W. Forbes,' Eli Lilly and Co., Indianapolis 6,Ind. REQUENT need for determining the
F ultraviolet speetrum of an ionizable compound in its several forms led to the construction of an accessory for the Cary Model 11 spectrophotometer which makes i t possible to change and observe the pH over the range of 0.5 to 13.5 while the cell remains in position in the spectrophotometer. Beckman No. 19167 glass electrode and No. 19168 calomel electrode pass through closely fitted vertical guide holes in a plastic cap which positions them into diagonally opposite corners of the 1 X 1 5 em. quartz absorption cell. The capillary tips of two burets (one for 2M hydrochloric or perchloric acid; the other for 2M potassium hydroxide) enter the cell in the remaining two corners, a t an angle that alloms manipulation of the stopcocks above the sample compartment cover. The burets are friction-mounted in fuse clips above the cover. The buret is lowered through the surface of the solution during addition of titrant hut raised while the spectrum is recorded. The capillary below the stopcock is long enough and of fine enough bore to allow approximate control of pH in the middle of the range even with unbuffered solution and more careful control when as little as 10-'M solute titrates in this range. Nonabsorhing buffer may he added if finer control of the pH requires its presence. A magnetic rotor beneath the cell is helt-driven from a motor mounted on the cover. A 5-mm. glass-encased follower in the bottom of the cell rapidly mixes the solution to a stable pH reading after each addition of acid or alkali. A stainless steel tube 1 mm. in diameter carries solvenbsaturated nitrogen gas to the surface of the solution.
x
Present address Shell Development
Co., Emeryville, Calif.
456
ANALYTICAL CHEMISTRY
Figure I .
Titration assembly
A hinged cover was added to eliminate error caused hy light leak, especially during operation with narrower slits (higher voltage settings). An improved version of the apparatus (Figures 1 and 2) has just been completed and is in use with a Cary Model 14 spectrophotometer a t Shell Development Co. It is smaller in the direction parallel to the beam, as it has been adapted to Cary instruments which have the multiple cell attachment. Stirring has been improved by enclosing the glass-encased magnetic follower within a vane rotor of Teflon and the drive motor has been enclosed in the base. The rotor has a small pivot cut into the bottom. The guide rods for the Gilmont ultramicroburets eliminate any chance of breaking the tips and also allow positioning of the burets a t any height. Although i t was necessary with the original apparatus to turn off
Figure 2. Cary Model 14 spectrophotometer the alternating current stirrer motor during the spectral scanning, the 6volt direct current motor, its power unit, or the magnetic rotor has no observable effect on the magnitude or stability of absorbance values with the revised apparatus. Application of these accessories has been directed toward obtaining spectra for correlation with molecular structure, but they have also been used for determination of pKa.
