Marine Chemistry, 6(1978) 281--288 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands 281 ON THE MERIT OF DEFINING AND APPLYING IONIC MEDIUM ACTIVITY SCALES -- A REPLY TO A COMMENT BY DYRSSEN, JOHANSSON AND WEDBORG SAM BEN-YAAKOV and EYTAN SASS Department of Electrical Engineering, Ben Gurion University of the Negev, Beer-Sheva (Israel) Department of Geology, The Hebrew University, Jerusalem (Israel) (Received February 17, 1978) The main points raised by Dyrssen et al. (this issue, pp. 275--279 in their comment on our paper (Sass and Ben-Yaakov, 1977) are summarized as follows: (1) A comparison is made between the ionic composition of Dead Sea waters (DSW) and the ionic composition of seawater (SW). (2) The expected value for K~ in DSW is estimated and compared to our experimental results and a suggestion is made that our values for pK ~ are too low (K~ too high) because they do not agree with the estimate. (3) The scatter in the ratio A/C calculated from our reported results for A and C is interpreted as a lack of precision in the determination of C. (4) The similarity between our treatment and the Gran function is pointed out. (5) Experimental results of an HC1 titration of synthetic DSW are presented along with the values of pK~* and pK* derived by an iteration method favored by Dyrssen et al. Since the scatter in p(K*/K*) derived by Dyrssen et al. is smaller than the scatter of p(K~/K~ ) calculated from our results, a conclusion is drawn as to lack of accuracy in our experimental and analytical procedures. (6) It is suggested that direct curve fitting of the data to the titration equation is better than the computation method used by us. (7) The advantage of ionic medium activity and pH scales is stressed, suggesting that our experimental procedure introduces unknown liquid junction potentials and poorly defined activity scale for H ÷. The following is a point-by-point reply to the comments of Dyrssen et al. (1) The composition of Dead Sea waters (DSW) Dyrssen et al. open their note with a statement concerning the composition of DSW (DSB in their notation). In order to correct misinterpretation of the