CONDUCTIVITY TESTS for REACTIONS WHICH GO to
END
an
E. L. GUNN Robert E. Lee High School and Lee Junior College, Goose Creek, Texas
'T
HE present article describes the design and use of a simple conductivity apparatus which has proved to be satisfactory in giving confirmation to the generally accepted reasons why double replace-
PROCEDURE
Previous to the experiment, solutions of the following compounds were made in distilled water, barium hydroxide, sulfuric acid, aluminum sulfate, and sodium carbonate. Each solution was prepared by dissolving the compound until the conductivity test of the solution was 0.50 ampere. The following double replacement reactions were caused to take place and conductivity tests were made in each case.
I +ZAI(OH)S
(1) Al~+++(S04-)s+3Ba++(OH-)-3BaS0,
(2) Ba++(OH-)s
(3) Nal+COe-
+ H2+S0,-
+ HntS04--
+ 2 HOH NaltS04- + HCOI BaSO,
I
11
CO2
+ Hz0
The procedure followed in each case was as follows. Each solution was tested separately as to conductivity just before the reaction. Then, successive portions of the first reactant were added to a fixed amount of the second, with vigorous stirrings, until the minimum deflection of the ammeter was observed. Continued addition of the first reactant beyond this point brought the effective conductivity back uo. of course. The electrode unit (0 was held in the solution being tested for the length of time necessary to rdad the ammeter, then placed in distilled water until the next test.
d
A ,
FOR CONDUCTIVITY TESTS FIGURE 1.-APPARATUS Aammeter V-voltmeter I' C-kctrade unit
RESULTS
ment reactions in solution go practically to an end through the elimination of two or more of the four varieties of ions involved. The effectsof precipitation, volatility, or formation of a slightly ionized substance, such as water, upon conductivity, give excellent support to the ionic explanation usually given of irreversibility. The experiment is simple and can be performed in a short time, either by student groups or as an instructor's demonstration.
In reaction (1) the precipitates of aluminum hydroxide and barium sulfate were clearly visible but not exce3sively copious. The conductivity test gave a minimum deflection which was barely sensible, therefore practically zero. This illustrates the type of reaction which goes practically to an end through precipitation. In this particular reaction both varieties of positive and of negative APPARATUS ions are eliminated as precipitates. Figure 1 is a schematic representation of the apThe minimum test for reaction (2) paratus used in this experiment in making the con- also was zero. This illustrates the elimiductivity tests. Direct current from an A.c.-D.C. recti- nation of two varieties of ions through fie1 was used. The reading of the voltmeter was con- the formation of a precipitate, barium stant a t thirty-two volts while tests were made. The hydroxide, and two varieties through ammeter used was read in one-hundredth-ampere divi- the formation of a slightly ionized prodsions. The electrode unit is shown in Figure 2. uct-water. 385
1
,3Cn
FIGORE 2ELECTRODE
UNIT A-air
pas-
Sag-p at i numeleetrades.
Tests for reaction (3) differ from the two previous ones in that the minimum conductivity was not zero 0.02 ampere. The sodium and sulfate but 0.17 ions remain in solution to carry current. Under the conditions of the experiment, the hydrogen and carbanate ions are most probably eliminated through the results of volatility. ~afC1NafNOaThe reaction, K'NOs-
+
=
+
example of the double reK+Cl-, furnishes a coed placement typein which ionic is attained. A conductivity test applied in this case gives convine. mg proof that the effective current-carrying ability is not diminished by reaction. This demonstrates that there has been no change in the composition of the ions in solution to affect their ability to carry current.
