A number of tests were carried out on a sample of swimming pool water to test its quality. (a) A colorimetric experiment was used to estimate the concentration of free chlorine in the sample. What is the general principle of all colorimetric experiments? (b) Identify a suitable reagent to test for free chlorine in swimming pool water and state the colour which develops when this reagent reacts with free chlorine. (c) Describe briefly how you would estimate the concentration of free chlorine in a sample using either a comparator or a colorimeter. (d) Give the name or formula of a free chlorine species in the swimming pool water. Give a reason why the concentration of free chlorine in treated drinking water is usually between 0.2 - 0.5 p.p.m. whereas in swimming pool water it should be between 1 - 5 p.p.m. (e) When 1200 cm³ of swimming pool water was filtered, the mass of the filter paper, upon drying, had increased by 0.78 g. When 250 cm³ of the filtered water was evaporated to dryness the mass of the residue obtained was 0.32 g. Calculate the concentration in p.p.m. (i) of suspended solids. (ii) of dissolved solids.

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A number of tests were carried out on a sample of swimming pool water to test its quality - Leaving Cert Chemistry - Question 3 - 2006

Question 3

A number of tests were carried out on a sample of swimming pool water to test its quality. (a) A colorimetric experiment was used to estimate the concentration of f... show full transcript

Worked Solution & Example Answer:A number of tests were carried out on a sample of swimming pool water to test its quality - Leaving Cert Chemistry - Question 3 - 2006

Step 1

What is the general principle of all colorimetric experiments?

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Answer

The general principle of colorimetric experiments is based on the relationship between the intensity of color and the concentration of the analyte in solution. Typically, this is expressed by Beer-Lambert Law, which states that absorbance (A) is directly proportional to concentration (c) of the substance: $A = ext{k} imes c.$ Here, A represents the absorbance, k is the proportionality constant, and c is the concentration of the colored species.

Step 2

Identify a suitable reagent to test for free chlorine in swimming pool water and state the colour which develops when this reagent reacts with free chlorine.

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Answer

A suitable reagent to test for free chlorine in swimming pool water is acidified potassium iodide (KI/HCl) or a DPD tablet (N,N-diethyl-p-phenylenediamine). The color developed when free chlorine reacts with DPD is a pink/red coloration.

Step 3

Describe briefly how you would estimate the concentration of free chlorine in a sample using either a comparator or a colorimeter.

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Answer

To estimate the concentration of free chlorine:

Comparator Method:
- Add the reagent to the water sample.
- Allow the color to develop for a specified time.
- Compare the color intensity with a standard color chart that correlates color intensity to concentration.
- Match the sample color against the standards to estimate the concentration.
Colorimeter Method:
- Prepare a standard solution with known concentrations of free chlorine.
- Add the reagent to the sample and measure the absorbance using a colorimeter.
- Obtain absorbance readings for both sample and standard solutions.
- Plot the concentration against absorbance for the standard to find the corresponding concentration of the sample.

Step 4

Give the name or formula of a free chlorine species in the swimming pool water. Give a reason why the concentration of free chlorine in treated drinking water is usually between 0.2 - 0.5 p.p.m. whereas in swimming pool water it should be between 1 - 5 p.p.m.

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Answer

A free chlorine species in swimming pool water is hypochlorous acid (HOCl) or hypochlorite ion (ClO⁻). The concentration is lower in treated drinking water (0.2 - 0.5 p.p.m.) because it is intended to be safe for human consumption, minimizing chlorination byproducts; whereas swimming pools require higher concentrations (1 - 5 p.p.m.) to effectively sanitize against pathogens and provide safe recreational water.

Step 5

When 1200 cm³ of swimming pool water was filtered, the mass of the filter paper, upon drying, had increased by 0.78 g. When 250 cm³ of the filtered water was evaporated to dryness the mass of the residue obtained was 0.32 g. Calculate the concentration in p.p.m.

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Answer

To calculate the concentration of:

(i) Suspended Solids:

Mass of suspended solids from the filter paper: 0.78 g.
Volume of water filtered: 1200 cm³ or 1.2 L.
Concentration calculation: $Concentration (p.p.m.) = \frac{0.78 \text{ g}}{1.2 \text{ L}} \times 10^6 = 650 \text{ p.p.m.}$

(ii) Dissolved Solids:

Mass of dissolved solids from the residue: 0.32 g.
Volume of water evaporated: 250 cm³ or 0.25 L.
Concentration calculation: $Concentration (p.p.m.) = \frac{0.32 \text{ g}}{0.25 \text{ L}} \times 10^6 = 1280 \text{ p.p.m.}$

A number of tests were carried out on a sample of swimming pool water to test its quality - Leaving Cert Chemistry - Question 3 - 2006

Worked Solution & Example Answer:A number of tests were carried out on a sample of swimming pool water to test its quality - Leaving Cert Chemistry - Question 3 - 2006

What is the general principle of all colorimetric experiments?

Identify a suitable reagent to test for free chlorine in swimming pool water and state the colour which develops when this reagent reacts with free chlorine.

Describe briefly how you would estimate the concentration of free chlorine in a sample using either a comparator or a colorimeter.

Give the name or formula of a free chlorine species in the swimming pool water. Give a reason why the concentration of free chlorine in treated drinking water is usually between 0.2 - 0.5 p.p.m. whereas in swimming pool water it should be between 1 - 5 p.p.m.

When 1200 cm³ of swimming pool water was filtered, the mass of the filter paper, upon drying, had increased by 0.78 g. When 250 cm³ of the filtered water was evaporated to dryness the mass of the residue obtained was 0.32 g. Calculate the concentration in p.p.m.

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