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6.
Transfer the solution from the beaker to the 100 ml volumetric flask. Dilute
the de-ionized water and mix thoroughly. This is your diluted extraction
solution.
7.
Filter 20 ml of the diluted extraction mixture through filter paper in a gravity
funnel into a flask or beaker.
8.
Wash the volumetric flask thoroughly with de-ionized water. Pipette 10 ml of
the diluted extraction solution into the flask, dilute to the mark, and mix
thoroughly. This second diluted extraction solution is the unknown solution.
Transfer the solution to an Erlenmeyer flask and label it.
9.
Place the second order filter lever in the white position and set the
wavelength to 508 nm. Fill a cuvette with de-ionized water and insert it into
the sample compartment. Blank the instrument according to the procedure in
Basic Operation.
10.
Fill another cuvette with the first standard. Insert the cuvette into the sample
compartment and record the absorbance.
11.
Repeat step 10 for the other three standards and the unknown.
Note:
The range of iron levels in the vitamin tablets can be quite broad. As a result,
the prepared unknown solution may not yield an absorbance value that can be
properly interpreted from the standard curve. Therefore, you may have to make
adjustments in the concentration of iron in the unknown solution by trial and error.
If the absorbance of the unknown turns out to be higher than that of standard 4, go
back to
Step 8
and use smaller amount (e.g. 5 ml) of the diluted extraction solution
in making the unknown. If the absorbance of the unknown is lower than that of
standard 1, use a larger amount (e.g. 20 ml) instead. Keep doing the above until
the desired unknown is obtained.
Calculations:
1. On a regular piece of graph paper, label the horizontal axis concentration and
mark it in equal intervals from 0 to the value of standard 4. Label the vertical
axis absorbance and mark it in equal intervals from zero to a convenient round
value above your highest data point.
2. Plot absorbance versus concentration for the four standard solutions. Draw the
best straight line through the four points and the origin of the graph.
3. Use the working graph to determine the concentration of Iron II ion in the
unknown solution. Using the dilution data from the experiment, calculate the
concentration of the diluted extraction solution and the mass of iron (in mg)
present in the vitamin tablet.
