14. Construcción del escenario apuesta
14.1 Objetivo 1
loosely referred t o as glass electrodes.
Generally grade A pipettes and grade
Bstandard
flasks were used in all standard volumetri c
work.
4 � 2
REAGENT SWhen c ommercially ava i l ab l e , analyti cal grad e reagent s we r e u sed , gen erally wi thout fu rther purifi c a t i on or a s say .
C arb onat e- free st andard s olut i on was
prepared acc ording to the m e t hod of Alb ert and S ergeant ,
( 1 9 6 2) . Exc e s s barium hyd r oxid e was add ed t o a solut i on
of pot a s si um hydroxide t o remov e carb onat e b y p re ci p i ta t i on a s barium carb onat e . The exce s s barium i ons w e re remov ed from t he solu t i on by pa s s age down a column of s trong cat i on
exchange r e s in (Amb erl i t e I R 1 20 ) in the pota s s i um form .
The r e sult ing potass ium hydr oxi de solut i on w a s s t andardi s ed agai ns t a solu t i on o f p o t a s s ium hydrogen phthalat e using a gla s s e l e c t rod e to det e c t th e end point. G enerally app roxi mat ely 0 . 2 M p o t a s s i um hydroxi de was u s ed.
St andard s . An 0.0 5 M s oluti on of B ri t i sh N a t i onal Phy si c s Lab oratory c ert i fi ed p o t a s s ium hydrogen phthal at e wa s u s ed a s a pri mary pH s t andard. ( In som e e arly work
analyti c al r eagent grad e p o t a s s i um hydrogen p ht halat e wa s u sed) . Radi omet er S 1 00 1 c onc entra t ed buffe r solut i on
( pH 6 . 50 ± 0 . 0 2 at 20°C ) wa s u s ed as a s e c ondary s t andard .
Wat er . Di s t illed wat er was ava i l abl e on t ap . Thi s was d e i oni s ed by p a s s age d own a mix ed bed i on exchange r e s i n
column short ly before i t wa s requi red. The c ondu ct i v i t y
- 6
o f the d e i oni sed wat er was l e s s than 1 x 1 0 S . The d i s t i ll ed , dei oni s ed wat er was stored in s ma l l polythene t anks .
e t al . ( 1 934 ) . Anhyd rou s c i tri c acid wa s di s s olv ed in an exc e s s of methanol and hydrogen chlori de ga s was bubbl ed int o the refluxi ng solu t i on ov er a p eri od of four hours. C ry s t al s of trimethyl c i trat e sep arat ed out on cool ing of the react i on mi xtu re and were coll e c t ed on a filter and washed w i th cold methanol. The tri me thyl c i trate wa s re- c ryst alli s ed from wat e r and from methartol. M e l t ing p oi nt , found 74.5- 7 5°C, l i t erature 7 6°C (Donald s on et al . , 1 934 ) , 73-73 . 5°C ( Loewens t ei n and Rob e rt s , 1 9 60 ) , 7 5- 7 6°C
( M ar t i n , 1 961 ) .
C i t r a t e was p repared by the method gi v en by S chroe t er and Schmi t z ( 1 90 2 ) . C i t r i c acid monohydra t e wa s di s s olved in exce s s m ethanol , a c a t alyt i c quant i ty o f
sulphuri c acid added and the mi xture refluxed fo r one hour. The rea c t i on m i xture wa s neu trali s ed wi th cal c i um carb onat e , filt ered and th e fi l t ra t e c onc entra t ed b y
evaporat i on under va cuum . Th e r e sul t i ng sol i d wa s
di s solved in hot wat er , the soluti on fil t ered and the e s t er precipi tat ed from th e solut i on wi th concentrat ed
hydrochlori c aci d . The e s t er wa s then twi c e re-
cry s t al l i s ed from wat er . M elt ing p oi nt , found 1 1 8- 1 23°C , l i t erature 1 2 5- 1 26°C ( S chroet er an d Schmi t z , 1 90 2 ) , 1 1 5- 1 1 7 0C ( Loewens t e in and Rob e rt s , 1 960 ) , 1 1 6- 1 1 8 °C ( M ar t i n , 1 9 6 1 ) . Equivalent wt , cal culat ed for dihydrat e 256 ,
c al culat ed for t h e monohydrat e 238 , found 254 , li t er ature 236 , ( Schroet er and S chmi t z , 1 90 2 ) , 2 57 ( M art in , 1 961 ) .
O ther C i trat e s were p repared by the m et hod o f
Loewen s t ein and R ob ert s ( 1 960 ) u sing p art i al saponi fi cat i on of ei ther t r i me thyl c i trat e o r symmetri c dimethyl c i trat e
3 5
and were not i s olat ed from solu t i on . The saponi fi cat i on s
were carri ed out b y the add i ti on of a measured quanti ty of
KOH shortly b efore pH t i t ra t i on wi th st andard hydrochl ori c aci d . The yi eld s of the vari ou s e s t er s hav e b e en calcul at ed
from the saponi fi cat i on rat e con stan t s
(
s e e App endix I I ) and are giv en i n T ab l e 4 . 1 .4 . 3 EXPERIMENTAL PROCEDURE
Kno�� wei gh t s
(
or volumes of s t andard soluti on s ) ofthe vari ous reagent s w e r e quant i t at iv ely t ransferred to a
s t andard 1 00 ml flask , di s solv ed , and mad e up t o t he mark . The c ontents o f the s t andard fla sk were t hen poured into a
t i trat i on v e s s el consi st ing of a wat er- j acket ed c oni cal
gl a s s flask wi th a cap a c i ty of ab out 1 2 5 ml . Wat e r was
c on t i nuously pump ed through the j acke t from a therm o s tat b a t h ,
(
Grant I n s t rument s , t hermo s t at/
pump , typ e SU 2 ) .0
T emp e rature s d i d not fluctuat e more than O o 5 C . The
cont ent s of the v e s s el were s t i r r ed magne t i cally u s i ng a t e flon encased follower . Oxygen-fr e e ni trogen ga s
(
availab l e commerc i ally in cyl i nder s)
was bubb led th rough a sint ered gla s s di s c into wat er at the t emperature o f th e t hermo stat b at h and was t hen admi t t ed to tre b ot t om o f the t i trat i on flask . The gas flow was ad j us t ed at the cyli nd e rout l e t . Th e s olut i on wa s sti rred and ni trogen bubbled t hrough i t for 1 5-30 minut e s . The gla s s electrode wa s carefully st andardi s ed against pota s s ium hydrogen
phthal at e buffer , rins ed wi th wat er and transferred t o t he s o lu t i on in th e t i trat i on ve s sel.
T i trant b a s e wa s added to the t i trat i on flask i n
Tab l e 4 . 1 Yi elds of C i trat e s Produced C alcul a t ed from th e rat i o of the
ra t e c onstant s at 25°C i n d i lu t e aqueous soluti on . Yi e ld
% .
E s t er - C i trat e
M ol e s of b a s e T ri me thyl Asym Sym C i trat e
p er mole of
Dimethyl
Monom ethylE s t er o . s 51 • 5 47.1 1 . 4 0 1 • 0 1 0 . 4 7 9 . 2 1 0 . 4 0 2 . 0 0 1 • 9 97 . 1 1 • 0 E s t er - Ci trat e M ol e s of b a s e p e r mole of E s t er 2 . 0 Sym Di me thyl 8 . 1 Asym M onomet hyl 83.9 C i trat e 8 . 0 37 ··-