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CONVERT VOLTAGE TO mg GAINED OR LOST
SUBROUTINES
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Figure 4. Example of computer generated thermogravirnetric plot. While retaining the pristine printout of the curve, the axes have been relettered for better clarity
THIS IS A WE16HT GAIN
P U T DATA W I N 6
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puter (GE 4020) which, in turn, drives the pen of the CALCOMP plotter. The resulting plot displays weight gained or lost per unit area (mg/cm2) us. time (minutes). An example of such a plot is shown in Figure 4. The computer program provides the flexibility necessary to adjust the data to a predetermined plot size and t o label the axes appropriately.
In addition to the application to thermogravimetry, the digital data acquisition system can be used for monitoring and recording a variety of phenomena which involve either time-dependent parameters or, in general, u p to three independent variables. Simple modifications of the basic hardware would, of course, be required for handling some specific tasks. All system components are commercially available as building blocks, as listed in Figure 1. Assembling of the system using commercial modules should be done with minimal design by any competent electronic staff.
ACKNOWLEDGMENT The assistance of S. Ludke and R. J. Herbs with the design and troubleshooting of the apparatus is gratefully acknowledged. Received for review March 29, 1973. Accepted June 25, 1973.
New Stirring Method for Microtitration J. R. Walsby Marine Research Laboratory, University of Auckland, R. D. Leigh, New Zealand A method of indirect stirring of a small volume of liquid during its potentiometric microtitration, is described. It has been used in the study of a marine worm's coelomic fluids chloride levels. The chloride levels were up to sea water concentration (35Oho) and therefore heavy silver chloride precipitate was involved during titration with silver nitrate. Precipitate accumulation about the microburet tip inhibits accurate titration, and none of the various methods of dealing with this problem, summarized by Ramsey, Brown, and Croghan ( I ) , were satisfactory. With small titration volumes, there is no room about the microburet tip for any moving mechanical or magnetic stirrer in the titrate and with potentiometric titration, care must be taken not t o disturb the indifferent electrode (anode). The (1) J . A. Ramsay. R . H. J. Brown, and P. C. Croghan, J. Exp. Bioi., 32, 822 (1 955).
method of Sanderson ( 2 ) of stirring by bubbling an air stream through the titrate, was unsatisfactory, because as the bubbles burst a t the surface, liquid containing chloride was lost from the titrate as minute droplets. With titration on a hydrofuge surface ( 3 ) ,the air stream directed a t the titrate body to effect stirring, did not efficiently disperse the precipitated silver chloride about the microburet opening and the indifferent electrode. Walsby ( 4 ) describes a flask for a reciprocating shaker in which the internal contours cause the contents to be swirled around as a body. The technique was developed for algal culture work and is also used for long term con(2) P. H. Sanderson, Biochern. J . , 52, 502 (1952) (3) J. Shaw, J. Exp. B i d , 32,321 (1955). (4) A. E. Walsby, Eiotechno/. Eioeng., 9,443 (1967).
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 1 4 , DECEMBER 1973
2445
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1 1
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,,Microburette
delivery tube
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Apparatus side view along the pivot axis with the solder k n o b striking the vessel to cause it to swing away. B. Transverse section through the apparatus across the pivot axis with vessel in vertical position during the swing Figure 1 . A.
densation reactions, in which conventional methods of stirring, to prevent clumping of nondissolved particles, have proved unsatisfactory. The requirements of this titration were similar, and small titration vessels were made which employed the same principle. The vessel (Figure 1) was straight sided with the base as a rounded cone protruding into its lumen. Apparatus Design and Procedure. The vessels were made from 12-mm i.d., medium wall thickness, borosilicate glass tubing. This tubing was blown to give a test tube round end and this was flattened in the flame. When the flat base was yellow to white hot, the tube was placed firmly and vertically base down onto a piece of asbestos board. The air in the asbestos directly under the hot base expanded and the increased pressure forced up the molten base as a smooth cone into the vessel lumen. The tube was then cut to give a vessel of external height, 2.0 cm. Once a known volume of titrate is delivered into the titration vessel, evaporation of water is irrelevant as long as sufficient volume of liquid remains to allow adequate dispersion of the precipitate. Volumes delivered from a breaking pipet in this study were found by back titration to be 7.95 g1 (*170). For practical convenience, this volume was increased by the addition of 0.3 ml of A.R. ethyl alcohol. This addition may affect the titration when titrating for very low concentrations of chloride with silver ni(5) K Schwarz, and C. Schlosser, Mkrochernie
13 ( N F 7 ) , 18 (1933)
trate (5), but here the range of concentrations was too high for any such effect to be important. Independent control titrations showed that the addition of small volumes of ethanol did not affect the end point. With vessels this small, the lateral motion required to effect the rotary swirling could be produced by swinging the vessel through a limited arc a t a controlled frequency. A rubber collar was cut from bunsen tubing and two pins were pushed through the collar from the inside, along a diameter. These were the vessel supports and pivots when the collar was fitted to the vessel. This type of collar allowed precise adjustment of the pivot height above the vessel's center of gravity. The pins pivotted in grooves of a fixed metal support ring within which the titration vessel swung. Regular swinging of the titration vessel is effected by its being struck by a soldered knob on the turning shaft of an electric motor. The turning speed of the motor is controlled through a rheostat as the striking frequency is critical. A small wave propagated in the titrate passes around the conical base rather than across the vessel and is reinforced each time the vessel is struck. The resultant stirring efficiently mixes the liquid and prevents precipitateclumping about the microburet and electrode tips. A rubber tubing collar fitted over the microburet delivery stem within the vessel, acts as an internal buffer, limiting the swinging travel. The collar also supports the silver anode wire which is pushed down through it to a position closely adjacent to the microburet tip. The collar is turned to align the anode and the delivery stem perpendicular to the pivot axis. The microburet tip and the anode are then positioned close to the side of the vessel in the greatest depth of liquid and adjacent to either of the pivots. With this precise positioning and buffering, neither the microburet nor the anode are struck by the swinging vessel. The electric motor is clamped so that it can be swung to or from the vessel. At the end of a titration, by lowering the pivot support ring away from the microburet, the vessel is easily removed for cleaning and refilling and can be readily replaced. Once the apparatus has been precisely assembled, a whole series of titrations may be performed rapidly without further adjustment. The electrodes are connected directly to the input of a potentiometric recorder (set to 500 mV full scale deflection) and the whole process of titration is highly automated. Received for review January 29, 1973. Accepted August 2, 1973.
Fluorescent Integrating Radiation Dosimeter W. A. Salmon and E. A. Chandross
Bell Laboratories, Murray Hill, N.J. 07974
The field of high-energy radiation processing of materials on an industrial scale is slowly but continuously expanding. A partial list of processes in operation includes the manufacture of heat-shrinkable film and tubing, the cross-linking of wire insulation, and the sterilization of medical supplies. A need exists for a simple indicator which would show a t a glance whether or not a supposedly 2446
processed object has indeed been exposed to the intended radiationdose. Various dye dosimeters are known which undergo a visible color change upon irradiation, i. e., the blue cellophane of Henley and Richman ( I ) which bleaches on exposure to (1)
E. J.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 1 4 , DECEMBER 1973
Henley and D. Richman, Anal. Chem., 28,1580 (1956).
