The leaves of the rhubarb plant are considered poisonous because they contain high levels of oxalic acid - Scottish Highers Chemistry - Question 3 - 2017

To determine the mass of oxalic acid in a rhubarb leaf, one viable method is the titration technique. This involves the following steps:

1. Preparation of the Sample: Begin by finely chopping the rhubarb leaves to increase the surface area, which facilitates the extraction of oxalic acid. The chopped leaves can then be soaked in distilled water to dissolve the oxalic acid into the solution.

2. Filtration: After allowing enough time for extraction, filter the mixture to remove plant debris, leaving a clear liquid that contains the dissolved oxalic acid.

3. Titration Setup: Prepare a standard solution of a strong base, such as sodium hydroxide (NaOH). Using a burette, the sodium hydroxide solution can be titrated against the oxalic acid solution. It is essential to use an appropriate indicator, such as phenolphthalein, which will signal the endpoint of the titration where the solution changes color, indicating that all oxalic acid has reacted.

4. Performing the Titration: Carefully add the NaOH solution to the oxalic acid solution until the endpoint is reached. Record the volume of NaOH used.

5. Calculating the Concentration: Using the titration results, calculate the concentration of oxalic acid in the solution. The reaction between oxalic acid and sodium hydroxide is a 1:2 reaction, as shown in the balanced equation:

   $$$$

ightarrow Na_2C_2O_4 + 2H_2O$$

Therefore, if 'V' is the volume of NaOH used and 'C' is its concentration, then the moles of NaOH used can be calculated as: $ext{Moles of NaOH} = C imes V$

Since the molar ratio of NaOH to oxalic acid is 2:1, the moles of oxalic acid can be determined by dividing the moles of NaOH by 2.

6. Final Calculation of Mass: Finally, using the moles of oxalic acid calculated, you can determine the mass using the molar mass of oxalic acid (90.03 g/mol): $ext{Mass of oxalic acid} = ext{moles} imes ext{molar mass}$

This method allows for accurate determination of the amount of oxalic acid in rhubarb leaves, ensuring safety and proper assessment of its toxicity.