Calcium and Magnesium ion concentration determination with EDTA titration

Summary

This video explains how to experimentally determine the concentration of calcium and magnesium ions using EDTA titration. EDTA, a strong compound, forms a 1:1 complex with metal ions. The experiment involves titrating a 50 mL aliquot of a dissolved limestone sample containing unknown calcium and magnesium concentrations. A buffer, potassium cyanide, and an indicator (calmagite) are added. EDTA binds with calcium and magnesium, causing a color change from wine red to blue, signaling the titration's end. The volume of EDTA used is calculated to determine the cumulative concentration of calcium and magnesium ions.

Takeaways

• 🧪 EDTA (ethylenediaminetetraacetic acid) forms a 1:1 complex with most metal ions, including calcium and magnesium.
• ⚖️ The experiment involves titrating calcium and magnesium ions in a limestone sample using EDTA.
• 📏 A 50-milliliter aliquot of the sample is used for each titration, measured with a burette for accuracy.
• 🧴 A buffer is added to make the solution basic before the titration process.
• 🔗 Potassium cyanide is used as a complexing agent to prevent interference from other metal ions.
• 🚫 It is important to maintain a basic pH to avoid the formation of toxic HCN gas in acidic conditions.
• 🎨 An indicator, calmagite, is added, which turns wine red when it binds with magnesium ions.
• 🔄 During titration, EDTA binds with free calcium and magnesium ions, causing a color change from wine red to purple and then to blue, signaling the endpoint.
• 📐 The amount of EDTA used in the titration allows calculation of the cumulative concentration of calcium and magnesium in the sample.
• 🧮 The total concentration is determined by multiplying the volume of EDTA used by its concentration and dividing by the volume of the aliquot.

Q & A

What is EDTA and how does it interact with metal ions?

EDTA, or ethylenediaminetetraacetic acid, is a strong compound that forms a one-to-one complex with most metal ions by binding its carboxyl groups to the positive charge of the metal ions.

What ions are being measured in this experiment?

The experiment measures the concentrations of calcium and magnesium ions.

What sample is used in this titration experiment?

A 250 milliliter sample of limestone dissolved in acid is used in the experiment.

What is the purpose of using a buffer in this experiment?

A buffer is used to maintain a basic pH in the solution, which is important for preventing interference from protons that could bind and form toxic HCN gas.

Why is potassium cyanide added to the aliquot?

Potassium cyanide is added as an auxiliary complexing agent to bind ions other than magnesium and calcium so they don't interfere with the titration.

What indicator is used, and how does it behave in the titration?

The indicator used is Calmagite (referred to as 'kaga mine' in the transcript). It binds with magnesium to form a wine-red color, and during titration, the solution changes color from wine-red to purple and then blue, indicating the endpoint.

What is the purpose of titrating the solution with EDTA?

The purpose of titration with EDTA is to form complexes with the free calcium and magnesium ions, eventually displacing magnesium from the Calmagite indicator until the solution turns blue, signaling the end of the titration.

How is the concentration of calcium and magnesium ions determined?

The concentration is determined by multiplying the volume of EDTA used by its molar concentration, giving the moles of EDTA. Since the reaction is in a one-to-one ratio, this also represents the total moles of calcium and magnesium ions. The cumulative concentration is then calculated by dividing the moles by the volume of the aliquot.

Why is maintaining a basic pH crucial in this experiment?

Maintaining a basic pH prevents the formation of HCN gas, which is toxic and could result from protons binding to cyanide in an acidic environment.

What volume of EDTA was used in the example provided?

In the example, an average volume of 38.5 milliliters of 0.01 molar EDTA was used to titrate the aliquot.