Mr. Robert Causey Linus Pauling 22 May 1963
Research on Electrical Conductivity of Solutions
During the last two years Dr. Catchpool, with the help last surlier of a student, has been measuring the electrical conductivity of certain aqueous solutions in the absence and presence of an anesthetic agent (chloroform). We hope that these measurements can he interpreted to provide information about the formation of small hydrated complexes (rdcrocrystals) in the solution.
Dr. Catchpool has obtained experimental values for solutions of potassium chloride, butyl ammonium chloride, tetramethyl ammonium chloride, and tetramethyl ammonium bromide. Some of the measurements are for solutions containing as much as four moles per liter.
He has found that the conductivity of the alkyl ammonium salts does not increase with increasing concentration so rapidly as that of potassium chloride, and it is affected by the presence of chloroform by a larger percentage amount than is that of potassium chloride solution.
I should like you to carry out the theoretical discussion of the complexes present in the solutions, in an effort to obtain some significant information from the conductivity measurements.
There is some difficulty in interpreting conductivity measurements of concentrated solutions. I suggest that we make an effort to interpret these measurements by using the potassium chloride solution as standard. We may assume that the potassium chloride solutions, even the concentrated ones, contain potassium ions and chloride ions, with no formation of complexes. By measuring dilute solutions of potassium chloride and of the other salts, the ratio of the limiting conductances of the salt A B and potassium chloride can be found. I suggest that for each concentration of salt and each concentration of anesthetic agent, the measured conductivity of the potassium chloride solution at that concentration and with that amount of anesthetic agent be corrected by multiplying by the ratio of the limiting conductants of the salt and potassium chloride, to obtain the conductants that would be expected for the salt if no complexes were present. The ratio of the experimentally measured conductivity of the A B solution and the value calculated in this way would provide information about the ex-tent to which complexes had formed in the solution of A B.
This treatment is far from rigorous, but I do not know how to make it more rigorous.
I should guess that several alternative theoretical treatments be carried out, as outlined below.
First, let us assume that the solution of A B contains some ions A+ and B-, and some neutral complexes, but no charge complexes. The neutral complexes would be A B plus some water molecules and perhaps molecules of anesthetic agent, A2 B2 plus these other molecules, A3 B3 and so on.
I suggest that in all of the equilibrium expressions you assume that the activity coefficients are all unity. This may be a considerable approximation but I doubt that it is worthwhile to try to correct for ionic strength, although you might think it worthwhile to sake this correction, using the Debye theory.
I suggest that you write down the equilibrium expression for the formation of the complex AM+1 Bm-1 from An Bn and the two ions. These equilibrium constants, K1, K2, ...., can perhaps be evaluated by comparison of the calculated conductivity as a function of concentration with the experimental results.
You sight find it worthwhile to try, as a start, a simplified theory in which the successive equilibrium constants are all given the same value. You can find an argument providing some justification for this assumption in the paper by Lassettre, Chem. Revs. 20, 259 (1937); he wrote this paper while he was a graduate student here.
The experimental results might justify the separate determination of K1 and K2, with the following equilibrium constants placed equal to K2; or perhaps the first three constants could be evaluated separately.
If you find that this theory gives a satisfactory fit to the experimental curves at one temperature, in the absence of anesthetic agent, you could then evaluate the same equilibrium constants at the other two temperatures, and see if the values of the change in enthalpy and entropy corresponding to the temperature coefficients seem to be reasonable.
In the same way, the values of the equilibrium constants in the presence of anesthetic agents could then be determined, and an effort made to discuss the effectiveness of molecules of anesthetic agent in stabilizing the successive complexes.
Dr. Catchpool has had in mind studying the effects of two different anesthetic agents. I think that, if good results are obtained for chloroform, it would be worthwhile to try fluothane, and then mixtures of chloroform and fluothane.
One would, of course, anticipate that some complexes containing cations and anions in unequal numbers would be present in the solutions. There is some question in my mind as to whether or not it would be worthwhile to try to formulate equilibrium expressions for these complexes also and to try to estimate the contribution that these complexes would make to the conductivity of the solution, and introduce this contribution as a correction.
I shall look forward to discussing with you the equilibrium expressions that you formulate for these systems.
Dictated by Linus Pauling
Signed in his absence:hpg
cc: J. F. Catchpool, M. D.
Karl Sato
May
22, 1963
