The h ydrogen ele ctrode function (pH respon se) , h ygroscopicity (water sorbing property), a nd che mical dLll'ability (surface alteration in solutions of various hydrogen ion concen trat ions) were m eas ured for a series of Na20-CaO-Si02 glasses. The res ulting d ata bo th confirm ed and extendcd t he previo us conclusion t hat glasses ha ving inadequ ate h ygroscopici ty a nd p oo r chem ical durability are incapa bl e of produ c ing sa t is factory electrodes for p H mea. uJ'emen ts. Each fa mily o f gla sses h a ving equal 8 i02 con ten t a nd var yin g Na20 :Ca O rat ios was found to ha ve a limi ted com posi tion ran ge t h at gave electrodes t hat approa ch ed t h e t heoretical voltage. H o wever, low h ygroscopi city, p OOl' ch emi cal durabili ty and d ev itrification limit t he number of u eful glasses to a small area [rom whi ch th e eutectic compos ition , 22 per cen t N a20 , 6 pe rce nt Ca O, 72 per cent 8 i02 (Cornin g 01 5) , is alread y known to be superi or [or i ts pH respon se. F or t he seri es of 70 pe rcen t 8i02 content, t he pH resp onse a nd c hemical durab ili ty cur ves appear to indicate the compositions at whi ch chan ges in the primary ph ase a re indicated by t he phase di ag ra m o f t he system .

Additional ,,·o rk ha been done on the correspondence between voltage departures (errors) of elecl rodes p repared fr om Com ing 01 5 g lass and t he change in t he chemical durabili ty of the g lass . The results con firm and extend t he earli er findin gs that voltage depart ures of the elcetrod e arc accompanied by durabili ty chan ges of t he glass. Th e pronoullced voltage departures in the a lkaline region an d in h ydrofluoric acid solution s are accompanied by conspie uou attack of the glass. Dilu te solu t ions of acids that do not form so luble compounds of s ilica cause swell ing of the glass. Th is s,,·el li ng is repressed as the concent ralion of lhe solution s is in creased . As i to be expected from the di stribution Jaw and Donnan membran e conside rations, th is repression of swellin g was also br ought aQout by high CQJ1cent rations of orga ni c acids and salts as well as b y the so-called "stro ng" a ids.The attac k of Corning 01 5 glass by alkaline solutions of different ion s was found to be in the orde r, Na+> Li+> K +> Ba++> NH.+, ,,"hich is the same order dicta ted for these ions by the voltage depar t ure data.

The pH ~·espo.n.s e, hygroscopicity, and chemical durability of a series of Na,O-PbO-Si02 (soda-lead oXlde-slhca) glasses have been investigated . The results obtained ar~ in complete acco:d WLth the data prevIOusly publls~e.d fOl: a series of N a20 -CaO-Ri02 glasses and show t~at. 1. Glasses of very: low hy~r~scoplclty YIeld electrodes whose pH responses fall appreCiably below ~he theoret.lcal 59 I11Ilhyolts per pH at 2.5 0 C; 2. electrodes prepared from glasses of P?or chemIcal dU:'ablhty also fal~ to d evelop the full theoretical voltage. . Vi ~en the app~ox lm ate compOSItIOns at wluch changes in the primary phase appear for the Na20 -PbO-SI02 system .are supenmposed upon the pH response and chemical durability c.urves, the marked ch[Lnge.s 111 t l? ese properties appear to reflect some of the criti cal compositions of the phase eqUIlibrium d iagram .. The chemical dura.bilit~ data obtaiI? ed b y the interferometer procedure indicate that the pioperty ?f swellmg 111 aCid solu tions IS much more universal for sili cate glasses than previously realized.

The th ickn ess of voltage-inhibiting films produced on glass slll'faces by h eat treatment afte r leach ing has been stud ied by means of t he glass electrode and the interferometer procedure previously used for determining the chemical dlll'abi li ty of glasses. The glass electrode gave conspicuous voltage departures (e rrors) a nd a lterations (swell ing) of the s urface t hat were just detectable by the interferometer. Th is corresponds to approximately 0.01 fringe, or four times t he unit cell dim e nsion for cristobalite. The inhibiting effect of film s of electric conductors, such as metall ic silver, has been qual itatively compared with the inhibiting effect of films prepared from electric nonconductors, s Llch as petrolatum and s ilicone stopcoc k grea se.

The effect of annealing and other heat treatments on the pH response of electrodes prepared f,:o m Corning 015 glass has been studied and a comparison made with the accompanying changes in hygroscopicity of the glass. New, unleached electrodes continued to give t he correct pH respon se after being annealed near t he cri tical temperature as indicated by the expan sion curve for Cornin 17 01 5 glass . Electrodes t hat were leached in 0.1 N HCI at SO° C lost much of thei r pH fun ction when given identical heat treatments. The pH respon se of a typical glass electrode, held 10 minutes at 500° C after being leached in 0.1 N Hel for 6 hou rs at SOO C, was reduced from 59 millivolts per pH (the theoretical value at 25° C) to 22 millivolts per pH. A few seconds in hydrofluoric acid solution restored the pH function of these "dead" electrodes, showing that the inhibiting effect was confined to the outer surface of the electrode bulbs. Evidence obtained by the in te rferometer indicated that the t hi ckness of this inhibiting layer was less than 5.SX 10-6 centimeter. Hygroscopicity determination s made on leached samples of t he powdered glass showed t hat the "sorption" power was greatly reduced by heat t reatment, whereas for unleached specimens this property was much less affected. From the evidence obtain ed it seem s reasonable to in terpret the loss in pH funct ion shown by glass electrodes on heat t reatm ent as being due to the formation of a thin non hygroscopic silica-rich layer. T he resulting electrodes behaved in a manner similar to electrode prepared from glasses of low h yg roscopicity.