A NEW BRIDGE CIRCUIT FOR CONDUCTOMETRIC TITRATIONS HARRY 1. SVEC Institute for Atomic Research, and Iowa State College, Ames, Iowa
APRACTICALUS~O~ the measurement of the conductance of electrolytes is the determination of titration end points in reactions involving neutralization, precipitation, or complex formation. These conductometric titrations are described in many laboratory manuals and textbooks for undergraduate students of quantitative analysis and physical chemistry. Most descriptions of apparatus involve the Wheatstone bridge circuit. This circuit is so universally described that chemistry students often get the idea that resistive impedance bridges are the only kind of useful bridge circuits. Another kind of bridge circuit was introduced in the physical chemistry laboratory a t Iowa State College to help do away with this notion. The new circuit involves inductive impedances in one leg of the bridge. It has the advantages of simplicity, low cost, and adaptability to a variety of conditions. R. S. Barnes' used a similar bridge circuit in a furnace-temperature controller. An adaptation of this circuit for conductometric titrations appears in Figrre 1. If an alternating current is applied to the bridge, balance is achieved a t the point at which a 180" phase shift occurs. This appears on the cathode ray oscilloscope screen as a straight line. The cell resistance is obtained from the relationship I t is not necessary to know the absolute values of 2, and Z2since only their ratio appears in equation 1. In practice, the autotransformer was mounted using a circular scale marked 0-100 and graduated in units. Thus when balance has been achieved Z1 equals the scale reading while
R = 300ohrns.carbon. l watt
Z = Power%t.Model I0
0
cathode ray oscilloscope 7
after paraffining to insure a conducting electrode surface. Electrical connection to the electrodes was by means of alligator clips. No difficulties because of electrode polarization were experienced when 400 and 1000 cycle a.-c. sources were used in the titrations. Dnring titrations the solutions were stirred thoroughly. Care was exercised to prevent vortex formation, which xouldcauseerraticresults. A diagrammatic representation of the cell appears in Figure 2.
stirrer-
z*= (100 - 2,) The conductance of the cell was calculated from the simplified expression
iz
=
c = R(100Z,- ZI)
graphite ....... .-.-...
(2)
The innovation was completed with a simple, inexpensive conductance cell which is well suited for titrations. The cell consists of a T constructed of large-bore glass tubing and fitted with graphite electrodes held in place by rubher stoppers. The electrodes were turned from solid graphite rods one-half inch in diameter. They IVere saturated ~ 6 t hparaffin before a fine mil1 use and the faces were scraped BARNES, R. S., J. Sci. ~ n s t . 28,89-92(1951). ;
rubber stopper
0
u
1
~nches
F~~U 2 .~
~h~ apparatus was tested by undergraduate physical chemistry students in the following manner, Solutions H,BO,, and (COOH), were prepared. o f ~ ~CH,COOH, l , Thesesolutionswere approximately0.1 NexcepttheHCl, which was further standardized with anhydrous Na2C03.
194
JOURNAL OF CHEMICAL EDUCATION
Twohundred millilitersof eachacidsolution were titrated that this cell is also feasible for nonaqueous systems, with 2 N NaOH which was freshly prepared by each especially where conventional platinum electrodes canstudent from a saturated solution. The base was first not be used. No quantitative data are available a t standardized against the HCI and then the other acids present. were titrated. Since the volume of acid was large and The inductive impedance bridge is well adapted for the heat liberated in the reactions not great, it was not student use. When readily available commercial autonecessary to thermostat the cell. The results of the transformers are used, the bridge is inexpensive and students were compared. Except for cases where a ruggedly constructed. This paper reports on the use of student was known to have made a serious error, the commercial autotransformers only, for which the average deviation of all results was less than 1 per cent. precision of balancing the bridge is 1 0 . 5 per cent. As an additional check on the method, the students This precision could be improved with an inductor conmade up solutions of NHlOH and titrated the acids. sisting of copper wire wound on a long iron wire core The same degree of precision was obtained with these and employed as a straight slide ire impedance. Such weak base titrations. Plots of the data for the various a slide wire might readily be encased in the housing for acid-base systems involved are typical conductometric a multi-turn helical potentiometer. If the housing of a titration curves. ten-turn potentiometer were employed along with a The author has checked the apparatus with precipita- commercial multi-turn dial, bridge balance should be tion, complex formation, and salt-acid reactions. The reproducible to less than 0.1 of 1 per results of these experiments indicate that the graphiteelectrode cell is adaptable to all the common aqueous The author has obtained such a ten-turn, helical inductance titration systems. Some preliminary work indicated on special order from the Ford Engineering Co.. Upland, Calif.
