The Effect of Light Intensity On Photosynthesis of Eel Grass
Aim
The aim of the experiment is to get acquainted with two different techniques for
measuring the concentration of dissolved oxygen in water – using Azide–Winkler method and
with digital Oxy-meter. In addition, the goal is to identify the effect of light intensity of eel
grass on the rate of photosynthesis.
Hypothesis
At low to medium light intensities the rate of photosynthesis is proportional to light
intensities, at high light intensities the rate reaches a plateau.
Planning
Photosynthesis is the process used by plants and some other organisms to produce all
their own organic substances (food), using only light energy and simple inorganic substances
[1]. The simplified chemical equation for photosynthesis is:
6 CO2 + 12 H2O
C6H12O6 + 6 O2 + 6 H2O
The rate of photosynthesis can be determined if the concentration of dissolved oxygen
in the solution is measured.
Independent variable:
- light intensity
Dependent variable:
- concentration of dissolved oxygen in water sample
Controlled variable:
- water temperature
- room temperature
Procedure:
Bottles with eel grass have to be exposed to different light intensities. To achieve that,
the same amount of eel grass and seawater is put into several bottles. Then each bottle is
exposed to the light at different distance from the reflector for the same time (15 minutes).
During the exposure, the water temperature is observed. In case the temperature of water
changes for more than 2°C, the experiment is aborted and the water is changed.
After fifteen minutes the water sample is taken using Winkler bottles and the
concentration of dissolved oxygen is measured using two different methods (Azide-Winkler
method, Oxy-meter).
1. Azide – Winkler Method
The Winkler method uses the following reagents (Gareth Williams: Techniques And
Fieldwork In Ecology), which have to be prepared before the experiment is conducted:
A. 45 g manganese (II) chloride in 100 cm3 distilled water
B. 70 g potassium hydroxide and 15 g potassium iodide in 100 cm3 distilled
water (Winkler reagent)
C. 50% v/v sulphuric acid (exact concentration is not critical)
D. 1.55 g sodium tiosulphate (IV) dissolved in distilled water and made up to
1 dm3
E. 25% w/v starch in saturated sodium chloride solution
The water sample has to be fixed (prevent changing of oxygen concentration)
immediately after it is being taken. Add 1 cm3 of manganese (II) chloride (solution A) and 1
cm3 of Winkler reagent (B), close and mix the bottle afterwards. The sample is now “fixed”
and can wait for some time to be analysed.
Using a pipette carefully add 2 cm3 concentrated sulphuric acid (C) and then mix the
bottle. Titrate 100 cm3 of the sample against standardised sodium thiosulphate (D) until pale
yellow. Add 0.5 cm3 of starch solution (E) and continue adding thiosulphate drop by drop
until the blue colour disappears.
The concentration of dissolved oxygen (c) can be calculated as follows:
c
1000 V2

V1
10
mg/l
where V1 is the volume of water sample and V2 is the volume of 0.0125 M sodium
thiosulphate solution used.
2. Oxy-meter
Usage of digital Oxy-meter is much more simple. Before measuring the concentration
of the dissolved oxygen in sample, the Oxy-meter has to be calibrated using the special
solution, which comes along with the Oxy-meter. After the calibration, the probe of the Oxymeter is plunged in the water sample and left there for few seconds allowing the electrode to
measure the concentration. The concentration of dissolved oxygen in the sample is written on
the screen.
Accessories:
1. Chemicals

manganese (II) chloride (MnCl2) – 45 g

potassium hydroxide (KOH) – 70 g

potassium iodide (KI) – 15 g

concentrated (50%) sulphuric acid H2SO4 – 100 cm3

sodium thiosulphate (IV) (Na2S2O3) – 10 g

starch – 100 cm3

sodium chloride (NaCl)

distilled water
2. Apparatus

balance

thermometer

Winkler bottles with corks (5x)

rack for tubes

250 cm3 bottles (2x)

pipette – 2 ml (2x)

birette

measuring cylinder (100 cm3)

reflector

transparent 1 l jar (6x)
Data Collection, Processing and Presentation
The results and the calculated value for the concentration of dissolved oxygen (c) from
the experiment using the Azid-Winkler method are presented in table 1.
s [m]
0.5
1.0
2.0
3.0
no light
1
2
3
4
5
V1 [ml]
135.24
131.22
132.65
136.75
131.22
V2 [ml]
6.2
10.0
20.0
8.5
17.0
c [mg/l]
4.6
7.6
15
6.2
13
Table 1: results gathered with Azide-Winkler method; s is the distance of the bottle from the reflector, V1 is the
volume of the sample, V2 is the volume of sodium tiosulphate solution used and c is the concentration of
dissolved oxygen
The calculated values of the oxygen concentration using Winkler method can be
compared with those gathered with Oxy-meter. The results are shown in table 2.
1
2
3
4
5
s [m]
0.5
1.0
2.0
3.0
no light
cwinkler [mg/l]
4.6
7.6
15
6.2
13
coxy-meter [mg/l]
2.5
2.5
2.5
2.5
2.5
Table 2: comparison of values of the concentration of dissolved oxygen gathered with Winkler method and with
Oxy-meter
Conclusion and Evaluation
The experiment was not a successful one. The hypothesis that the concentration of
dissolved oxygen decreases with increasing distance of the bottle from the reflector cannot be
proved, neither disproved, because the gained results are not reliable as it is seen in table 2.
Both methods used are very common in scientific measurements. The Azide-Winkler
method is one of the most accurate ones. However, the obtained results are not reasonably
connected, probably because of the errors connected with the method. The concentrations of
the needed solutions were probably little different from the right ones, because of difficult
conditions when conducting the experiment. The experiment was done outside, at very high
outside temperatures. Therefore, all the solutions were evaporating and their concentrations
were changing constantly.
To get reliable results, the analysis of the sample should be done inside, preferable in a
laboratory with the most precise equipment. Therefore you can avoid the problems mentioned
above.
Also the results obtained with Oxy-meter are not reliable. This is because the electrode
in the probe was probably malfunctioning. The electrode was saddled with different
precipitates and therefore could not measure the concentration of dissolved oxygen
accurately. The electrode needs to be replaced and then the Oxy-meter will produce true
concentration values.
References
1. Andrew Allot: Biology for the IB Diploma; Oxford University Press, 2001.
2. Gareth Williams: Techniques And Fieldwork In Ecology; Collins Educational, 1987.