into the flask. Adjust the pH as outlined above. One can also add fluoride standards to the crucible before fusion. Because of the approximately logarithmic response of the electrode, a typical series of standard solutions contains 1, 3, 6, 10, 30, 60, and 100 pg of fluoride per flask. Figure 1 illustrates the deviation of the standard curve from strict logarithmic form. The reproducibility of the curve is checked with a standard or two which contains about the same amount of fluoride as the samples; the potentials are reproducible to 1 mV. RESULTS AND DISCUSSION
Table I compares results obtained for three samples analyzed by pF measurement and by anion exchange separation with a spectrophotometric finish (4). Experiments were performed on portions of one tungsten sample to test whether any volatile fluoride compounds were escaping during fusion (4). The results indicate that no fluoride is lost. Three other observations were made about the fusion reaction. First, the reaction is quite exothermic and begins at
a temperature of 380”420” C ; this initiation temperature apparently coincides with the decomposition of sodium nitrate. Raising the temperature too rapidly, or starting with the temperature greater than 400 ” C, produces too vigorous a reaction which spatters the products and causes a loss of fluoride. Second, a white vapor seeps out of the crucible during reaction of the sample, but no fluoride was detected in it. The vapor gives a basic reaction in water and is probably sodium oxide carried with the nitrogen gas escaping from the reaction. Finally, if sodium is replaced by potassium, the products show a considerable tendency to creep out of the crucible. Also, nitrate can be replaced by chlorite, chlorate, and perchlorate, all of which give smoother but slower reactions. A single operator can do 8 to 12 determinations per day by this procedure. This procedure will determine 2 to 100 pg of fluoride in a gram sample with a standard deviation which is about =t2 pg at 95 % confidence. RECEIVEDfor review February 23, 1967. Accepted June 15, 1967. Work performed under the auspices of the U S . Atomic Energy Commission.
Scale Expansion and Increase in Sensitivity in Rotational Viscosimetry Eberhard Zimmermann Physiologisch-Chemisches Institut der Uniuersitat, 87 Wiirzburg, W . Germany
RECORDING VISCOSIMETERS which are commercially available measure changes in viscosity between 0.5 and 50 centipoises on a recorder span of 15 cm. In biologically important substances viscosity changes fall in the range of 0.5 to 5 centipoises; under these conditions only 10% of the available recorder span would be utilized. This report describes the modification of an industrial rotational viscosimeter which greatly increases the sensitivity and thus extends its use to viscosity measurements in physiological regions. The sensitivity of this kind of instrument depends primarily on the geometry of the rotating cylinder and its chamber, as the distribution of shear speed between parallel planes or cylindrical surfaces is not linear and may be regarded as constant only across a relatively narrow cross-sectional distance. If the instrument, which consists of coaxial cylinder combinations, maintains constant angular velocity, the shear speed and the viscosity will determine a torsional moment. This torsional moment is measured on an electrical dynamometer and is subsequently recorded (1,2). In order to increase the sensitivity of such an instrument, several problems must be considered. Experiments designed to study changes in structure or structural stability of biological substances are, in most cases, carried out in solvent systems of different characteristic viscosities in a temperature range of 20’ to 90” C.
(1) M. Hediger in “Messungen rheologischer Eigenschaften,”
Contrares-IndustrieprodukteGmbH, Stuttgart-Vaihingen,Bulletin 6704-652 (1966). (2) R. B. Martin, in “Introduction to Biophysical Chemistry,” McGraw-Hill, New York, 1964, p. 164.
If the sensitivity is increased by the simple expedient of a fixed, linear relation between the dynamometer and the recorder, it will reach a point where it will be limited by the viscosity of the solvent. In a more viscous solvent, a relatively large part of the recorder span is no longer available for the experimental change in viscosity due to the solute. The electronic displacement of the zero point with a potentiometer circuit permits a limited full-scale recording range to be registered but requires repositioning for each measurement. Furthermore, in most cases it is not possible to predict how large the viscosity change will be so that several zero point corrections may be necessary during the course of an experiment. A satisfactory solution to these problems has been accomplished by the following modifications. A rotatory viscosimeter from the Firm Haake, Berlin, was adapted to a 12-channel compensation recorder (Philips, Type PR 3210 A/OO). The chart width of the recorder was 25 cm, full scale, and corresponded to a potential change of 100 mV. The sensitivity of the torsion dynamometer was calibrated by potential division (560 KO: 10 KQ) at the voltage output with a potentiometer so that 100 mV output potential (corresponding to 20 scale units on the meter of the basic instrument) conditioned a full scale recorder response. The load through the recorder lead (input resistance, 560 KQ) is therefore minimal. The potential variation due to the parallel connection of the Compensation circuit to the Diode ECO 1339 is insignificant. Correction of the zero point by the use of the 10 KQ potentiometer is necessary in order to accommodate the recorder zero point to the zero point of the viscosimeter. In this way viscosity changes of 1.31 centipoises can be expanded to a scale 25 cm in width with regard to the paVOL. 39, NO. l l , SEPTEMBER 1967
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Figure 2. Diagram illustrating decrease in viscosity with increasing temperature As viscosity decreases and the pen reaches the end of the scale, the pressure switch is actuated automatically displacing the pen to the next recordingrange. Full scale in diagram is 85 cm
mm. If the flexible drive shaft from the dynamometer is mechanically fixed, the error is reduced from 2 to + 0.5-1 Since the viscosity-temperature ratio is an important factor in the measurements described, a simultaneous recording of the temperature of the system was made with the use of a thermistor. With these modifications negative and temperature-dependent changes of dynamic viscosity can be recorded at a shear speed of 2620 sec-l with a scale over 100 cm in width (Figure 2).
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Figure 1. Wiring circuit for torsion dynamometer with step switch combination
rameters and the geometry of the apparatus. In order to fully utilize the instrument for measurements in regions of higher viscosity at the same sensitivity, the recorder was equipped with a pressure switch at the end of the scale. It is actuated by a stepping switch (Type Travid-SR 24E, a two-armed magnetic switch with return contact) and supplies a counter potential to the recorder of 100 mV per step (Figure 1). The response of the stepping switch ensures a return in 0.4 second over all of the 10 steps. A DC-stabilizer (Philips, Type PEA 4222/06) is used to minimize fluctuations in the line voltage to the viscosimeter as the error of measurement in the region of 1 centipoise is due to the deviations in the rotating cylinder drive of 0.005
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ANALYTICAL CHEMISTRY
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ACKNOWLEDGMENT
The author wishes to express his appreciation for the technical assistance of Hans Feineis.
RECEIVED for review February 24, 1967. Accepted June 23, 1967. WORKsupported by the Deutsche Forschungsgemeinschaft.
