18
axis absorbance and mark it in equal intervals from 0 to a convenient round
value above your highest data point.
2. Plot absorbance versus concentration for the four standards. Draw the best
straight line through the origin and the four points.
3. Using the working graph determine the concentration of the unknown solution.
4. Multiply the concentration of the unknown by 1000 to get the concentration of
glucose in the original soft drink.
Experiment 5
Determination of a Species in a Solid Sample
In order to determine the amount of a species in a solid sample, it is necessary to
make an extraction solution containing that species with a proper solvent and then
measure the concentration of the species in the solution using the same method
described in the last experiment.
As an example, you will determine the amount of iron in a vitamin tablet. Iron II
forms a colored complex with 1, 10 phenanthroline, which absorbs light energy at
508 nm. A working curve will be prepared for standard solutions of Iron II-
phenanthroline complex, and the concentration of an unknown sample will be
determined from the working curve data.
The materials required for this experiment are a vitamin tablet, 50 ml 0.1M HCL (aq)
solution, 50 ml 0.1g/l Fe
+2
(aq) solution, 10 ml 1% (w/v) HONH
3
CL (aq)
(hydroxylamine hydrochloride solution, 10 ml 1M NaC
2
H
3
O
2
(aq) sodium acetate)
solution, 10 ml 1% (w/v) 1,10- phenanthroline (ortho) solution, a 100 ml beaker, a
50 ml burette, five Erlenmeyer flasks, a 50 ml graduated cylinder, a 10 ml pipette,
six round cuvettes, a hot plate, a stirring rod, a gravity funnel, and some filter paper.
Procedure:
1.
Fill the burette to the top calibration line (50 ml) with the Fe
+2
stock solution.
2.
Deliver 5 ml of the stock solution form the burette into a clean 100 ml
volumetric flask. Measure 10 ml of the acetate solution and 10 ml of the
phenanthroline solution into the graduated cylinder respectively and add to
the flask. Allow the mixture to stand for five minutes, dilute the flask to the
mark with de-ionized water, cap the flask, and mix the diluted solution
thoroughly. Transfer this first standard to the Erlenmeyer flask, calculate the
concentration of the standard and label the flask.
3.
Rinse the volumetric flask with de-ionized water and repeat
Step 2
for 10 ml,
15 ml, and 20 ml of the stock solution from the burette. These are standards
2, 3, and 4.
4.
Place the vitamin tablet into a 100 ml beaker. Measure 50 ml of the 0.1 M
HCL (aq) into the graduated cylinder and add it to the beaker.
5.
Place the beaker on a hot plate and heat the extraction solution to boiling.
Gently boil the acid extraction solution for 15 minutes. Break apart the tablet
with a clean stirring rod while boiling. Remove the beaker from the hot plate
and allow the solution to cool to room temperature.
