Determination of Chlorine Level in Water

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BSc Pharmaceutical Technology Practical - No 5

Preparation of a Primary Standard Solution

Aim:to prepare a standard solution of pure silver nitrate and use it to determine the concentration of chloride ions in a sample of tap water and another sample of bottled water.

Chemicals:

* High grade purity silver nitrate
* Potassium chromate indicator
* Tap water
* Bottled water
* Distilled water

Apparatus:

* Laboratory oven
* Dessicator
* Conical flask
* Watch glass
* Spatula
* Glass rod
* Burette
* Funnel
* Volumetric flask
* Volumetric pipette

Setup:(see diagram under Discussion)

Procedure:

1).Weigh accurately approximately 4.5g of pure silver nitrate in powder form. 2).Place the watch glass with the powder in the drying laboratory oven for 30 minutes at a high temperature (60 – 100oC). 3).Take the sample out of the oven and place in the dessicator for about 10 minutes to cool down in dry conditions. 4).Weigh the powder once more to see the mass of actual silver nitrate now that it has lost the water of crystallization. 5).Wash the crystals into the volumetric flask using distilled water. Fill the flask with distilled water up to the 250ml mark shaking the solution well. 6).Place some of the solution of silver nitrate in the burette up to a suitable mark and note down. 7).Measure accurately 25ml of tap water and place in the conical using a volumetric pipette. Add three drops of potassium chromate indicator. 8).Place the conical flask underneath the burette and titrate the sample of water with the silver nitrate solution. Stop the titration when the end point (a slightly reddish colour) appears in the flask. 9).Repeat steps 7). and 8). until you have a rough estimate and 3 precise and accurate titre values. 10).Tabulate the results and use them to find the average titre value for this titration and so work out the concentration of chloride ions in the tap water being analysed. 11).Repeat steps 6). to 10). this time using bottled water instead of tap water.

Precautions:

i).Make sure all silver nitrate powder has been washed off the watch glass into the volumetric flask in order to reduce transfer losses and thus errors in the calculations. ii).Before performing the titration, rinse the volumetric flask and burette with the solutions that they will contain so as to reduce contaminations and volumetric differences. iii).Perform the titration very slowly and carefully since the chloride ion concentrations are expected to be very small and thus the endpoint can be overshot easily.

Errors:

i).The laboratory oven was not prepared beforehand and preheated, and so the temperature reached was not the ideal on since it would have taken too long to reach optimum temperatures. ii).One cannot be certain of the purity of the silver nitrate powder being used, particularly if the batch is not a freshly prepared one. Also exposure to air and environment during weighing may be a source of new contaminants.

Results:

AgNO3 + Cl- AgCl + NO3-

Mr of AgNO3 = 170gmol-1

Initial weights

Weight of watch glass=19.55g
Weight of watch glass + crystals=24.07g
Weight of crystals=4.52g

After drying and dessication

Weight of watch glass + crystals=24.06g
Final mass 0f crystals=4.51g
Mass of water of crystallization lost =24.07- 24.06=0.01g No. of moles of silver nitrate =4.51 / 170=0.0265mol
Molarity =0.0265 / 250 x 1000=0.106M

For tap water:

| Approximate| 1st Titration| 2nd Titration| 3rd Titration| Initial volume| 0cm3| 1cm3| 2cm3| 3cm3|
Final volume| 1cm3| 2cm3| 3cm3| 4cm3|
Titre value| 1cm3| 1cm3| 1cm3| 1cm3|

Average titre value=1cm3

0.106 moles of AgNO3in250cm3
? moles of AgNO3in1cm3
0.106 / 250 x 1 =4.245 x 10-4 moles of AgNO3

Ratio of no. of moles Ag+:no. of moles Cl-
Is1:...
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