Ionization of hydrofluoric acid at 25 C

Abstract

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...

Fluoride Incorporation and Apatite Solubility

“…Thereby, the ion HF2", which is important in aqueous solutions [Patel et al, 1971], may also occur in FA. These proposals result in a number o f ways o f how a deviation from stoichiometry in FA after equilibration in an aqueous solution might be realised.…”

Solubility and dissolution rate of silica in acid fluoride solutions

“…The pK for the dissociation of HF in solutions has been determined both conductometrically (Davies and Hudleston, 1924;Ellis, 1963;Roth, 1939;Wooster, 1938) and potentiometrically (Baumann, 1969;Broene and deVries, 1947;Hefter, 1984;Kleboth, 1970;Kresge and Chiang, 1973;Patel et al, 1971;Vanderborgh, 1968;Wynne-Jones and Hudleston, 1924). The reported values for the pK range from 2.82 to 3.33 at 25°C.…”

BIOCHEMICAL DISPOSAL OF EXCESS H+IN GROWING PLANTS?

“…Specifically, we consider fiuoroacetate synthesis: some Australian and South African plants {inter alia, species of Acacia, Dichapetalum, Gastrolobium and Oxylobium) contain up to 12-5 mg fiuoroacetate g~' dry matter, or 162 mmol fiuoroacetate kg ' dry matter (Peters, 1972); fiuoroacetate is a putative 'chemical defence' component (Peters, 1972;Mead et al, 1985). Since the pK^ of HF is 3-45 (Weast, 1979;Patel, Moreno & Patel, 1971) and that of fiuoroacetic acid is 4-76 (Sharpe, 1972), the synthesis of fiuoroacetate from fiuoride and gaseous COj at neutral pH can be represented as:…”