Required equipment → tools and chemicals

1. NaOH 0.01 M solution
   • It can sometimes be difficult to obtain a 0.01 M solution. A 0.1 M solution is much more common. If only 0.1 M NaOH solution is available, prepare a more dilute 0.01 M version by measuring out 100 mL of 0.1 M NaOH, adding 900 mL of distilled water, and mixing thoroughly.
2. Phenolphthalein 1% solution
   
3. Beaker 100 mL
   
4. Graduated syringe 1 mL
   • graduation interval: 0.01 mL (= 10 μL)
   • ideally with plastic pipette tip
   

Procedure for determining the concentration of carbon dioxide in aquarium water

1. Add 50 mL of aquarium water into a beaker.
2. Add 2-3 drops of 1% phenolphthalein solution and mix the sample by gently swirling the beaker.
   • If the sample turns pink (and this color does not disappear after 30 seconds), then the sample does not contain any CO₂, and there is no point in continuing.
3. Draw exactly 1 mL of 0.01 M NaOH solution into the syringe.
4. Carefully add the NaOH solution to the sample drop by drop from the syringe and mix the sample gently with a circular motion of the beaker after each drop.
5. Once the sample turns a faint pink color (and this color does not disappear after 30 seconds), stop adding more drops of NaOH solution to the sample.
   

   Starting point

   

   Slow down

   

   Ending point

   

   Over shot
6. Read the amount of NaOH consumed (in mL) from the syringe.
   • Let's refer to this amount as V_NaOH

Explanation the logic behind this method

• CO₂ makes water slightly sour (acidic).
• You add a special dye (phenolphthalein) that's colorless in sour water.
• You slowly drip in a strong soapy liquid (NaOH solution) until the water just barely turns pink.
• The amount of soapy liquid you had to add tells you exactly how much CO₂ was making the water sour in the first place.

Calculation of the CO₂ concentration

Formula:

• V_NaOH = Volume of NaOH solution used in the titration (in milliliters, mL)
• c_NaOH = Concentration of NaOH solution (in moles per liter, M) = 0.01 M
• 44 = Molar mass of CO₂ (in grams per mole, g/mol)
• 1000 = Conversion factor (milligrams per gram, mg/g) AND (liters per milliliter factor for V_sample)
• V_sample = Volume of your water sample (in milliliters, mL) = 50 mL

Example:

• Let's say that I used 1.25 mL of 0.01 M NaOH to neutralize carbonic acid (H₂CO₃) in my 50 mL sample of aquarium water:

  CO2 =	1.25 × 0.01 × 44 × 1000	= 1.25 × 8.8 = 11 ppm
  	50

• So, I have 11 ppm of CO₂ in my aquarium.

Alternative procedure with a smaller sample size (= 10 mL):

• If I use a sample of aquarium water that is 5 times smaller (10 mL) but the same concentration of NaOH (0.01 M), I will logically have 5 times lower NaOH consumption (0.25 mL):

  CO2 =	0.25 × 0.01 × 44 × 1000	= 0.25 × 44 = 11 ppm
  	10

• However, the CO₂ concentration in my aquarium should be the same: 11 ppm.
• Note: This alternative method is less accurate.