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Iodometric
Titration Method for Estimation of Iodine
in Salt
Principle
The
iodine content in
iodated salt is
estimated by titrametric
procedure known as iodometric titration.
Free iodine reacts with sodium
thiosulphate solution to give a light
yellow colour complex. This
colour complex combines
with soluble chemical starch which indicates the presence of sodium
iodide.
Equipment and Chemicals
Equipment
i.
Laboratory balance
ii. Gas
burner or kerosene stove
iii.
Reagent bottles with stoppers - 250ml,
500 ml, and 1000 ml
iv.
Measuring cylinder with stopper, 50 ml
v. Wash
bottle, 500ml
vi. Glass
stirring rod
vii.
Conical flask with stopper, 100ml
viii. Glass or plastic funnel
ix.
Burette, 10 ml
x.
Burette stand
xi. Clock
or watch
xii. A
closed box, cupboard or drawer to keep the conical flask
Chemicals :
i.
Sodium thiosulphate, (Na2S2O3 5H2O)
ii.
Concentrated sulphuric acid,(H2SO4)
iii.
Potassium iodide,(KI)
iv.
Soluble chemical starch
All
the chemicals should be analytical grade
and double distilled water
should be used free of
iodine and other contaminants.
Preparation of reagents
a) Sodium
thiosulphate (0.005 M) :
1.24 g
sodium thiosulphate was dissolved in
1 litre
hot double distilled water. This volume was sufficient for testing 200 salt samples. The solution
was stored in a cool, dark place.
b)
Sulphuric acid (2N H2SO4):
5.56 ml concentrated sulfuric acid
was added dropwise into 90 ml of chilled
double distilled water and
the final volume was made upto
100 ml
with double distilled water. This
volume was sufficient for testing 100
salt samples.
CAUTION: To avoid violent and dangerous reaction always
add the acid to water, never water to acid.
c) Potassium
iodide (KI):
100 g potassium iodide was dissolved in 1000
ml double distilled water. This volume was sufficient for testing 200
salt samples. This was stored in a refrigerator.
d) Saturated Salt Solution :
100 ml double distilled water was taken in a
conical flask and sodium chloride was added until the salt was insoluble. This solution was heated till the NaCl
crystals formed on the sides of the
vessels. After cooling down the saturated salt solution
at room temperature the
supernatant was transferred
in a clean bottle.
(e) Soluble Chemical Starch:
10g
soluble chemical starch was
dissolved in 100 ml hot double distilled water. Then the volume was
adjusted upto 1000 ml with saturated sodium chloride solution. This was
sufficient for testing 200 salt samples.
The starch solution was prepared fresh
every day.
Procedure:
10 g of salt was dissolved in 50 ml double
distilled water. Then 1.0 ml
of 2 N sulphuric acid and 5.0 ml of
10% potassium iodide was
added. On shaking, the solution
turned a yellow colour. This was incubated at room
temperature in the dark for 10 minutes. During
the incubation period
sodium thiosulphate
solution was poured into the burette and
the level of solution was adjusted to zero. The samples
were removed from the dark and titrated
against the sodium thiosulphate solution until it turned into a very light yellow colour (pale
yellow). Subsequently, a few
drops (1-5ml) of 1% starch solution was added. The solution turned a deep purple colour. Finally, it was
titrated until the solution became
colourless and the final reading was observed.
Result:
From
table given below, the iodine
content of the sample
in parts per million was calculated accordingly.
The results
were recorded in a register, under the following parameters
- Date
of collection
- Date
of testing
- Sample
number
-
Type of salt
-
Cluster number
- The
level of iodine in the sample.
Precautions
i)
Adding sulphuric acid
to a solution of iodated
salt liberates iodine, which is titrated with sodium
thiosulphate. Potassium iodide
(KI) is used because of the low solubility of
iodine and more iodine is dissolved in order to maintain the equilibrium.
ii)
Potassium iodide solution is added to keep the iodine in the dissolved state.
iii)
The reaction mixture
should be kept in the dark for 10 minutes
before titration because
light accelerates a side reaction in
which iodide ions
are oxidized to
iodine by atmospheric oxygen.
iv)
Starch solution must be prepared freshly.
v)
Starch is used as an external indicator.
vi)
The starch solution must be added near the end
of the titration, when
very little amount of iodine
is left
and the solution has a
faint-yellow colour. If starch is
added earlier, the iodine-starch
complex becomes very strong
and reacts too slowly
with sodium thiosulphate,
resulting in false
high readings.
vii)
The titration should be done in a comfortably cool room because
iodine is volatile and the sensitivity of the
starch indicator diminishes as the temperature rises.
Table
_________________________________________________________________
Burette
Parts per Burette Parts per
reading
million reading million
____________________________________________________________________
0.0
0.0 3.0 31.7
0.1
1.1 3.1 32.8
0.2
2.1 3.2 33.9
0.3
3.2 3.3 34.9
0.4
4.2 3.4 36.0
0.5
5.3 3.5 37.0
0.6
6.3 3.6 38.1
0.7
7.4 3.7 39.1
0.8
8.5 3.8 40.2
0.9
9.5 3.9 41.3
1.0
10.6 4.0 42.3
1.1
11.6 4.1 43.4
1.2
12.7 4.2 44.4
1.3
13.8 4.3 45.5
1.4
14.8 4.4 46.6
1.5
15.9 4.5 47.6
1.6
16.9 4.6 48.7
1.7
18.0 4.7 49.7
1.8 19.0 4.8 50.8
1.9 20.1 4.9 51.9
2.0
21.2 5.0 52.9
2.1
22.2 5.1 54.0
2.2
23.3 5.2 55.0
2.3
24.3 5.3 56.1
2.4
25.4 5.4 57.1
2.5
26.5 5.5 58.2
2.6
27.5 5.6 59.2
2.7 28.6 5.7 60.3
2.8
29.6 5.8 61.4
2.9 30.7 5.9 62.4
Burette Parts per Burette Parts per
reading
million reading million
6.0
63.5 8.0 84.6
6.1
64.5 8.1 85.7
6.2
65.6 8.2 86.8
6.3
66.7 8.3 87.8
6.4 67.7 8.4 88.9
6.5
68.8 8.5 89.9
6.6
69.8 8.6 91.0
6.7
70.9 8.7 92.0
6.8
71.9 8.8 93.1
6.9
73.0 8.9 94.2
7.0
74.1 9.0 95.2
7.1
75.1 9.1 96.3
7.2
76.2 9.2 97.3
7.3
77.2 9.3 98.4
7.4
78.3 9.4 99.5
7.5
79.4 9.5 100.5
7.6
80.4 9.6 101.6
7.7
81.5 9.7 102.6
7.8
82.5 9.8 103.7
7.9
83.6 9.9 104.7
Method for
Internal Quality Control adopted
A known positive
iodized salt sample was obtained and by performing multiple analyses on this
positive salt sample, a concentration
range was established and used for internal quality control purposes.
Once a
sufficient number of these test results
were obtained, the samples mean concentration (X)
in ppm and standard deviation
(SD) was also calculated. The 95% confidence interval was then calculated and
used as the operating control range, as follows:
Sample Mean
(X) ± 2(SD)
The X - 2(SD) =
the lower confidence limit (L)
X + 2(SD) = the
upper confidence limit
(U)
The
operating control range
is (L,U).
This internal
quality control sample was run with every batch of the test samples and the internal
quality control was successful when the results of this sample were between the
lower and upper limits
(i.e, the L-U range), for 95% of
test results. If the results were
outside the established range they were considered potentially suspicious and
the whole batch was repeated.
The quality
control chart was prepared to ensure the validity of the results obtained.
A regular linear graph paper was
used to prepare these plots. The mean salt iodine concentration (in ppm) of the
internal quality control sample was
plotted as a continuous horizontal line on
the Y-axis. The lower concentration value (L) was plotted below the mean
line on the Y-axis scale and the upper
concentration value (U) was plotted above the
mean line on the Y-axis scale.
The X-axis was used to plot the date on which the internal quality control
sample was analysed. This chart was used
to plot
the specific analysis date,
and salt iodine concentration obtained
for the control every time it was tested. If the value of the
internal quality control sample was
between the two limit lines, then the test
was deemed in control,
and all results were accepted.
Any internal quality control value that was plotted outside the two
limit lines then the test was considered as out-of-control, and the entire
batch was repeated.
+ 2SD(U)
Mean (X)
- 2SD(L)
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