H eterogeneous eq uili bria a t the glass-solution in terface we re investigated in co nnection with t he pH r esp o nse of glasses. The preferentia l leaching of a lka li an d oth er cations fr o m the glass surface by ac id aq ueous solu t ion s, leav ing a s ilica-rich layer to act as a nonmigratable negatively charged ion, caused a n un e\'en d istr ibu t ion of t he migratable ions of t h e system in accordance with t he d ictates of t he Donnan membra ne equilibria an d the "equation of p rodu cts." The un even d istribution of Ag (KH 3)1i, and Br-io ns for various experimental con dition s wa s readi ly foll owed potentiomet r icall y for Corn in g 015, a series of Ka20-CaOSi02, a nd a series of Na20-Si02 glasses. T he Don nan membrane theory is compat ible with the vario us experimentall y observed phen omena su ch as t he s we llin g of the glass s\Il"face, repr ession of swelli ng by h igh con ce ntrations of elcctr olytes, vo ltage departure~ in a lkalin e an d hydrofluoric-aci d solutions, vo ltage departures in the "superacid" region, and asymmetr.v p otentials. H owever, t h e Don nan membr a ne con sideration s do not accou nt for t h e p refere nt ia l voltage response of t he glass electr ode t o hydr ogen ions wi th t he complete or parLial exclusion of other ions.

Electrodes prepared from glasses of low hygros copicity, high electrical resistance and unsatisfactory pH response in aqueous solutions have been inves tigated in an effort to deter-!l1i~e if .th e~r failure to develop t he f~ll pH response is due merely to a lack of sensitivity of t he lI1dlcatll1g lI1strument or t o a real faIlure of these glasses to respond to hydrogen-ion activity. By increasing t he sensitivit y of the indicating instrument and decreasing t he t hickness of the electrode membrane in order to lower the electrical resistance, many of t hese glasses have been shown t o develop the full pH response of 59 millivolts per pH at 25° C . Howe ver, theoret ical and experimental evidence indicates that glasses below a limit ing hygros copicity probably do not develop full pH response regardless of the thickness of the glass membrane t he resistance of t he ele ctrode, or the sensitivity of t he indicating instrument. In fact glas~ electrodes for which the thickness, electric resis tance, and surface are retained esse~tially unaltered t hroughout t h e experime nts exhibit large departures from t he hydrogen electrode upon the progressive removal of water .