ference in the results using the 100-pm probe and the 65-pm probe is probably due to a slight difference in the positions of the tips of the probes relative to the flame. CONCLUSIONS The quantitative sampling from a reactor in which the steady state flow rate does not exceed 10 cmisec, and in which the sample must pass through heated zones and remain in those zones for any period of time, must meet certain criteria. Of those interpretive problems set forth in reference ( I ) , the effects of concentration gradient and quenching are of the most concern here. The optimum sampling probe should be at least 2-mm i.d. and have an orifice which, when operated at a pressure of one half the external pressure (critical orifice), permits the cross section of gas being removed to be at least the cross section of the probe. This will minimize the aerodynamic disturbance to the flame and yet achieve adequate quenching of the stable species. The probe orifice should not exceed a value which will allow more than one per cent of the flame gases to enter the probe. This will assure that the concentration gradient will not be a problem and that a point source
sample will result. For a gas chromatographic analysis of the probe sample, higher pressure samples will give more sensitivity. Thus, it would be advantageous to operate at pressures just below that which will assure complete quenching. Heated sampling lines could be used to reduce water adsorption on the walls of the lines, although under dynamic sampling after a conditioning period, this should not be a problem. Heating the sample lines may be a problem in that some compounds normally stable at room temperature may become unstable at higher temperatures. Teflon sampling lines are preferred to stainless steel to reduce adsorption. Free radicals play an important role in preignition events. Conventional types of probes such as described in this paper will not quench free radicals. Thus, further reaction of free radicals can take place in the probe to form stable species. If the free radical is one that forms a stable product which is being measured, then an error in the analysis will result.
RECEIVED for review December 3, 1971. Accepted May 1, 1972. Paper presented at the 162nd National Meeting of the American Chemical Society, Washington, D.C., September 12-17,1971.
Critical Evaluation of the Karl Fischer Water Method, End-Point Detection System, and Standardization Thomas H. Beasley, Sr., Howard W. Ziegler,l Richard L. Charles, and Perry King Mallinckrodt Chemical Works, Post Ofice Box 5439, S t . Louis, Mo. 63160 The classical Karl Fischer method for determination of water can be used for precise assay purposes under carefully controlled conditions. Variations of the “electrometric end-point” procedures commonly used with Karl Fischer titrations were evaluated by nonaqueous polarographic techniques, resulting in a better insight into the cell behavior. The electroanalytical parameters for end-point detection have been defined without arbitrary or empirical calibration. A simple automatic amperometric titrator, used to determine the optimum operating parameters, is described. Sodium tartrate dihydrate, commonly used for standardization of Karl Fischer reagent, was found to be an unreliable fundamental reference because of the occlusion of about 0.3% absolute (2% relative) water inside the crystal structure. This water is not removed by drying at 150 OC but is measured by the Karl Fischer method using the recommended conditions.
THISINVESTIGATION was initiated to resolve discrepancies found in the assay of some compendia items where the Karl Fischer ( K F ) water result is used to obtain a value on the anhydrous basis. After developing a technique for obtaining a precise and reproducible end point, a relative bias of minus 2 was found between standardizations of KF reagent against sodium tartrate dihydrate when compared with distilled water as the fundamental standard. Such a bias is insignificant for trace (
