afeature
Compiled by
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CHEMICAL PROJECTS Research Ideas for Young Chemists JAY A. YOUNG, King's College, Wilkes-Borre, Penno. JOHN K. TAYLOR, National Bureau of Standards, Washington, D. C.
Precipitation from Homogeneous Solution See GORDON, L.,
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SALESIN,E. D., J. C h m . Educ., 38, 1 G (1961).
The particles of a precipitate formed by mixing solutions of two reagents often exhibit different physical properties, depending upon the manner of mixing of the two liquids. In general the more slowly the liquids are mixed, the larger tho particles of precipitate which form. I t is possible in many instances to generate the precipitating reagent within the solution containing the component to be precipitated. In these cases the effect is similar to that which would be produced by very slow mixing of two solutions in the conventional way. Usually precipitates formed hy this process have large, well-defined erystdline st,ructures and in general are much more suitable for the purposes of quantitative analysis. ~
The precipitation of nickel dimethyglyorimate from homogeneous~olutionis described in the article.
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Questions:
Suggest a. reason why the onset of precipitation of nickel dimethyglyoximate should he delayed when the pH is changed from 7.9 to 7.1. Test your hypotheais by further laboratory work. Devise a procedure for the precipitation of another substance from homogeneous solution. What factors which you can cantrol in your procedure affect the properties of the precipitate or the onset of precipitation? Account for some or all of these effects and test your hypotheses by further laboratory work.
The Turbidimetric Determination of Lead See PERKIX, O.,JR., ET AT.., J . Chem. E ~ I ~ 38, c . ,3 5 8 4 0 (1961). The auantitv of nreci~itatein a t u r l d susnension wn be determined by comparison with a turbid suspension of equal volume containing a known amount. of precipitate, using a calorimeter or spectrophotometer. In this snnlytical procedure care must be taken to insure that the pnrtirles of precipitate are of approximately the same size in bnt,ll the unknown and in the standard suspensions. In some cases these necessary P r e cautions are effected by an elaborate time consuming procedore, hut in the analysis described in this artirle these have heen minimised. In addition to tho usual reagents and apparatus avl~ilahlein any laboratory, n Duboscq ealorimet,er or more elaborate elmtronie instrument will he required. A suitable substitute device could be designed by an ingenious student: essentially, a light source, n test tube holder, a barrier layer photo-cell, and a sensitive ammeter-dl hut the latter enclosed in a light tight box-would suffice.
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Analysis of Aspirin: See PROCTOR, J. S.,
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Questions: ----..-. ~~
Devise a turbidimetrir procedure for the determination of any precipitate, other than lead molybdate. Can you modify the procedure given in the artirle in order to ~ b t n i nresults lower per cent errors tllan those reported? FO, example, precipitation from homogeneous 8olution, in wllich t,he particle size is controlled by regulation of p~ or of ti,c concentration ,f be see 1,. A N D E. D. S n ~ s s i J. ~ ,CHEM.I ~ c c . ,38, 16 (1961) for GORDON introductory details, When the absorbency of a turbid suspension is plotted against conaentration of precipitat,e, a straight line is not obtained; the data do not follow Beer's law. Can you account for the deviation from Beer's law? Test your hv~othesishv further Inhorntory studies.
A Conductometric Titration
ROBERTS, J. E.. J . Chenz. Edzrc., 38, 471 (1961).
Unlike most titratians, in a conduetometric titration it is not necessary t o stop adding titrant a t the end-point. The end point is determined hv " a. eraohicitl analvsis of the conductance of the tit,rnted solution versus titrant added over a range which includes data obtained both before and after the end point is reached. Also, no indicator is required. As usually perfonned, however, the procedure is tedious, since several conduetometric measurements must be made.
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But by using an inexpensive capacitance tester, available in kit form from well-known suooliers of electronic kits. the measure may be quickly and easily performed the desired analysis completed by a graphical analysis of the data. I n the article, the procedure for a conduetometric determination of aspirin is described.
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A306 / Journal of Chemical Education
Questions:
Select other available suhst,ances, instead of aspirin, nnd determine the amount, or concentration, present b r a cnnductametric titration. Suggested common substances are vinegar, haking powder, other pharmaceutical products in addition to aspirin, rack salt, otller housel,old Can you use a conductometric t,itration to assist in the solution of a problem listed in another project, in this series? The authors suggest that the potentiamet,er furnished with the capacitance tester kit is not strictly linear, a t least a t the extreme end8 of the winding. Is this true for your tester? If so, do you think it would be desirahle to replace the potentiometer which is furnished with a more precisely-linear potentiometer? Support your conclusion with data obtained in the laboratory. N ~ hi^~ project ~ is: partieulsrly recommended to students who are interested in electronics and its relation to the experimental study of chemistry.
