Determination of water hardness (titration)

Summary

This video script details a scientific experiment to determine water hardness, cautioning about potential hazards. It outlines the process of collecting a water sample, using a buffer solution, sodium EDTA, and eriochrome black T indicator in ethanol for titration. The experiment measures the color change to determine hardness, with results expressed in degrees German hardness, a unit specific to Germany. The script also explains converting this to millimoles per liter for scientific accuracy and calculating the concentration of alkali ions present.

Takeaways

• 🚫 The experiment involves hazardous compounds and the presenter disclaims responsibility for any damage caused by replicating the experiment.
• 💧 To prepare for water hardness testing, water should be allowed to run briefly to avoid additional contamination from the pipes.
• 🧪 Three solutions are essential for the experiment: a pH 11 buffer solution, an aqueous sodium EDTA solution, and an eriochrome black T indicator in ethanol.
• 🔬 The titration process requires an Erlenmeyer flask, a stir bar, a magnetic stirrer, and a buret filled with EDTA solution, ensuring air bubbles are removed.
• 📝 Accurate measurement is facilitated by the meniscus, which helps in reading the volume precisely in the buret.
• 🔑 A 50 mL sample of water is taken for the test, and the indicator is added, with adjustments made if the initial amount is insufficient.
• 🌡 The addition of 10 mL of buffer solution prepares the water sample for titration.
• 🔵 The endpoint of the titration is indicated by a color change to blue, signifying the reaction between EDTA and the hardness ions in the water.
• ⏱ It's important to wait a few seconds between additions of EDTA solution to allow for the color change to occur.
• 📊 The experiment involved measuring six samples, calculating the mean value, and converting the results to degrees German hardness.
• 📚 Degrees German hardness is a unit specific to Germany and outdated; the scientific unit for water hardness is millimoles per liter (mmol/L).
• 🔍 The concentration of alkali ions (calcium or magnesium) in the water is calculated by relating the volume of EDTA solution used to the volume of the water sample.

Q & A

What is the purpose of the experiment described in the transcript?

The purpose of the experiment is to determine the water hardness using a titration method with EDTA as the titrant.

Why is it important to let the water run for a short while before taking a sample for water hardness measurement?

It is important to let the water run to ensure that any additional contaminations, such as heavy metal ions that may have settled in the pipe, are flushed out, as these can influence the measurement.

What are the three solutions required for the water hardness determination experiment?

The three solutions required are a buffer solution with a pH of 11, an aqueous solution of sodium EDTA, and a solution of eriochrome black T indicator in ethanol.

Why is it necessary to fill the buret above the highest marking initially?

The buret is filled above the highest marking initially to trap air bubbles. These bubbles are then removed by quickly opening the stopcock and collecting the excess solution, allowing for an accurate adjustment to the highest marking.

How does the meniscus affect the reading of the buret?

The meniscus causes a broad line on the back of the buret to taper, which helps in reading the values more precisely by aligning the eye with the lowest point of the concave curve.

What is the significance of the buffer solution in the titration process?

The buffer solution maintains a stable pH of 11, which is necessary for the EDTA to effectively bind with calcium or magnesium ions, the indicators of water hardness.

What happens when the titration is complete and the solution changes color?

The color change to blue indicates the endpoint of the titration, signifying that all the calcium or magnesium ions have reacted with the EDTA.

Why is it important to wait a few seconds between additions of the EDTA solution during titration?

Waiting a few seconds between additions allows for the color change to occur, as the reaction between EDTA and the ions may take some time to be visibly noticeable.

How is the concentration of alkali ions in the water sample calculated?

The concentration is calculated by multiplying the volume of EDTA solution used by its concentration, then dividing by the volume of the water sample to get the concentration in moles per liter or millimoles per liter.

What is the relationship between degrees German hardness and millimoles per liter?

14.45 degrees German hardness is equivalent to 2.58 millimoles of alkali ions per liter, which is a more scientifically accurate unit for expressing water hardness.

Why might the water hardness values vary even if they are close to the values provided by the water utility?

Values may vary due to factors such as distance from the water source, which can introduce additional contaminants or affect the water's composition.