EXPERIMENT # 6

OBJECT
To determine end point by conductometry
CHEMICALS
1. NaOH 0.1M
2. Distilled Water
3. Sample A !
APPARATUS
1. "onductometer
2. "ell
3. !urette
#. pipette
$. %olumetric &las' 100ml
OBSERVATION
Sample- A
Sample B
Volume of NaOH (ml Co!"u#$a!#e (m%
1 1.255
2 1.301
3 1.384
4 1.52
5 1.644
6 1.778
7 1.895
8 2.03
9 2.14
10 2.31
11 2.45
12 2.66
13 2.72
14 3.04
15 3.13
16 3.21
17 3.36
Volume of NaOH (ml Co!"u#$a!#e (m%
1 61
2 60
3 55.8
4 52.5
5 49.4
6 47.1
7 46.2
8 45.7
9 45.4
10 44.6
11 43.3
12 43.6
13 39.8
14 38.9
15 34.7
16 32.4
17 30.8
18 24.8
19 22.9
20 22.6
21 22.6
22 20.8
23 17.5
24 16.8
25 16.3
26 15.48
27 14.92
28 14.59
29 12.89
30 11.59
31 11.09
32 9.8
33 9.18
34 8.44
35 7.87
36 7.77
37 7.38
38 7.35
39 7.46
40 7.66
41 7.85
42 7.96
43 8.18
44 8.32
45 8.42
46 8.6
47 8.86
48 8.94
49 9.06
50 9.22
51 9.44
52 9.56
53 9.79
Discussion(
Spe#&f&# Co!"u#$&'&$(
Water is a %ery poor conductor o& electricity. T)e presence o& ionic species
*electrolytes+ in ,ater increases t)e conductance considerably. Solutions o&
electrolytes- li'e metallic conductors- obey O)m.s la,. T)us- t)e current I passin/
t)rou/) a solution o& a particular electrolyte is proportional to t)e applied potential
di&&erence V as /i%en by t)e e0uation(
R
V
I =
Here 1 is t)e resistance o&&ered by t)e solution in o)ms *+
Or
I
V
R =
*1+
T)e resistance is directly proportional to t)e len/t) - o& li0uid t)rou/) ,)ic) t)e
current passes- and in%ersely proportional to its area o& cross2section A. 3t t)ere&ore
&ollo,s t)at(
A
R

=

*2+
Here is t)e constant o& proportionality and is called t)e specific resistivity. 3t is a
constant &or an a0ueous solution o& a /i%en electrolyte o& &i4ed concentration at a
particular temperature. 3n S3 units- resisti%ity )as t)e units o& o)m meter * m+. 3&
area o& cross2section is increased- more ions ,ill be able to &lo, and resistance ,ill
decrease.
T)e conductance L is de&ined as t)e reciprocal o& t)e resistance e4pressed in units o&

51
or Siemens *S+.
R
L
1
=
*3+
T)us- t)e conductance o& a )omo/enous body o& uni&orm cross2section is proportional
to t)e cross2sectional area A and in%ersely proportional to t)e len/t) - /i%en by(

A A
L =

*#+
Here t)e reciprocal o& resisti%ity is replaced by anot)er constant- ,)ic) is called
t)e specific conductivity ,it) units
51
m
51
or S m
51
,)ic) can be e4pressed as(
R
k
RA
= = =

=
A
k

*$+
Here k is called t)e cell constant.
T)e conducti%ity o& a /i%en solution in cell o& arbitrary desi/n and dimension can be
obtained by &irst determinin/ t)e cell constant k *t)e e&&ecti%e %alue o&

6A+ by
measurin/ t)e resistance o& 'no,n conducti%ity. T)e standard solution used &or t)is
purpose is 0.2000 M solution o& potassium c)loride *7"l+. A&ter t)e cell constant k
)as been determined- conducti%ities can be calculated &rom e4perimental resistances
usin/ e0uation *$+.
Measurement of Electrolytic Conductivity
T)e &irst attempts made to calculate t)e electrolytic conducti%ity employed DC or
direct current. 3t ,as soon noticed t)at t)is approac) lead to %arious complications &or
e4ample electrolysis lead to t)e remo%al o& ions &rom t)e solution and t)ere&ore to an
increase in t)e resistance. 3n addition- t)e release o& /ases at t)e electrodes *notably
)ydro/en and o4y/en+ o&ten /a%e rise to an increase in resistance in t)e %icinity o& t)e
electrodes. T)e use o& D" circuitry is impractical &or determinin/ conducti%ities &rom
measurement o& resistance o& t)e solution in a conducti%ity cell- since t)e electrodes
become 0uic'ly 8polari9ed8: t)at is- electrode reactions ta'e place ,)ic) set up an
em& opposin/ t)e applied em& leadin/ to a spuriously )i/) apparent cell resistance.
T)ese complications o& polari9ation can be pre%ented by usin/ AC or alternatin/
current- so t)at t)e 0uantity o& electricity carried durin/ one )al&2cycle is insu&&icient
to produce any measurable polari9ation. Similarly /reat impro%ements ,ere made- at
t)e same time- by employin/ platinum electrodes co%ered ,it) a colloidal deposit o&
8platinum black8 )a%in/ an e4tremely lar/e sur&ace area to &acilitate t)e adsorption o&
t)e tiny 0uantities o& t)e electrode reaction products produced in one )al&2cycle so
t)at no measurable c)emical em& is produced.
T)e resistance o& a conducti%ity cell ,it) an ionic solution can be measured %ery
accurately by use o& a "Wheatstone-bridge" circuit employin/ )i/) &re0uency
alternatin/ current.
Factors Affecting Conductivity
T)e conducti%ity o& a solution o& an electrolyte increases as t)e temperature is raised.
;or e4ample- t)e conducti%ities o& 0.1 M solution o& 7"l are 1.12 S6m and 1.2< S6cm
respecti%ely at 2<1 and 2<= 7.
T)e conducti%ity o& a particular electrolyte at a &i4ed temperature depends upon(
a+ t)e number o& ions present in unit %olume o& solution
b+ t)e speed at ,)ic) t)e ions oscillate in step ,it) t)e applied alternatin/
%olta/e
c+ ioni9ability o& t)e electrolyte
Variation of conductivity with concentration
a) For Strong Electrolyte
"onducti%ity does not solely depend upon t)e number o& ions present in unit
%olume o& t)e solution. "oncentrated solutions o& %ery soluble electrolytes s)o, a
ma4imum conducti%ity %alue ,)ic) t)en decreases on &urt)er concentration o& t)e
solution. T)e reason lies in t)e &act t)at ionic distances becomes less as
concentration increases. T)is results in a /reater attraction bet,een positi%e and
ne/ati%e ions- and ,it) a /reater reduction in ionic speeds. T)e p)enomenon is
called 8ionic interference8.
b) For Wea Electrolyte
As t)e concentration o& ,ea' electrolyte is increased- t)ere is an increase in t)e
number o& molecules- but a small proportion o& t)em are ioni9ed. !y increasin/
t)e concentration o& a ,ea' electrolyte- t)e conducti%ity reac)es a ma4imum
%alue &ollo,ed by a steady decrease. T)is is t)e same e&&ect as ,as obser%ed &or
stron/ electrolyte but in t)is case- ionic inter&erence is not responsible as a ,ea'
electrolyte produces %ery &e, ions and &e, ions can not inter&ere to a considerable
e4tent.