1 AIDS FOR ANALYTIt%ASGHEMISTS Effective Temperature in Stopped-Flow MeasurementsA Potential Source of Error with Certain Instruments P. K. Chattopadhyay and J. F. Coetzeel Department of Chemistry, Unicersity of Pittsburgh, Pittsburgh, Pa. 15213 THEPURPOSE of this communication is to alert users of certain designs of stopped-flow instruments to a potential source of error in measurements carried out a t other than ambient temperatures. The problem is caused by a temperature gradient existing in some flow systems manufactured from Kel-F o r possibly other materials having poor thermal conductivity. We have encountered this difficulty (which, fortunately, is easily overcome) with a Durrum Model D-110 stopped-flow spectrophotometer equipped with a Kel-F valve block. The manufacturer does point out ( I ) that for measurements requiring better temperature accuracy over a wider temperature range, an optional stainless steel flow system is recommended. I n this communication, we are concerned with measurements involving reagents that attack stainless steel, and which therefore require the Kel-F flow system. In this design the glass-walled drive syringes a r e totally immersed in the circulating thermostat liquid, but the Kel-F valve block is only partially immersed, although the flow channels between the drive syringes and the mixing jet are located in the lower part of the valve block, well below the surface of the thermostat liquid. It may be unexpected t o some users (as it was to us) that when the thermostat liquid is a t other than ambient temperatures the temperature of the reactant solutions in the valve block is significantly different from that in the thermostat liquid well. A valuable feature of this instrument is that the position of the stop block can be adjusted to vary the volume of each reactant solution dispensed per measurement from a maximum of 0.36 ml down to very small vo!umes. The minimum volume permissible might appear to be that required t o displace all of the reaction product from the observation cuvette with fresh reactant mixture. The capacities of components of the flow system are as follows ( 2 ) ; valve block, 0.285 ml per channel; mixing jet, 0.039 ml; cuvette inlet, 0.008 ml; and cuvette, 0.068 ml for stopped-flow o r 0.20 ml for dual-purpose stopped-flow temperature-jump cell. Consequently, if less than ca. 0.34 ml per channel is dispensed with the 0.07-ml cuvette, o r 0.41 ml with the 0.20-ml cuvette, part o r all of the reactant solutions will come from the valve block where the temperature may be different from that of the thermostat liquid. Under such conditions, significant errors may arise. In the determination of the activation parameters of reactions, this error may escape recognition because the error in the 1
Please address all correspondence to this author.
(1) Durrum Instrument Corporation, Palo Alto, Calif., Bulletin
3.5
3.E
log k
3.2
3s
2.7
. IO~T-'
Figure 1. Apparent second-order rate constant as a function of temperature for the reaction o f nickel(I1) with 2,2'-bipyridine in aqueous solution
Ambient temperature = 27 k 1 "C. Closed circles: volume of cuvette = 0.07 ml, volume of reactant solutions dispensed per flow channel = 0.36 ml; open circles: volume of cuvette = 0.20 ml, volume dispensed per channel = 0.21 ml; square: same value for two sets of conditions. Values obtained for AH*: solid line, 11.7 + 0.2; dashed line, 10.1 f 0.2 kcal mole-'
assumed effective temperature varies approximately linearly with the difference between ambient and thermostat temperatures; consequently, the linearity of Arrhenius plots is not affected. However, the Arrhenius slopes obtained are lower than they should be. As a n illustration, we present in Figure 1 two Arrhenius plots obtained for the reaction of nickel(I1) with 2,2'-bipyridine in aqueous solution. Full details will be presented elsewhere (3). In one set of measurements, all of the solution in the cuvette had originated directly from the drive syringes, while in the second set all of the solution had come from the valve block. F o r both sets the ligand concen-
131.
(2) Durrum Instrument Corporation, personal communication, 1972.
(3) P. K. Chattopadhyay and J. F. Coetzee, to be published. ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2117
tration was kept constant a t 2.5 X lO-5M while the nickel(I1) perchlorate concentration was varied through 5 values from 1.25 X to 5.0 X 10-3M at each temperature. For both sets the resulting plots of pseudo-first order rate constants (reproducibility 1 2 %) were linear and passed through the origin at all temperatures. However, the slope of the one Arrhenius plot is 13% lower than that of the other, and is consistent with a deviation of the effective valve block temperature (Tb)from the thermostat temperature (T,) given by: 7'' T , - 0.13(Tt - To),where T, is the ambient temperature. It is unlikely that this simple relationship will apply over a much wider temperature range than that studied here. I t was verified by removing a valve and inserting a thermocouple into a flow channel that the solution there was closer to ambient temperature than was the thermostat liquid. However, it is difficult to measure meaningful temperatures in this way, because the thermal conductivity of the surroundings under actual experimental conditions is not readily simulated in the absence of the valve and in the presence of the thermocouple; furthermore, a longitudinal temperature gradient exists in each flow channel, with the temperature deviating more and more from that of the thermostat with increasing distance from the drive syringe. Consequently, additional tests were carried out at a thermostat temperature of 45.0 f 0.1 "C and an ambient temperature of 27 i 1 "C. Using a
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0.20-ml cuvette and dispensing first 0.21, and then 0.36 mlper channel, the apparent rate constants were 7.1 X lo3and 7.8 X l o 3 1. mole-' sec-I, respectively. On changing to a 0.07-ml cuvette and using a second pump with intake from the thermostat well to circulate liquid around the cuvette (as recommended by the manufacturer), dispensing 0.21 and 0.36 ml per channel gave apparent rate constants of 7.6 X lo3 and 8.0 X lo3 1. mole-' sec-', respectively. Varying the level of the liquid in the thermostat well from two-thirds full to completely full, and making measurements a t intervdls from hour to 2 hours after the thermostat liquid had reached a temperature of 45.0 "C, had no discernible effect. Finally, solutions at ambient temperature introduced into the drive syringes reached a temperature of 44.9 "C within 15 minutes. From the above results, the best value of the rate constant at 45 "C is 8.0 X 1031.mole-lsec-l. We conclude that when instruments of the design described here are used at other than ambient temperatures, sufficient volumes of reactant solutions should be dispensed t o ensure that all of the solution being measured has originated directly from the drive syringes.
RECEIVED April 14, 1972. Accepted May 23, 1972. Financial support by the National Science Foundation under grant GP-16342 is gratefully acknowledged.
Simple Inexpensive Flowmeter for Use with Fluorine and Other Corrosive Gases A. B. Waugh Chemistry Department, Unicersity of Melbourne, Parkdle, Victoria, Australia 3052
P. W. Wilson Chemical Technology Dicision, Australian Atomic Energy Commission, P.M.B., Sutherland, N.S. W., Australia 2232 MOSTFLOWMETERS for use with fluorine are both expensive and unreliable, and moreover can often only be used over a narrow range of flow rates. The simple differentialmanometer type flowmeter described here is made from materials resistant to fluorine. In the literature, a number of differential-manometer flowmeters are described, but their restricting capillaries are usually made from glass tubing and therefore are not suitable for use with fluorides (1). Capillaries made of materials other than glass normally have to be purchased and are expensive. The material most suitable for use in a fluorine resistant flowmeter is polytetrafluoroethylene (Teflon, Du Pont), but n o simple way to make a small hole in this material was available previous to this work. A method for making small holes in Teflon has been described but it is complicated and time-consuming (2). EXPERIMENTAL
Figure 1 is a diagram of the flowmeter we have used successfully. The U-tube is made from l/& 0.d. Kel-F tubing and is filled with Kel-F oil. The T-unions are Monel metal and the inlet and outlet tubes are nickel. Presumably, other materials could be used in these components depending on the particular gas being metered. ( I ) G. Brauer, "Handbook of Preparative Inorganic Chemistry," 2nd ed., Academic Press, New York, N. Y . , 1963, p 84. (2) H. P. Raaen, ANAL.CHE44.. 34, 1714 (1962). 2118
ANALYTICAL CHEMISTRY, VOL.
TEFLON CAPILLARY RLSTRICTIOII
I
Figure 1. Fluorine-resistant differential manometer The restricting capillary is made as follows (Figure 2). With a small drill (approx. l / d n . diameter) holes are drilled in both ends of a piece of Teflon rod until only a l / d n . thick plug remains in the center of the rod (Figure 2 4 . Metal needles are inserted into these holes. One needle is grounded; the other needle is connected t o a high-frequency discharge coil, in our case, Edwards H. F. Tester, Model T.l. (Figure 2B). The high-frequency coil is turned on and is held in position until a discharge occurs through the Teflon plug. A discharge within the Teflon rod is easily seen since it emits a bright purple glow. If a low flow rate is required, the coil is removed quickly. If the discharge is maintained for a longer period, a larger hole is made and the
44, NO. 12, OCTOBER 1972
