Estimation of Chlorine in Household Bleach.

Danny Basil Kannan Vincent

Estimation of Chlorine in Household Bleach

In this investigation, I am looking to determine the amount of chlorine present in household bleach. However, in order to do this I would have to calculate the mass represented by chlorine in the mass of the household bleach. After doing that, I would then have to represent this as a percentage.

In order to calculate the mass represented by the chlorine in the mass of the household bleach, I would have to titrate this solution, along with an excess of potassium iodide and ethanoic acid against sodium thiosulphate from the burette using starch solution as an indicator. The reason for this choice of indicator is that iodine is turned blue black by the starch solution, and this therefore helps to indicate any colour change that occurs.

I began my investigation by measuring out 10mls of household bleach using a measuring cylinder. I poured this into a volumetric flask, and used distilled water to ensure that all 10mls of the bleach were in the volumetric flask and not in the measuring cylinder. This was then topped up to 250ml using distilled water in the volumetric flask. I had to make sure that the bleach solution is mixed completely and shook the flask from top to bottom. This results in air bubbles forming along with some froth, meaning I had to leave the flask to settle.

While I was leaving the bleach to settle, I must transfer 30 cm3 of sodium thiosulphate from its beaker to the burette. Before doing that, I had washed the burette with a little bit of sodium thiosulphate, which would allow the solution to run smoothly into the tip of the burette. Having done that, I would need to see where the bottom of the meniscus lies, i.e. V1, unless it lies on zero, and then record this value. Also a funnel should be used to ensure that all 30 cm3 of sodium thiosulphate is in the burette.

Back to the bleach, where I would have to transfer 25mls of the bleach from the volumetric flask to a clean conical flask. To do this, I would have to use a pipette-filler to extract 25mls of bleach from the flask to the conical flask. At this stage, I would need to ensure that the burette is clamped with the side showing the measurements facing me; the conical flask is placed underneath the tap of the burette and is placed on a white tile.

To this, I must add 30 mls of potassium iodide and ethanoic acid. ...

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To this, I must add 30 mls of potassium iodide and ethanoic acid. Potassium iodide is used to give us the amount of chlorine produced, (i.e. the mass of the chlorine in the bleach) and ethanoic acid is used to ensure that the experiment is carried out in acidic conditions, which would suit the starch solution. Two or three drops must be added, as this is only an indicator.

The investigation begins as my partner, allowing the sodium thiosulphate to enter the conical flask dropwise, opens the burette tap. As this investigation was carried out in pairs, I did the swirling. This must be done until the indicator changes colour completely. This is known as the ‘end-point’ of the titration. In this case, the colour of the solution would change from blue-back to cream/colourless. After stopping the tap, I would have to read the burette again (V2). Then I would have to subtract this from the previous reading of the bottom of the meniscus (V1) to give the volume of sodium thiosulphate required to react with the ethanoic acid.

This titration should be repeated, so an average titre is obtained. From this volume, we can now determine the percentage of chlorine present in household bleach. My results are shown in the table below: -

During the experiment, the bleach liberates chlorine readily when reacting with the ethanoic acid: - ClO-(aq) + 2H+ (aq) + Cl- (aq) → Cl2 (aq) + H2O (l) (1).

The amount of chlorine present in the household bleach, which contains Sodium Chlorate (I) can be found by allowing the bleach to react with an iodide solution to react with an iodide solution to form, and then titrating with thiosulphate solution: -

ClO- (aq) + 2H+ (aq) + 2I- (aq) → I2 (aq) + Cl- (aq) + H2O (l) (2).

2S2O32- (aq) + I2 (aq) → S4O62- (aq) + 2I- (aq) (3)

Now I can begin to work out the percentage of chlorine in household bleach.

I know that the concentration of the sodium thiosulphate is 0.100M. This means I can calculate the number of moles present. From the investigation, I know that 20.1cm3 of sodium thiosulphate is required to neutralise the ethanoic acid.

Therefore,

This shows that there are 2.01 X 10-3 moles of sodium thiosulphate. 2 moles of thiosulphate will react with 1 mole of iodine, meaning that 2.01 X 10-3 moles of sodium thiosulphate would react with [0.5 X (2.01 X 10-3)] moles of iodine. The fact that there were 1.005 X 10-3 moles of iodine in reaction (3) means that the same amount of moles was present in reaction (2). This means that the same amount of moles of ClO- and Cl2. In the calculations below, you will see that I have multiplied the first answers I received by ten. The reason for this is that the mass at first is low due to the fact that this is a sample transferred from the volumetric flask at first as a 25ml sample. To get the number of grams present in the 250ml, I have multiply by ten. The number of moles in a 25ml sample will be ten times smaller than the number of moles present in a 250ml sample. Therefore this means that the mass of chlorate and chlorine present in a 25ml sample will be ten times smaller than the mass of chlorate and chlorine present in a 250ml sample.

Now I have got the original masses of the chlorine and the chlorate, so therefore I can calculate the available chlorine in the bleach.

EVALUATION

During this investigation, there were many improvements that could have been made to the investigation. These improvements could include more co-ordination between my partner and I, which had resulted in the first attempt of the back titration, where I had closed the tap a bit late due to a late call from my partner to call ‘stop’ when the end-point had been reached.

Normally, the amount of sodium thiosulphate solution required to neutralise the amount of ethanoic acid is the same, but in the case of my investigation, they differ by quite a large range. They differ by 1.1, which is quite big, but the gap is not big enough to affect the results I get.

Despite that, no anomalies had occurred throughout the investigation. Possible changes to how investigation was carried out could be that the attempts could have been done separately. My investigation was carried straight after one another, as a result of a lack of sodium thiosulphate, after washing the vessels, which was needed to prevent any interference from the previous attempt. This could have led to more accurate results and better reading of the meniscus. My partner and I had completed the investigation quickly meaning that the investigation lacked some accuracy. So to change this, I would have preferred to complete the investigation with a bit more haste. Also to improve my investigation, I could have used more technologically advanced equipment, but unfortunately these were unavailable.