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relationship between pH value and rate of photosynthesis

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Background information

pH influences the rate of photosynthesis. University students and scientists have conducted experiments showing how pH affects the rate of photosynthesis. The result shows that in pH 7.5 solution leaves produce oxygen the fastest and those in pH 4.5 the slowest( using pH 4.5, 5.5, 6.5 and 7.5). (Source from the internet: www.madsci.org/posts/archives/mar2000/952732207.Bt.q.html )

pH of the solution is determined by the H+ ion in the solution. In pond water, there are many ions present at well. Do they influence the rate as well?


Test the relationship between the pH value of water and different ions present to the rate of photosynthesis in plants.


The closer the pH value is to neutral(pH=7) and fewer ions present, the faster the rate of photosynthesis.





pH of pond water using data logger

Pond water of different locations in Singapore

  • Amount of sampled Water
  • Light intensity
  • Resistivity of the wires

Equipment and apparatus

18 watt lamp

Data logger

Water sample from different

 parts of Singapore


Hole punch

12 ml syringe

Universal indicator


20 ml beaker

Measuring cylinder

Methyl orange



Leaves of Ervatamia

Screened methyl orange



  • Test the pH of different pond water using data logger
  1. pour 100ml of sampled water into five different beakers
  2. attach the pH sensor to the data logger
  3. dip sensor end of the data logger into the sampled water
  4. allow the sensor to stay in the water for a time of one minute
  5. record the pH level of the water
  6. repeat (5) for each water sample
  7. repeat the process twp more times and calculate the average pH level for each water sample( as to increase the accuracy of the date logger)
  • to prove data logger gives the correct range of pH and amount of ions present
  1. add all different types of indicator to a small portion of each sampled water
  2. record the observation
  1. to know which contains more ions than others
  1. add NaOH and acidified image01.png, phenolphthalein respectively to each sampled water
  2. observe whether precipitate forms or not and compare the amount of precipitate

Data collection and observation

Table of pH level of collected sample of pond water

location of water sample

1st reading

2nd reading

3rd reading

Average pH value

NUS Prince George Park pond





ACS(I) pond





Botanical Garden swan lake





Bukit Timah nature reserve





De-ionised water





Observation when different chemicals are added

Location of sampled water

Universal indicator

Methyl orange

Screened methyl orange


HNO3 + AgNO3



Light green

Yellowish orange


Small quantity of white ppt

Small quantity of white ppt

No visible changes


Light green

Darker shade of yellowish orange


Larger quantity of white ppt

Small quantity of white ppt

No visible changes

Botanical Garden

Lime green

Lighter shade of yellowish orange


Larger quantity of white ppt

Small quantity of white ppt

No visible changes

Bukit Timah

Lime green

Lighter shade of yellowish orange


Larger quantity of white ppt

Larger quantity of white ppt

No visible changes





Dark Green

Larger quantity of white ppt

Small quantity of white ppt

No visible changes

Universal indicator: very dark green

Methyl orange: orange

Screened methyl orange: dark green

Phenolphthalein: colorless

Rate of photosynthesis

(This is the biology practical hence details are excluded)

Location of

pond water

Number of

discs used

1st reading of time

needed for 50% of

discs to rise/min

2nd reading of time needed for 50% discs to rise/min

Average time needed for 50% of the discs to rise/min

NUS pond





ACS(I) pond





Botanical Garden pond





Bukit Timah pond





De-ionized water






  • The closer the pH level of the water is to neutral, the faster the rate of photosynthesis. In this case water from ACS (I) has the fastest rate of photosynthesis since its pH level is the closest to neutral value.
  1. While precipitate appears when phenolphthalein was added may be due to the other ions present in the water, because pod water has many more ions than normal chemicals, hence the sample water appeared to be cloudy when indicator was added.
  2. it is obvious that the more cloudy it is, the more types of ions present in the water. In this case, NUS pond water has fewest ions since only a small portion of white precipitate was observed.( the more ions present the slower the rate of osmosis from outside solution to cell sap in the root, and hence the slower the rate of photosynthesis.)
  3. According to biology concept, NUS pond water has fewest ions therefore supposedly it should be the one with highest rate of photosynthesis. However in our experiment it was not the case.

Limitations in the experimental method

  • water sample were not collected at the same time on the same day.
  • It rained before we collected the sample, hence the water was slightly more acidic than usual since the rainwater is acidic due to the present of image02.png
  • The number of leaf discs used was not constant.
  • There is no control of water at pH=7 used.
  • The leaf discs were left in open air for a time before being added to the beakers to be tested for the rate of photosynthesis. The leaf discs would have started photosynthesizing before being placed in the water samples, causing a shortened time and hence lead to inaccurate results.
  • The pH values of the water samples received from the date logger may be flawed as the sensor of the data logger was washed with the de-ionized water after being dipped in each different sample of water. Hence the results for the pH for the water sample may be slightly more acidic than it really is, since the de-ionized water is the most acidic one according the results recorded.


This is my first time doing a self-designed experiment. For all the past practical I have done under internal assessment are all practical in which instructions are given so that students only need to follow what is written on the paper word by word and carry out the experiment.

However this time was not the care. No instructions, no apparatus suggestions, no procedure—nothing is prepare for me. However we really found it an enjoyable and memorable experience despite all the disagreements caused when we were doing the practical.

Needless to say we have encountered many problems during the process. The most significant one was that due to the limited amount of water sample we had, no matter how much chemicals we have added, a positive change did not appear. As a result we have tried a lot of funny things, and we all burst out into laughter.

There was one thing that we all felt it was a pity. We did not carry out any titration or testing the concentration of H+. this should be something that has been planned before we started. When we finally realized that we should have done at least one set of titration, the time has almost gone.

Hence by reflecting upon what we did on group 4 project day, I should say that it was a success to certain extend, although we did not get all the favored results. However we could have planned the whole experiment better before that. Moreover all the uncertainties when small quantity of sample water used, should be taken into consideration before the experiment is carried out.

This student written piece of work is one of many that can be found in our GCSE Green Plants as Organisms section.

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