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2nd year students of pharmacy

Instrumental chemical analysis laboratory

Conductometry measurement

Practical guide

Teacher: Dr. Attila Bnyeiresearch fellow

University of Debrecen, Institute of Physical Chemistry

Debrecen, 2009


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Determination of equivalence point in titration using

conductometry

In this lab we measure electric conductance.

Conductance (L or G) is the reciprocal of the electrical resistance. G = 1/R (Siemens)

Conductivity: = G*C where C is the cell constant, whose unit is cm-1

.

Molar conductivity or equivalent conductance of a solution with concentration of m molar: m =

c

*1000

Table 1 Conductivity of KCl solutions

Conductivity () / S cm-1

Concentration/M

19oC 20

oC 22

oC 24

oC 25

oC

1.000 0.1001 0.1021 0.1059 0.1098 0.1118

0.1000 0.01143 0.01167 0.01215 0.01264 0.01288

0.01000 0.001251 0.001278 0.001332 0.001386 0.001413

Conductometry can be used as an end point signaling tool in titrations when molar conductivity ofthe solution changes significantly in the chemical reaction. For example the

A+ + B- + C+ + D- AD + B- + C+

reaction will result decrease of conductance. Acid-base titrations are good candidates for using

conductometry to signal the end point of titrations.

Typical titration curves

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5 6 7 8 9 10

V (ml)

G

(mS) HCl - NaOH

AgNO3 - KCl

AcOH - NH4OH


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Problem: Determine API content of Vitamin C tablets of two differentmanufacturers with conductometry and iodometry suggested byPharmacopoeia.

In this lab first the general method of analytical method development is reviewed. General

questions of validation, quality management and assurance, specific properties of ascorbic acid areconsidered in a 30 min discussion during the dissolution of the samples.

Literature:

1. European Pharmacopoeia 4, Council of Europe, Strassbourg, 2001, p.52., p.674.

Sample preparation

Vitamin C tablets of two different manufacturer should be dissolved in approx. 30.0 cm3

0.025 M

HCl. The tablets contain 200 mg of ascorbic acid by the manufacturers specification. After

swelling and dissolution of the tablets (30 min) fill the volumetric flask to 50.0 cm

3

and the solidprecipitate should be filtered off (sample solution). Dilute 20.0 cm3

of filtrate to 50.0 cm3

using

0.025 M HCl (diluted sample solution).

Measurement

1. Titrate 50.0 cm3 0.025M HCl solution with 0.1M NaOH solution. At least 20 cm3 baseshould be added in approx 0.5 cm

3portions. Measure the conductivity of the solution after

addition of each portion of NaOH solution.. At least 20 cm3

base should be added.

2. Titrate similarly the first and second diluted samples solution of 50.0 cm3. At least 25 cm3

base should be added.

3. Measure 10.0 cm3 of the first sample solution (NOT diluted) and 5.0 cm3 starch solutioninto a baker and titrate with 0.025M iodine solution. Add the I2 solution in 0.5 cm

3portions

and measure the conductivity (G) after the addition of each portion. At least 17 cm3

iodine

solution should be added. Note the equivalence point from the color change.

4. Titrate 10.0 cm3 of the second sample and 5.0 cm3 starch solution with 0.025 M I2 solution,note the equivalence point.

Hints: The solution should be stirred. Air bubbles may cause instability of the conductivity. In this

case decrease stirring rate.

Calculation

Plot the measured conductivity (G) versus added base volume. All points can be plot on the same

paper using different notation (four curves). Determine equivalence points. Calculate the exact

concentration of NaOH solution. (Titration #1.) For Vitamin C samples consider that ascorbic acid

is a weak organic acid. One of the samples contains sodium ascorbate, too. Molecular weight data

for ascorbic acid and for sodium ascorbate are Mw= 176 g/mol and Mw= 198 g/mol, respectively.

Compare the ascorbic acid content determined by the two different method and specification.


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