Unlts ot an Equillbrlum Constant To the Editor:

Helfferich [1987,64,1069] has effectively replied toQuin- tero's uncom~romising reiection of an equilibrium constant having units;but i t i s n o t only chemicalengineers who give them units; so do chemists. Thus, the IUPAC manual on Quantities, Units and Symbols in Physical Chemistry (The Green Book) allows the use of Kc and& havingunits, and a dimensionless or thermodynamic K, which is the ualue of either Kc or K,.

Besides the ~ o i n t made bv Helfferich, an equilibrium con- stant having appropriate G i t s can, under some conditions (elementarv Drocesses, or anv reaction a t equilibrium), he equated toihe ratio kJk-l of rate constants in forward and reverse directions. This useful relationship is lost if K is renuired to he dimensionless unless. as surelv no one would ~~ ~- ~~ ~

advocate, we take rate constants to be dimensionless also. The best oolicv is to define eauilibrium constants in terms . .

of ratios of concentrations or pressures, in which case they have units. I alwavs require my students to state the units; thin is more convknieni than stating standard states, and also it avoids errnra that easily arise when the dimensionless quantities are used.

If, for a logarithmic relationship, a dimensionless quantity is required, this is readily obtained, as with any other physi- cal quantity, simply by dividing by the unit quantity. One can conveniently indicate that this has been done by adding a superscript to show that the quantity has been made unit- less, in other words, that one is taking the ualue of the quantity.

If Quintero's argument were valid, that a quantity must be dimensionless because we sometimes have to take its loga- rithm, we would have to require many other physical quanti- ties to be dimensionless; examples are pressure, electric cur- rent, and rate constant. Units are valuable scientific assets: let us not relinquish them until we have to.

Keith J. Laidier university of Ottawa

Ottawa. ON KIN 6N5 Canada

"Redox Demonstrations and Descriptive Chemistry, Part 1. Metals" To the Editor:

The equation for the formation of the bisthiosulfateferra- te(lll) ion stated immediawly under Discussion Procedure 1 of the tested demonstrations in the article "Redox Demon- .- ~-~~ ~~ -~~ ~~~

strations and Descriptive Chemistry Part 1. Metals" 11987, 64(3), 716-7171 should be written as Fe3+ + 2 S ~ 0 3 ~ - - [Fe(S203)2]- instead of Fe3+ + 2S20z2- - [Fe(S203)2]-. AU other equations are correctly balanced.

William D. Hill, Jr. N. C. Central University

Durham. NC 27707

88 Journal of Chemical Education