Water experiments

To determine Total Suspended Solids (in p.p.m.) in a Water Sample via Filtration. To determine Total Dissolved Solids (in p.p.m.) in a Water Sample via Evaporation. To Measure the Amount of Free Chlorine in Swimming Pool Water Using a Comparator / Colorimeter

Describe how the concentration of suspended solids in Sample A of swimming pool water could have been measured

Known volume of water sample //

through filter paper //

of known mass //

dry in an oven //

find new mass of filter paper and residue

Calculation - 2015 Q3 part b)

[ remember 1p.p.m. = 1mg / L ]

What solution was titrated against a known volume of Sample C (swimming pool water) to determine its total hardness?

EDTA / ethylenediamenetetraacetic acid

When some of Sample C was boiled, filtered, and re-analysed for hardness, the result was 175 p.p.pm.

Suggest a reason why the second result was significantly lower than the first, 220 p.p.m.?

Temporary hardness removed / only permanent hardness remaining

Identify a reagent used to detect free chlorine in a water sample

DPD (diethylphenylenediamine) /

KI (potassium iodide) and H⁺

Describe how a comparator or a colorimeter could have been used to estimate the concentration of free chlorine in Sample D of swimming pool water

Comparator

Add reagent (DPD) /

stir (dissolve, mix) /

compare colour with chart /

read concentration (ppm)

Colorimeter

Prepare standard solutions of reagent /

note colorimeter readings /

plot graph of concentration versus reading (absorbance) /

read concentration (p.p.m.)

Which of the test results led to pools A and D being considered unsuitable for swimming?

Free chlorine

What problem might arise for swimmers in

i) pool A.

ii) pool D?

i) Infection / microorganisms

ii) taste / smell / nausea / eye (skin) irritation / skin burning / breathing problems

What is the general principle of all colorimetric experiments?

Intensity of colour / absorbance / transmittance //

proportional to concentration

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

KI (potassium iodide) → brown / red /

DPD tablet → pink

Give the name or formula of a free chlorine species in swimming pool water.

Chlorine (Cl₂, dichlorine)

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.

Greater concentration needed to kill pathogens added by swimmers /

drinking water has much fewer pathogens /

drinking water is less contaminated

swimming pool water more contaminated

would be dangerous to drink / bad taste

Calculation 2006 Q3 part e)