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Easily Built, Accurate Apparatus To Measure Conductivity Miguel ~uzrnan'and Davld Puga Departamento de Quimica, Universidad Catolica del Norte Casilla 1280, Antofagasta, Chile A low-cost, easily constructed conductivity apparatus has been shown in this Journal (1).The accuracy and precision of the measurement can be augmented without 10sing the previous characteristic when an additional circuit is added that increases the frequency to 1,250 Hz and stabilizes the voltage during the measurement. In this way, the error by electrolysis and the large variance in measured values are avoided. The proposed electronic circuit shown in Figure 1consists of an AC-DC adaptor (A) (3-12V) easily purchased in
' Author to whom correspondence should be addressed.
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stores; two capacitors (C) of 0.01 pF;an operational amplifier (B) 114 LM 324; resistances R,,R1,Rzof 11kn; and a conductivity cell (Dl with graphite electrodes extracted fmm D-type cells and insulated according to the simple system. The resistance of a solution is determined by measuring the output voltages with a high-impedance digital multimeter between points x, y and y, z and is calculated with equation: R (solution)= & x 11kn vw (1) If the cell constant is known, molar concentration, specific and molar conductivity can be determined. Methods of measuring conductivity can be found in any book of physical chemistry laboratory experiments. The following was calculated by the two systems.
$ Figure 1. Schematic diagram of the apparatus.
GROUND
Solubility Products of CaC03 The specific conductivity of saturated aqueous solution of CaC03 and of the water used to prepare the solution was calcuY lated. The molar conductivity for infinite dilution of the salt a t the same temperature, 25 OC, was extracted from the Handbook (2). The solubility product was calculated to be 1.17 x 10" with the simple system, and 0.98 x 10" with the proposed system. The Handbook value is 0.87 x 10" which show the aecuracv of the proposed system.
Volume 70 Number 1 January 1993
71
The Ionization Constant of CHCOOH
0 PROPOSED SYSTEM
Solutions of concentrations 0.05 M, 0.0514 M, 0.05116 M a n d 0.051250 M were prepared and the specific molar 'U 4.74 conductivities a t 25 'C were 6 calculated. The degree of dish (75xfo-5 sociation was calculated according t o the aforementioned ? 4. 76 r=0.62 and using the molar conductivitv a t infinite dilution taken from t h e Handbook, -4.78 a n d plotting log M d l - a2 0 10 i13 30 40 against (Ma)m for both systems (see Fig. 2). By extrapolation of the straight lines, the ionization constant of the acid was determined. The result ~~~~-~ was 1.69 x lo4 for the simple Figure 2. The ionization constant of the acid determined by extrapolation. system and 1.75 x loJ for the Literature Cited proposed system. The Handbook value is 1.754 x 10" which shows the accuracy of the proposed method. The cor%nich,T,;sa~ R,J . Ckm,Edu ,
