AN AUTOMATIC TITRATOR J. P . PHlLLIPS University of Louisville, Louisville, Kentucky
A PHOTOELECTRIC automatic titrator suitable for demonst,rations in elementary quantitative analysis can he constructed easily a t little expense. The device (Figs. 1 and 2) consists of a cadmium sulfide photoconductive cell in series with a d.c. voltage of 20-200 volts and a relay that closes a solenoid valve controlling the buret. Although a 45-volt battery is used in the illust,ration, much more flexible performance can he obtained from a power supply such as the Heathkit PS-3 because the sensitivity cf the photocell increases with increasing voltage. For demonstration the photocell is mounted in a hole, '/a inch diameter, drilled in an inch-thick block of wood; friction tape may be placed over the leads a t the back to protect the cell from stray light and to hold it in place. The block is placed against one side of the t,itration beaker, mounted on a magnetic stirrer, so that all light reaching the photocell must pass through the heaker. Daylight or an artificial source of reasonahle intensity mill serve as light source; the higher t,he operating voltage the less the light that is needed. In t,itrations of dark colored solutions little or no light reaches the photocell, and the battery or power supply passes insufficient current to operate the relay. If the solution becomes decolorized or lighter in color a t the end point of the titration, the resistance of the photocell decreases enough to pass current to operate the relay closing the solenoid valve. The buret is connected to the top of the solenoid valve tube by
B , 45-volt B hattern (see text). P. Phatooanduotive cell, Claire. CL-2. R. Relay. Sizma 4F.
VOLUME 34, NO. 4, APRIL. 1957
Figure 2. Amembled Automatic Titrator. All components Excegt the Solenoid valve are Mounted on Top of the Battery
rubber tubing and a delivery tip is similarly attarhed to the bottom. The flow rate must be rather slow and the solution vigorously stirred to prevent going beyond the end point. The principal defect of t,he device is the lack of any vray to slow the delivery rate in the vicinity of the end point. In most of our experiments a relay closing a t a minimum of about 500 microamperes (Sigma 4F) has been used; clearly, a more sensitive relay mould give superior performance. The solenoid valve was a surplus item obtained for $2.50; it is a 10-watt, normally open valve with brass tube for connection to quarterinch pipe. A great many varieties of solenoid valves are readily obtained; the only important special requirement for this application uvould seem to he corrosion resistance. The entire device is portable, being carried in two sections by disconnecting the clip (Fig. 2) and the wire leading from the relay to the 110-volt line. A satisfactory titration for demonstration is iodine titrated with sodium thiosulfate, preferably with starch in the iodine.
