NOTE on DETERMINATION of EQUIVALENT WEIGHTS N. M. SHAH Karnatak
College, University of Bombay, Dharwar, India
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F T H E different methods of determining the While magnesium requires hardly 4-5 ml. of conc. equivalents of common elements, that of dis- HC1, in the case of Al, about 5 to 6 times that amount placing hydrogen from an acid by reaction with of the acid has to be taken to hasten the reaction. This method is especially suitable for demonstration a metal like Zn, Al, Mg, etc., is of high pedagogical value from various standpoints. Naturally, there- purposes. The reaction is completed in a very short fore, an experiment involving this method is usually time. I t is rather strange that few books on practical given to an elementary chemistry class. chemistry devote any attention to it. The essential part of the experimental procedure consists in collecting the hydrogen evolved. Two methods are commonly followed: (1) direct collection and measurement of the gas evolved; (2) collection and measurement of the equivalent volume of water dis--, placed by hydrogen. The object of this note is to compare the methods, pointing out their relative merits with regard to (a) apparatus used, ( b ) their suitability for large freshman classes, and ( 6 ) the results obtained. The direct measurement of the volume of hydrogen can be easily accomplished by collecting it in a graduated eudiometer tube.* The eudiometer filled with suitable acid and water is inverted over a weighed piece of the metal taken, in a dish containing some water. The only precaution necessary is to avoid the entry of air bubbles during the above operation.f The whole arrangement is shown in the accompanying diagram (Figure 1). The volume is read a t barometric pressure The second method requires a little more elaborate by transferring the eudiomeapparatus. Numerous authors of books on practical ter to a cylinder full of water chemistry1 advocate the use of either a liter flask or a and adjustingthe levels inside bottle fitted with a two-hole stbpper, as the "siphon and outside to coincidence bottle" in which hydrogen is collected, displacing an (Figure l b ) . equivalent volume of water. This method requires very This arrangement of apparatus, though good in itsimple apparatus which can self, is not very satisfactory, because: (1) often it be supplied to a large class; happens that there is a slight leak, leading to errors in it is easy to work with; and the result; (2) the elemeritary student becomes disthe results are very satisfacappointed a t his inability to set up an air-tight apparatory, as will be shown in the tus; (3) there is a great deal of recurring waste of corks table given (uide infra). and glass tubing. 16 The only difficulty enFIGURE This deficiency in apparatus has been successfully countered is the extremely slow reaction in the case of a metal like aluminum, remedied. It is found that an ordinary gas-washing but this can be rectified, the amount of the acid to be bottle (Drechsel's p a t t e d of about !m-ml. capacity used depending upon the nature of the metal taken. S a ~ e sas an excellent substitute for the PurPoses of this experiment. The diagram shows the complete * In case graduated eudiometers are not available, an ordinary $ See: BRUCE AND HARPER,"Pmtical chemistry," Macmillan glass tube (diam. about 1 cm.), closed at one end, works very well. In the diagram, an ungraduated tube is shown. In this Co.. 1925, p. 243; A. Smm, "Laboratory outlines of elementary case, an extra measuring cylinder (100 ml.) is to be supplied. chemistry," G. Bell & Sons, Ltd., London, 1915, pp. 41, 135; t For complete details of the experiment, see SHAH, "Practical HOLMYARD, "Practical chemistry." G. Bell &Sons.Ltd., London, chemistry," 2nd edition, The Students Own Book Depot, Dhar p. 29; LWDLAMa m PRESTON. "Experimental chemistry," Edward Arnold & Co., p. 43; and others. war, 1933, pp.37-8. 492
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arrangement of the apparatus (Figure 2). All other arrangements are the same as previously described.* The reaction flask (shown in the diapam) can be conveniently replaced by a hard-glass test tube, as it has been experienced that it can be pressed into the cork very tichtlv without much daneer of breaking. This apparatus also serves very well for experiments on the determination of molecular weights of gases insoluble in water.*
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T*eLe S H O W ~ N THB G RBSULTS 0s EWIYALENT DB.TE~~M~NATIONS
orr Meld
DraPBRENr
M E ~ * LB S Y T H E ABOVBMBTHODS
EudiomrUr Mclhad
Zinc
31.46
Magnesium Aluminum Tin
11.97. 12.1 9.1
Siphon Bolrle Method, urina .. %. .! Ymr-xaru."g old pattern bottle as siphon bottle 29.6, 3 0 . 2 RR
...
s
.. 8.46 ..
siphon bottle 3 1 . 8 , 32.12 31 a 11.9,12.2 9.02 58.1
SUMMARY
1. In the equivalent determination of metals by the displacement method, the direct collection of hydrogen by eudiometer gives good results. The method is suitable for large classes because of the simple apparatus required. It is a very convenient experiment for demonstration purposes.
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* For complete details of the experiment, see SHAH,106. cit.. loc. cit., p. 136.) pp. 40-3. (Also cf A. SMITH,
into theacid, t o start thereaction.
2. The apparatus for the siphon-bottle method has been modified. Excellent results are obtained by using an ordinary gas-washing bottle as a siphon bottle. The author wishes to acknowledge his sincere appreciation for the helpful ciriticism of Mr. R. L. Alimchandani.
