MPI
Investigation :
Starch Calibration Curve
The
concentration of a solution can be determined
colorimetrically. This is done by comparing it with a
series of standardised colours. First a series of
solutions is prepared by serial dilution of a stock
solution.
In this
investigation the concentration of unknown starch
solutions is to be determined using iodine solution as
an indicator.
A. The
Visual Method
Materials
-
15 test tubes with bungs in a rack
-
10cm3 graduated pipette and pump
-
1cm3 graduated pipette and pump
- Marker pen
|
-
50cm3 1% starch solution in a beaker
-
50cm3 distilled water in a beaker
-
20cm3 dilute iodine solution
- 4 unknown starch solutions (A, B, C and D)
|
|
-
Set up a series of starch solutions as follows:
|
Starch concentration
/ % |
Volume of 1% starch
/ cm3 |
Volume of distilled water
/ cm3 |
| 1 |
10 |
0 |
| 0.9 |
9 |
1 |
| 0.8 |
8 |
2 |
| 0.7 |
7 |
3 |
| 0.6 |
6 |
4 |
| 0.5 |
5 |
5 |
| 0.4 |
4 |
6 |
| 0.3 |
3 |
7 |
| 0.2 |
2 |
8 |
| 0.1 |
1 |
9 |
| 0 |
0 |
10 |
-
Label
each tube and add 1cm3 iodine solution to
each tube replace the rubber bung and turn each tube
once.
-
Take
four more test tubes label them and add 10cm3
of solutions A, B, C and D. Add 1cm3 iodine solution
to each tube replace the rubber bung and turn once.
-
Compare
the colours of the unknown with the standard
solutions. Estimate the concentrations of the unknown
solutions.
B. The
Colorimetric Method
The
colorimeter is a device which will measure the amount
of light of a particular wavelength passing through a
coloured solution. The wavelength is set by shining
light though a filter or by using an LED. The coloured
liquid is placed in the path of the light and the
amount of light passing through is measured by a
photocell.
The colour
of the light is often recorded as the wavelength of the
light measured in nanometres (nm).
|
Colours |
Wavelengths / nm |
| Red |
625
– 665 |
| Orange |
605
– 625 |
| Yellow |
590
– 605 |
| Green |
505
– 590 |
| Blue |
436
– 505 |
| Dark Blue (Indigo) |
420
– 436 |
| Violet |
397 – 420 |
Materials
|
TI Graphing Calculator with DataMate program installed
CBL2 interface
Colorimeter sensor |
11 cuvettes
Marker pen
Pipette |
|
Starting
the DataMate Program and calibrating the sensor
1.
Suck up
liquid from a test tube and fill a colorimeter cuvette
until it is 1 cm from the top. Wash the pipette, take a
sample from the next tube and fill a fresh cuvette.
When labelling the cuvettes mark one side only near
the top. Handle the cuvettes near the top
too.
2.
Use the
following steps to start the DataMate program on your
calculator:
Press
(for TI-73, 82 and 83 press
),
then press the calculator key for the number that
precedes DATAMATE. Press
.
An introductory screen will appear, followed by the
main screen.
3.
Plug the
colorimeter probe into channel CH 1 on the CBL2
interface.
4.
Start the DataMate program. Press
to reset the program. DataMate will detect the sensor
and display the current sensor reading.
|
Wavelength selection on the colorimeter
The
Vernier colorimeter uses four LEDs which shine at
a single wavelengths: 430nm, 470nm, 565nm and
635nm. Which colours do these correspond to?
Select the colour of your choice on the
colorimeter. An green indicator light will tell
you which one you are on.
To
decide which colour to select you need to
consider the colour of your solution (iodine plus
starch is blue so which light does it let
through?). Then select a different colour from
this.
To calibrate to colorimeter
Make sure the colorimeter has been switched on
for about 5 minutes.
Insert a cuvette, filled with distilled and
iodine solution in this case, this is called
the blank. It will be regulated to show 100%
transmittance or 0 absorbance. The clear side of
the curvette must face the arrow at the back
of the cuvette slot.
Press the CAL button until the red LED begins to
flash. Release the CAL button. When the LED stops
flashing, the calibration of the colorimeter is
finished.
|
Collecting data
-
Using the cursor buttons,
or
(be patient it’s a bit sluggish!)
select
MODE,
press
(scroll up to get to the last
item on the menu).
-
In the
SELECT MODE menu press
 :
EVENTS WITH ENTRY.
-
Press
 :
OK
to return to the main screen.
-
Set the cuvette with the first sample in the
colorimeter, take care not to touch the sides
where the light will pass.
-
Select
 :
START to begin
data collection. Press
to record your measurement. When you are asked
to enter value, enter the starch concentration
of your first sample. Press
again.
-
Place the next cuvette in the colorimeter and
press
.
Then type in the next starch concentration.
Press
again and you will find the calculator
producing an autoscaled scattergram of the
measurements.
|
 |
-
Continue to take measurements of all your
samples.
-
You may stop data collection at any time by
pressing the
key. When you stop data collection you will
see the complete auto-scaled graph.
-
To store your data, if you are satisfied with
it, return to the main screen by pressing
.
Press
 :
TOOLS, then select
:
STORE LATEST DATA RUN. This stores the data
in lists. In this case “temperature” in L1
and the colorimeter “absorbance” in L2
with a copy of it in L3.
To check this press
 :
QUIT, then
,
then
and, finally, select
:
EDIT… You will see a spread sheet with your
data in it in L1, L2 (and L3).
|
The Open Door Web Site 