An IMPROVED CONDUCTIVITY APPARATUS W. S. McGUIRE Northeastern University, Boston, Massachusetts
INCE the determination of the conductivity of various solutions is a commmon experiment in elementary chemistry courses, a large variety of instruments has been developed for this purpose. The one most frequently used consists of a lamp, in series
but also because of the failure to give quick, consistent readings, a fault due to the polarization effects at the electrodes. After various attempts to work out a more satisfactory and comparatively inexpensive instrument, the one illustrated has been developed. I t consists merely of an inexpensiveradio milliammeterin series with a pair of electrodes, using as a source of power the output side of 6.3-volt radio power transformer, the input side of
with a pair of electrodes of either platinum or carbon, which plugs into a 110-volt line, such as the one formerly used in this laboratory.' This has the disadvantage of showing only marked differences of conductivity, although Brown and Bickford2 have described an improved form using two lamps and a resistance by which more satisfactory comparisons can be made. Another type is that sold by several apparatus houses, which makes use of some sort of current-measuring device in order to obtain semiquantitative results. As these invariably employ a six-volt battery as a source of current, they have not been entirely satisfactory, partly because of the added cost and charging nuisance,
' McGurns AND Z ~ E W T T ISch. , Sci. Math., 36,364 (1936) BROWN AND BICKPORD, J. CFLEM. EDUC., 14,384 (1937).
which can be connected to any ordinary 110-volt A.C. line.
With this instrument there are no polarization or other electrode effects. Thus, except in the case of certain corroding solutions, a few cents' worth of chrome1wire can be used in place of the more expensive and usually less rigid platinum. Since these wires cannot be sealed in glass in the usual manner, the end of the glass tube is first closed to the desired diameter, the wire inserted and clamped in some manner while hot sealing wax is poured in to the depth of several inches, furnishing a reliable and chemically inert seal. By using solutions of not more than half-normal strength, satisfactory comparative results can be obtained which do not require the student to estimate the different grades of brightness. If desired, the experi-
ment described by Petersona can be repeated, showing very well the increased equivalent conductivity upon dilution of a strong electrolyte such as sulfuric acid. In this case we have found the ordinary 180-ml. electrolytic beaker, pyrex No. 1025, to be a very satisfactory container for the solution. While the cost is somewhat higher than the simple lamp-electrode set-up, there is no reason why, if the parts are purchased through a radio supply store giving the usual educational discount, it cannot be built in most school laboratories for less than three dollars for materials. PETERSON. 1. CHEM.Eouc.. 9, 923 (1932),