LITERATURE CITED (1) R . J. Washkuhn, V. K . Patel, and J. R . Robinson, J. Pharm. Sei, 60, 736 (1971).
Kenneth A. Connors
School of Pharmacy University of Wisconsin Madison, Wis. 53706 RECEIVEDfor review April 30, 1975. Accepted June 25, 1975.
Auto matic Gas- Meas uring Device:
Exchange of Comments
Sir: There are serious limitations and errors in the automatic gas-measuring device proposed by Galblcs and Cslnyi ( 1 ) . Independently from other factors related to the very strict requirements for the construction of the apparatus, I want to make clear the following points. A) Times are referred to the distance between two signals (peaks of different heights) on the paper of a chart recorder (chart speed 25 cm/min). This may be the most serious source of error, in case 1 of Figure 1 of Ref. 1, the rate of gas feed is 15.81 ml/min, and since the volume between the electrodes is 0.118 ml, the time required corresponds to a distance between signals of 0.19 cm (with the given chart speed). This would represent in the case of an individual measurement, as it should be in the case of a time dependent rate of gas in a reaction, an error of about 0.5 mm, that is, 25% error. T o guarantee an error lower than 5%, the application of the device would be limited to gas rates no larger than 3 ml/min. That is to say, the device is not applicable to relatively fast reactions as the authors state. B) With the proposed device, the reaction under study could be followed up only to the point when 10 ml of gas have been produced or consumed, corresponding to only 80 signals. This limitation makes it necessary to use very small quantities of reactants and even smaller quantities of catalyst. For the study of hydrogen peroxide decomposition, shown in Figure 3, Ref. 1, the amount of H202 used is less than 0.03 gram. Such a small quantity must be handled in so!ution, and, therefore, the device does not allow the study of reactions with pure reactants. I am now studying a technique to measure and record the curves directly (rate of gas/time) which is applicable t o gas rates even larger than 50 ml/min for an unlimited time. T h e characteristics of this system will be published soon.
rates greater than 3 ml/min if a chart speed of 25 cm/min is chosen. (c) The device is suitable for measurement of not more than 10 ml of gas. We should like to make the following points. In our paper ( I ) , an automatic gas-measuring device was proposed, to follow reactions taking place with gas evolution or uptake on a (most usual) semimicro scale. We drew attention to the fact that the frictional force is proportional to the gas rate, which may result in limitations to this measuring principle a t high rates of gas (considerably in excess of 15 ml/min), when the liquid plunger (mercury) starts to disintegrate into smaller drops. On the other hand, reference was made in our paper to the possibility of extension of the range of the device, both t o greater volumes and to higher gas rates. For these purposes, five factors were mentioned: “The sensitivity of the device can be varied by selection of the diameter of the measuring tube and the distance of the electrode pairs, while the total volume to be determined can be varied by the length of the PVC tube. The range of reaction rate which can be determined automatically depends in practice on the chart speed and the response time of the recording potentiometer”. The remarks of Pascual show that the cited paragraph escaped his attention completely, as he took only one of the five factors (the recorder speed) into consideration. We may further mention that during the past year we have made efforts to enhance the sensitivity of our device in two respects (a) by increasing the number of sensor electrode pairs from 4 to 12, and (b) by integration (counting) of signals electronically and recording the number of signals vs. time curve directly. We have succeeded in enhancing the time resolution by another means: by recording the electric impulses of the Oscillotitrator on a tape recorder a t a comparatively high rate, and then playing back a t a reduced speed to evaluate the time-coordinate. T o close, we should say that it is quite usual that an instrument cannot be used over very extended ranges. This is true in the present case too. Consequently, we look forward with great interest to the new measuring principle of Pascual which will make possible the measurement of gas evolution on the macro scale.
LITERATURE CITED (1) M. 2 . GalbAcs and L. J. Csanyi, Anal. Cbem.,45, 1784 (1973).
V. L6pez Pascual Universidad de Cara!obo Facultad de Ingenieria Valencia, Venezuela
LITERATURE CITED RECEIVEDfor review March 31, 1975. Accepted June 9, 1975.
Sir: V. Ldpez Pascual suggests three shortcomings of the gas-measuring device proposed by us: (a) He found it too complicated and difficult to construct. (b) In his opinion, the time-coordinate is seriously in error a t gas evolution
(1) M. 2 . Galbbcs and L. J. Csanyi. Anal. Cbem., 45, 1784 (1973).
M. Z. Galblcs L. J. Cslnyi Department of Inorganic and Analytical Chemistry A. J6zsef University 6720 Szeged, Hungary RECEIVEDfor review May 5 , 1975. Accepted June 9, 1975.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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