1. The total hardness values of a water supply before and after it had been passed through a laboratory deioniser were compared - Leaving Cert Chemistry - Question 1 - 2010

An appropriate indicator for this titration is Eriochrome Black T. The colour change observed at the end point of the titration is from wine red to blue.

The general purpose of buffer solutions is to stabilize the pH of a solution and resist changes in pH when small amounts of acids or bases are added.

Using the wrong buffer could lead to inaccurate endpoint determination due to changes in pH affecting the reaction between EDTA and metal ions. This could result in incomplete complexation, thus giving erroneous titration results.

1. Swirl to mix: The solution in the titration flask should be swirled to ensure proper mixing of the titrant (EDTA) with the sample solution.
2. Wash down: After adding the EDTA from the burette, wash down the sides of the flask with deionized water to ensure complete reaction and avoid losing any titrant.

Using the formula: ext{Total Hardness (M)} = rac{50.0 ext{ cm}^3 imes 20.0 ext{ cm}^3}{1} = 92.2 ext{ mg/l} Adjusting for the total solution, we find: ext{Concentration} = rac{1.84 imes 10^{-3}}{0.00184} imes 1000 = 184 ext{ mg/l} Thus, the total hardness in grams per litre is 0.184 g/L.

The total hardness is thus calculated as follows: $ext{Hardness (mg/L)} = 184 ext{ mg/L}$

For the water after ion-exchange: Calculating using the new average:

• Volume of EDTA used was 48.0 cm³
• The hardness is recalculated using a similar method, yielding a result of: ext{Total Hardness} = rac{48.0 imes 20.0}{0.00184} ext{ g/L}

The water is unsuitable because hardness ions (calcium and magnesium) remain even after passing through the deioniser. If the deioniser needs to be replaced, it indicates that the resins have lost their capacity to remove these ions effectively.