Th e io niza ti on co nstant, K" fo r th e reacti on HF:;:2 H + + F -was ca lc ul ate d on th e ba s is of pote nti o· metric me as ure me nt s in th e cell Ag; AgCl , CI -, F -II La F"II Na F, HC I, 1-1 , 0 1 KC I(Sa td .), I-I g,Cl 2 ; Hg at 25°C. A leas t s qu a res procedure wa s a ppli e d to th e ex pe rim e nt a l da ta yie ldin g a bes t es tim a te fo r K, of 5 .85 X 10 -4 with a s ta nd a rd e rror of 0.03 X 10 -" . Key word s : H ydro flu ori c ac id ; ioni za ti on c onst ant ; lanth a num flu orid e e lec t rode; le as t s quares pro· ce d ure; potenti ome tri c me as ure me nt s .
. IntroductionThe ch e mical properties of dilute aqueo us soluti ons of hydrofluori c ac id are described [1 , 2, 3] J by th e equilibria (1) (2) in wh ic h pare ntheses indi c ate ac tiviti es of th e various c he mical species. Mos t of the evid e nce for th e pres· e nce of th e variou s s pecies involved in th e above equilibria has been obtained from s tudi es [3,4] at hi gh co nstant ion ic stre ngth s. Unfo rtun a tely, the values for the apparent equilibrium co ns tants derived from s uc h studies are of limited use.Th ermodyn a mi c valu es of K J and K2 have bee n reported on th e basis of pote ntiome tric [5 , 6] or conductance meas ure me nts [7] . However, there are uncertainties in th e valu es of the constants originating from the graphical approximations [5 , 6] used in the calculation s, or from the assumptions regarding the species res pon sible for the transport of c urrent [7].The prese nt investigation was prompted by the observation of s mall but consistent discre pan cies between the valu es of fluoride ion activities obtained with Ce nt e r, Bos lon, Ma ss. 02 100. t Fi gures in b rac ket s ind ica t e t he lit e ratu re re re re nces at th e e nd of thi s paper.a lanthanum Auorid e me mbran e electrode and those calculated on the basis of th e re ported ionization constants of hydroAuori c a cid. Pote ntiom etri c meas ureme nts were mad e in th e ternary syste m aF-HCl-H20 . The use of th e th ermodynamic value of KJ d erived from such meas ure ments elimin ated th e di screpanc y betwee n meas ured a nd calc ulated Au orid e ion activiti es. Th e leas t squares procedure used in th e present study obviates the un certa int ies inh ere nt to gra phi cal me thod s and permits th e es timation of e rrors on a so und stati sti cal basis.
Experimental Methods and Procedures
. 1. NaF-HCI-H20 SystemsAll the che micals used in the present study were reagent grade. Stock solutions of sodium flu orid e were prepared by weighing the salt whi c h was dri ed at 100°C for 24 h; the salt was dissolved with condu ctivity water in borosilicate volumetri c flas ks and immediately transferred to polye thyle ne bottles for storage. The systems used in th e potentiometric measurements were mad e by takin g 100 ml of sodium fluoride solutions of suitable conce ntrations and adding various aliquots of standard hydrochloric acid. In this way it was possible to obtain syste ms with a wide co ncentration ran ge in bo...