The Effect of Light Intensity On Photosynthesis of …



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:

[pic]

[pic]

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 Oxy-meter 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] |V1 [ml] |V2 [ml] |c [mg/l] |

|1 |0.5 |135.24 |6.2 |4.6 |

|2 |1.0 |131.22 |10.0 |7.6 |

|3 |2.0 |132.65 |20.0 |15 |

|4 |3.0 |136.75 |8.5 |6.2 |

|5 |no light |131.22 |17.0 |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.

| |s [m] |cwinkler [mg/l] |coxy-meter [mg/l] |

|1 |0.5 |4.6 |2.5 |

|2 |1.0 |7.6 |2.5 |

|3 |2.0 |15 |2.5 |

|4 |3.0 |6.2 |2.5 |

|5 |no light |13 |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.

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