CHEMICAL PROJECTS Research Ideas for Young Chemists JAY A. YOUNG, King's College, Wllkes-Barre, Penna. JOHN K. TAYLOR, National Bureau of Standards, Washington, D. C.
Density Measurements with a Magnetically Controlled Float See CARTAN, F., A N D ANACEER, E. W., J . Chem. Edue., 37,36(19GO). One of the most precise methods for the determination of the densities of liquids makes use of a magnetically-controlled float. A glnss float containing a small magnet or a piece of soft iron is completely immersed in tho liquid and a. current is passed through a solenoid until the float remains etationmy in the liquid. I t can be shown that the current required is a linear function of the density of the liquid. The article describes a simple flotation apparatus that could be constructed lw students. The theorv is described in sufficient detail and the edibration of the instrument done should be an interesting and instructive exercise. The authors descrihe several rtpplicttiuns of the apparatus to hath physico-chemical measurements and to chemical analysis. Accordingly, a wellconstructed deneimet,er could be used by a number of students to sbudy n variety of interestingproblems. Querlionr: Construct flotation apparatus as described, calibrate it, and verify its arrurnry and precision by measuring densities of a perks of liquids whose values are known.
Study the factors that affect the reproducibility (precision) of measurements with your apparatus. These may be identified from the theory of the method, and verified experimentally. Make modifiostions as possible to improve the equipment. Use the apparatus to monitor the purification of liquids by distillation, for example. What are the limitations of such a procedure? Select an interesting application among those suggested by the authors and study a related problem with the apparatus. "Densiametrie" titrations are mentioned as a. possible application. Compare the results obtained in this manner with those found by more conventional methods, with respect to precision snd accuracy. An interesting modification of the apparatus described to make possible rapid routine measurements of density is described by A. R. Richards in Ind. Eng. Chem., Anal. Ed., 14, 595 (1942). Such an apparatus could he constructed for analysis of antifreeze solutions, the results correlated with freeaing point determinations, and compared with commercial hydrometers.
Float Method for the Analysis of Solutions See O s s u n ~J. , O.,
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CAOI,I . C., J. Chem. Educ., 38, 578-80 (1961).
No other technique readily available to the beginner is as precise as the use of s. suspended float to determine the concentration of salute in a solution. With care and with apparatus which can be easily built, sensitivity of the order of 1 X 10g of solute /ml of solution can be attained. A hollow cylinder of glnss, about the size of a grain of wheat, or smaller, is constructed from glass tubing and closed st both ends. By trial and error, adding or removing bits of glass with the aid of a tiny blow torch construet~dof small bore glass tubing, tiny floats are fashioned which have the desired density-using liquids of q proximately the right density to test the float from time to time as it is fashioned. The flont is calibrated by immersing it in ~olutionsof known conecntmtian, and determining the temperatures a t which, for each aolnbion., the float remains susoended within the liauid. Then, within the range of temperatures provided hy the calihration, quite precise determinations of solute concentration can he quickly established. A water bath in which temperature can be controlled to about +0.005"C is also required. I t can be constructed from a glasswalled fish nqunriwn (since the sides of the water hath must be transparent,). A powerful stirrer is needed, as is a, thermosensitive device constructed of glass tubing and filled with mercury (or with mercury and a liquid with a high coefficient of thermal expansion; see the cumulative indexes of J. CHEM. Enrrc. for suggestions), coupled to a. relay and electrical heater. For very precise temperature control, some investigators have found i t desirrtble to continuously cool the water bath while it isintermittently warmed by theelectrical heater. ~
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Questions: Use the flont method for the analysis of solutions as a technique for answering one of the questions asked in another project in this series.
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Journol of Chemicol Education
Equation (1) in the article is derived upon the assumption that the density of the glass float does not change with temperature. Is this a. valid assumption? If not, can you derive a more rigorous equation, using data you have obtained which describe the change in density of one of your floats as a function of temperature? The authors suggest that the temperature he directly determined a t which the float remains motionless, suspended in the liquid whose density is of interest. Is this necessary? For example, can you measure the rate of fall of the float a t one temperature, and its rate of rise a t a slightly lower temperature and from these data. determine the unique temperature a t which the float would he stationary? If so, is this procedure more, or less, precise; is it easier, or more difficult to accomplish? Are there any other modifications in the recommended procedure which will increase the precision or ease of the experimental work? When viscous solutions are centrifuged, for example, solutions containing a high concentration of sugar, i t might he expected that the concentration of the solute would vary along the length of the centrifuge tube. Does this actually happen? If so, can you determine a relation between the magnitude of the phenomenon and the duration, or revolution rate, or both, of the centrifuging? Clu yo11 t l r t t ~ i:I"?. widr~.cc.for vnyviuy rc,r~rrnlrntion; of druttriurn wide (I.rivy smtcr, i n wntvr i n m diilrrmi sourre?. nwi1:hlr to yu19 I f 50, do ihr w r i ~ t i o n ai u thv ..rl.ounis of tht: heavy isotope of hydrogen correspond to what you expected or do they contradict your expectations? Account for unexpected results if these are obtained, and test your explanation with further laboratory work.
