BY 1YILI)l~:RD. IlhSCROF'r
hi page 430 of the second edition of Xeriist',< ' Thcowtist-h/, pass.!ge : TI-e h a v e liitlierto excluded tlie possibilit!. of the sol\-ent takiiix part iii the reaction aiid we will 11ow tleteriiiiiie lion- such a plieiioi~ieiioiii y to lie treated. T h e simplest case is that in nliic.li there is only one kind of iiiolecnle A in solution which reacts with the iiiolecules of tile solvent, water, for illstance! i n n-liicli case it foi-iiis a li!,cirate ; n-e are then considering the reaction (
c i i ~ ~ i n'i cis to be found the follon-ilia
X CH.@ I-:;
I
s*
-1 i i H : O ,
1 ,
\vliicli has tlie forin of a tlissociatioii equation. Let (. be the coiiceiitratioii of tlie li!.drated, cI that of the deli\driited molecules of the siibstmce, -1aiitl r2 the coiiceiitratioii of tlie Lvater, tlieii we 1iai.e tlie eqiiatioii Kc.
~
r,c,,' .
Son- it is to be noticed that (-2 is 1-ei-X large relatiI.elj. to r and and therefore. especiallj. \\.lien the solution is ,stifficic-iitl!. diltite, the active mass of the sol\.eiit will cliaiig-e but slightl!. if tlie equilibrinni i.q displaced iii one direction or the other. i. e. t/ic trr.tii,r iur7.s.r (!f thc solz'ciit ii j r g ~ - f i ~ ~ ( 7 lm/i.sfni?f; [11 ~ l i t - n c follo\vs e proportionalit!. betweeen c aiirl cI1and in a dilute .solution fhc fi(~i-i.ciit(r~~~t' c f / l t ~ r ( i - n t t . r Jsiihsttrucc i s fhcrlyiire iiid(j?vli/driit (!/' tic(, l . ~ ) ~ / ~ . ~ ~ ~ ? fai . g consequence fi~)ii~ of the iiiass law n-hich 112s not hitherto receil-ed due attention.'' T h i s coiiclusioii is of siufficieiit iniportaiice to call for :i caref u l esa~iiination. The first ohjectioii to be iiiatie to it is tliat tlie tacit assimption has been iiiade that 110 compouiltls (.If tlie for111 &I, H - 0 c n n exist. It i 5 a fact that inost of the substances \\.it11 (.,?
~
' Reprinted ivitliout cliaiige from page
2 1 9 of the firht etlitioii.
wliicli we are faiiiiliar aiid which form hydrates do not crystallize with submultiples of n-ater ; but tlie n-ater in the argument \vas merely selected so as to have a concrete case to discuss. If ive assiiiiie tlie existence of a h j d r a t e of alcohol, for instance, contaiiiiiig two or more of n.ater, Seriist’s reasoning shows that it would be decomposed ljy alcohol but not by water. This, however, is a hypothetical case \vliicli probabl\- does not really interest anybody. iiiore iiiiportaiit qiiestioii is whether Seriist’s conclusions apply also to hydrated salts. good many people are fairly certain that acetic niili!drid is lij-drated when dissolved in water ; they feel confidelit also that nietaphosplioric acid takes up water when dissol\-ecl aiid the!. slirew-dl!. suspect that something similar takes place with snlfur cliosid and triosid. I n view of tliese facts, the!. see no reasoii why sulfates, for instance, might not retniii tlie so-called n-ater of constitution ’ when in solution. Tiifortuiiatel!~, it is not clear from tlie t e s t what S e r n s t tliiiiks aliout the case of Iiyclrated salts. He lias omitted all reference to the possibility of electrolj-tic dissocintioii aiid yet foot-note that he adds inakes it iiiiprobable that lie intends to exclude electrolytes. I t will, however, lie iiiore profitable to consider what conclusions are permissilile it1 regard to 1ij.drates in solution rather than to specnlate as to Xeriist’s iiieanitig. T o simplify matters n e will assiiiiie tlie validity of the iiiass law, tlie gns laws, and Ostn-ald’s dilution law. lye have tlieii two reactions takiiig place siiiiultaiieoiisly wit11 changing concentration : a possible delij-dratiou and a cliaiige iu the electrolytic dissociation. Let A be the total amount of salt present a t any iiioiiieiit, .I- tlie aniouiit of an1i)drous undissociated salt, J’ tlie amount of dissociated salt, and C the aiiiount of water at a n y given tiilntioii. T h e amount of hydrated salt \vi11 therefore be A - .r - ~ -aiid y we shall have two equations for t?qnilihriniii : ~
,lssuniing with Kernst that C 1-is a constant for dilute so1utic)n. n e may write
from which follows that the percentage hydration d e c r w w i \\ ~ t h decreasing concentration or that 11: drated salts are clecoiiipo4 by water. T h e qitestioii at oiice comes iip as to the effect of the 1)ie.eiice of 11) drates in soltitioil 011 the Oitwald dilution la\\ A< usually esprewecl. ITliat \\e determine from the coiidiicti\ i t \ measurements is the percentage dissociation, ant1 the Oat\\ ~ l d dilution law is therefore written :
w11icIi is iiot the same as equation ( I ) or equation ( 3 ) . It i\ however true, in case these two ec~iiationshold, for subtractiiig equation ( I ) froiii (4)u e have
wliich becomes identicd with equation (5) provided \ve write E(
KI-drated ion will remain the same, provided there is 110 secondary electrolytic dissociation. CLImci/
i
-ti i ,c.r.iif-l
