16
The materials required for this experiment are 500 ml 0.04 g/l thymol blue solution,
40 ml 1 M HCL (aqueous), a 50 ml burette, a 10 ml graduated cylinder, a 100 ml
volumetric flask, five round cuvettes, and four Erlenmeyer flasks.
Procedure:
1.
Fill the burette to the top calibration line (50 ml) with the thymol blue stock
solution.
2.
Deliver 5 ml of the thymol blue solution from the burette into the volumetric
flask. Measure 10 ml HCL (aq) in a graduated cylinder and add to the flask.
Dilute the flask to the mark with de-ionized water, cap the flask, and mix the
diluted solution thoroughly. Transfer this first standard to an Erlenmeyer
flask, calculate its concentration, and label the flask.
3.
Rinse the volumetric flask with de-ionized water and repeat step 2 for 15 ml,
20 ml, and 30 ml of HCL. These are standards 2, 3, and 4.
4.
Place the wavelength at 545 nm and the second order filter in the white
position. Fill a cuvette with de-ionized water and insert it into the sample
compartment. Blank the instrument according to the procedure in
Basic
Operation
.
5.
Fill another cuvette with the first standard and insert it into the sample
compartment. Measure the absorbance of the first standard at this
wavelength.
6.
Repeat step 5 for the other three standards.
Calculations:
1.
On a sheet of graph paper, label the horizontal axis concentration and mark it
in equal intervals from 0 to 0.02 M. Label the vertical axis absorbance and
mark it in equal intervals from 0 to a convenient round value above your
highest data point.
2.
Plot absorbency versus concentration for the four standard solutions.
3.
Using a transparent ruler, draw a straight line from the origin such that there
are equal numbers of points above and below the line. This is the B
eer’s law
plot.
Experiment 4
Determination of the Concentration of a Solution
You will prepare a Beer’s Law plot from a series of molybdenum blue standards and
determine the amount of glucose in a soft drink. The glucose is able to reduce Cu
2+
to Cu
+
under the reaction conditions. The Cu
+
is then used to reduce
phosphomolybdic acid (dodeca molybdophosphoric acid) (PMA) to molybdenum
blue, which absorbs light at a wavelength of 780 nm. The amount of molybdenum
blue formed is directly proportional to the amount of glucose originally present in the
solution.
The materials required for this experiment are 50 ml 2 g/l glucose stock solution, 12
ml alkaline copper titrate solution, 12 ml phosphomolybdic acid (PMA) solution, 10
ml unknown soft drink sample, two 600 ml beakers, a 50 ml burette, six Erlenmeyer
