LETTERS
To the Editor: I have read with interest the article by 11. vanSwaay and R. F. Lolley in the July issue [42, 381 (1965)l describing a simple constant-flow device for use in titrimetric analysis. It seems to me that if the specific gravity of the titrant differs from that of water, there should he variations in the rate of flow which depend on the quantity of titrant within the polyethylene sleeve, and I have found that this appears to be the case experimentally. For some years I have been obtaining constant flows using a Marriotte flask Const~ctedof materials readily available in most laboratories (see figure). After the air begins to bubble from the constriction, the pressure driving the liquid through the capillary will he that of the hydrostatic head between the upper end of the capillary and the constricted tubing tip (neglecting pressure changes within the capillary). Variation of this hydrostatic head and of the diameter of the capillary will produce a very wide range of liquid flows. I t should he noted that no error is introduced if the liquid to bemetered has a specific gravity differing from that of water. If desired, the sketched device can he altered easily to permit refilling the bottle during use. Holes should be made in the stopper to accommodate two additional glass tubes. One of these, of as large a diameter as can be used conveniently, should be fitted with a funnel a t its upper end; its lower end should he at a level below that of the constricted tube. Through the other hole is inserted a smaller diameter glass tube; its upper end should he fitted with a small stopcock which is connected in turn to a water aspirator, and its lower end should he just below the stopper. To add more liquid to the bottle while the device is in use, one turns on the aspirator and opens the stopcock carefully so as to increase slightly the rate of bubbling from the constriction. Care should be exercised not to increase the bubbling rate excessively. The liquid which is to he added can now be poured into the funnel, (not so fast that bubbling at the constriction stops). After the liquid addition has been made the stopcock is closed and the aspirator turned off.
K. J. RADIXER 12 MARTIN PLACE LITTLE FALLS,NEWJERSEY
446
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Journal o f Chemical Education
To the Editor: The suggestion in J. CHEM.EDUC.,43, 264 (1966), that a razor blade be used as a sample for student analysis, may be an easy way out for the instructor, hut I feel that it is questionable pedagogy and unfair to the student and to his prospective employer. Quantitative analysis should he taught as closely as possible to the way it is practiced. The answer should not he known in advance; the student should he graded on his performance; and his work should be done under conditions that preclude cheating and insure that the grading be correctly and fairly done. Iron and steel are never analyzed for iron. They are commonly analyzed, hut only for the minor components. Rather than have students analyze a steel for iron, it would he better to analyze a primary standard such as electroytic iron or ferrous ammonium sulfate-materials for which the constancy is assured. Every scientist should be brought early in his career into brutal contact with the impersonal characteristics and the immutable laws of nature. Thus only can he he subjected to the discipline that will develop the critical appraisal of physical situations and personal performance, and foster the self-reliance that is essential to independent thought and to professional employment. The quantitative analysis and physical chemistry laboratories are the ideal training ground for these purposes. Fortunately, good iron ore samples are available. The 450 series marketed by the G. Frederick Smith Company of Columbus Ohio, is composed of a sufficient number of samples for even the largest classes. The prices are high, equal apparently to those of the standard samples of the National Bureau of Standards, hut the samples have been meticulously prepared and so thoroughly tested for homogeneity as to make them above reproach. The relative standard deviation in parts per thousand is less than 2.0 on each member of the series, as determined by analysis of a t least six specimens taken after 24 hours of blending of 200-mesh ore. On sample 453, for example, the figures are: percent Fe, 59.76; standard deviation, 0.054; relative standard deviation, 0.91. Using these samples a teacher can assess the performance of his students with confidence and honesty.
