To carry out a titration between a strong acid and a weak alkali, to eventually find the unknown concentration of sulphuric acid. However, the sodium carbonate (weak alkali) is solid, and you need two liquids to carry out a titration.

Finding out How Much Acid There is in a Solution

Planning: -

Introduction: -

During the extraction of a metal from its ore, sulphur dioxide is often produced. This is converted to sulphuric (VI) acid and is sold as a useful by-product. I am going to be given a sample of sulphuric acid which is thought to have a concentration between 0.05 and 0.15 mol dm3. I have to find out its accurate concentration by carrying out a titration between sulphuric acid and sodium carbonate. It is an acid-alkali titration. A titration is the “determination of material concentration by the successive addition of measured amounts of standard reagents to a known volume or weight of solution until a desired end point is reached.” Acids and alkalis are classified as “strong” or “weak” depending on the extent to which they form ions when dissolved in water. Sulphuric acid is a strong acid which means it is completely in the form of ions in dilute solution. Sodium carbonate is a weak alkali, which means the process is only partially complete. Methyl orange is the best indicator to use to find the end point of a titration between a strong acid and a weak alkali.

Na2CO3 (aq) + H2SO4 (aq) Na2SO4 (aq) + CO2 (g)

One mole of sodium carbonate is needed to neutralise one mole of sulphuric acid. It has a 1:1 ratio.

Aim: -

To carry out a titration between a strong acid and a weak alkali, to eventually find the unknown concentration of sulphuric acid. However, the sodium carbonate (weak alkali) is solid, and you need two liquids to carry out a titration. This means I will make the sodium carbonate into a solution in order to carry out the titration, by dissolving it in distilled water. The concentration of the sodium carbonate solution is 0.1 mol dm-3

Apparatus: -

Spatula (to measure out the sodium carbonate)
Balance (to weigh the sodium carbonate accurately to 2.65g)
Watch glass (to place the sodium carbonate upon)
250cm3 beaker (to dissolve the sodium carbonate)
250cm3 graduated volumetric flask (to make up the sodium carbonate solution once it’s dissolved)
Distilled water (put in a minimum to make the solvent dissolve. Use distilled water to wash out the watch glass and beaker, to ensure all the sodium carbonate gets transferred and the solution is accurate)
Burette
Sulphuric acid (the amount needed will be determined by doing a rough titration to see how much sulphuric acid is needed).
Pipette filler and pipette (to transfer sodium carbonate solution to a conical flask)
3 drops of Methyl orange (to show colour change-end point of titration)
White tile (to show colour change clearly for more accuracy so we can determine the exact point the titration ends. It will change from green to clear.)
2.65g of sodium carbonate (this will be turned into a solution by adding 250cm3 of distilled water).The concentration of the sodium carbonate solution: 0.1 mol dm-3
A small pipette to accurately add the distilled water to the graduated volumetric flask, and to accurately add the sulphuric acid to the burette to zero.

Control variables: the sodium carbonate solution that is made up will be kept in the same volumetric flask and labelled with my name. The same sulphuric acid batch will also be used as there is no guarantee the concentration of another batch is identical. The temperature will also be kept the same as the titrations will be carried out on the same day. If the temperature is increased, the liquid will expand and it will make the test inaccurate.

Method: -

Wash out the watch glass, graduated volumetric flask and the 250cm3 beaker with distilled water to get rid of any contamination. You must rinse out every piece of apparatus you use, especially glassware as it may be contaminated from previous experiments, and to ensure all solids get transferred properly.
Put the watch glass on the balance and record its weight.
Re-zero the balance and weigh out 2.65g of anhydrous sodium carbonate using a spatula.
Transfer this to the 250cm3 beaker and wash any remaining powder on the watch glass into the beaker with distilled water to ensure all the sodium carbonate gets transferred, making sure the concentration of sodium carbonate is accurate.
Dissolve the sodium carbonate using the minimum amount of distilled water (less than 100cm3). Stir it with the glass rod in the 250cm3 beaker until it has dissolved.
Transfer the dissolved sodium carbonate to a volumetric flask using a funnel. Wash out any remains on the funnel using distilled water to ensure accuracy of the sodium carbonate solution.
Add distilled water to the volumetric flask until the solution is close to the graduation mark. Then use a pipette to make sure the bottom of the meniscus of the solution sits on the graduation mark. This ensures that the concentration of the sodium carbonate solution is accurate.
Show it to Miss Hyde and label it to make sure it doesn’t get mixed up with anyone else’s, making it a fair test. Then invert the volumetric flask (obviously with the lid on) and shake it to mix the solution thoroughly.
Set up a burette and place it on the stool to ensure it is at eye level (so you can accurately read off the amount of sulphuric acid in the burette). Make sure you wash it out with distilled water first to ensure there is no contamination. Place a beaker underneath the burette and place a funnel at the top. Fill it with sulphuric acid solution using the funnel to prevent sulphuric acid solution from splashing into your eye. The beaker underneath it is there in case any sulphuric acid solution falls out (as the stopcock may be left vertical instead of horizontal). Fill up the burette to zero exactly. Use a pipette as you get closer to zero to ensure the bottom of the meniscus is on zero. This is extremely important as you will not know the amount of acid that went into the conical flask to turn the sodium carbonate solution clear. Your results will be inaccurate if the sulphuric acid solution is above or below zero.
Use a pipette filler and pipette to transfer 25cm3 of the sodium carbonate solution from the 250cm3 graduated volumetric flask to the 250cm3 conical flask. Hold the pipette at the top, because if you hold the bulb, it heats it up and may cause the liquid to expand, leading to inaccuracies. Remember to wash out the pipette using distilled water. Place the pipette in the sodium carbonate solution and put the pipette filler on top. Do not hold the pipette above the liquid as it sucks air back up. Fill up the pipette above the graduation mark and then take it out of the volumetric flask. Let out a bit if it is still above the graduation mark, to make sure the bottom of the meniscus is exactly on the graduation mark and transfer it to the conical flask. As the volume of the pipette is greater than the pipette filler, you will have to keep removing it and resetting it, then replacing it back on the pipette.
Add 3 drops of Methyl orange solution to the conical flask. The solution will turn green. This indicator is used to indicate the end point of the titration because it is the best indicator to use during a titration between a strong acid and a weak alkali. Place this conical flask under the burette. Place a white tile underneath the conical flask to see the colour change. I’m using three drops so that the green colour is darker and more concentrated, so that when it starts turning lilac/clear, it will show up more.
Add the sulphuric acid solution from the burette to the conical flask by turning the stopcock on the burette vertical. Keep the flow of acid into the conical flask rapid, until the solution starts turning lilac at the centre, as it is more concentrated under the tap. Then turn the stopcock slightly horizontal until the sulphuric acid comes out drop by drop extremely slowly. Start swirling the conical flask every time a drop of sulphuric acid is added. When the conical flask is swirled and the colour doesn’t go back to green, and remains clear, it is the end point of the titration.
Read off the amount of sulphuric acid on the burette and record it. Ensure it is at eye level, and read the number the bottom of the meniscus is sitting on. This will allow you to find an approximate value for the volume of sulphuric acid needed to react with the sodium carbonate solution.
Carry out the titrations until you have recorded three results within 0.1cm3 of each other. Obviously use the same labelled sodium carbonate solution and sulphuric acid batch to make your results fair, accurate and reliable. This means you have achieved results that will allow you to find the accurate concentration of sulphuric acid.

Risk Assessment: -

Wear safety glasses to prevent sulphuric acid and other chemicals splashing into your eyes.

Wear a lab coat as methyl orange stains and you need to protect your clothing in case other chemicals fall onto them.
Stools must be tucked underneath the table and bags put away to ensure you don’t trip and fall.
Loose objects must be removed/tied back (i.e. Hair) as it can interfere with the experiment and loose objects can hook onto apparatus and tip them over.
Don’t sit down as you will not be able to move out of the way ...

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Risk Assessment: -

Wear safety glasses to prevent sulphuric acid and other chemicals splashing into your eyes.

Wear a lab coat as methyl orange stains and you need to protect your clothing in case other chemicals fall onto them.
Stools must be tucked underneath the table and bags put away to ensure you don’t trip and fall.
Loose objects must be removed/tied back (i.e. Hair) as it can interfere with the experiment and loose objects can hook onto apparatus and tip them over.
Don’t sit down as you will not be able to move out of the way if anything spills, and it makes it inaccurate if the burette isn’t at eye level.
Burette is put on the stool to make sure it is at eye level and a container is put underneath it to catch any sulphuric acid, so that it doesn’t spill out.
Use a funnel when pouring in acid to prevent it splashing in your eye.
Rinse hands to make sure the chemicals are removed, so that you don’t touch your face or mouth and harm yourself.
Refrain from chewing pens as chemicals may be on them from the table.
Wipe away immediately if anything is spilt as it is dangerous if skin comes into contact with the chemicals.
If glass is broken, inform the teacher. Obviously broken glass is dangerous and you could cut yourself.
Running is prohibited in the laboratory to prevent injury, as you could fall over or hit something as you run past.

The plan I have devised is likely to produce accurate and reliable results because I am washing out all my glassware to prevent contamination. My solution concentration will be accurate because I am using electronic equipment to determine the mass of sodium carbonate and I am washing all of it into the solution to make up an accurate solution.

Any errors in my experiment are fixed errors in the measuring devices e.g. Balance, beaker, burette, pipette. As all measuring devices contain fixed errors, and the same errors will be present throughout the experiment, this fixed error is cancelled out. These errors cannot be removed, unless specialist equipment is used.

My results will be reliable because I am repeating the titrations several times until I produce three results that are within 0.1cm3 of each other. This will allow me to accurately determine the concentration of sulphuric acid.

I will try to keep the temperature constant by not touching the containers the liquids are in. This means my body heat will not increase the temperature and expand the liquid, although this will only happen in extreme cases, it still minimises error in my experiment.

I will also carry out a rough titration first to work out the amount of sulphuric acid needed to react with the sodium carbonate solution. This helps make the actual titration much faster and more accurate and reliable, as I will know the exact amount of acid to use, and I will roughly know when to slow down the addition of sulphuric acid to the sodium carbonate solution to a drop.

I am using the best indicator for reactions between strong acids and weak alkalis. This means the end point of my titration will be determined accurately, so that I can correctly work out the concentration of sulphuric acid.

In devising my plan, I checked against another plan from the “student support coursework guidance” sheet, and my plan seems reasonable. I think it can be relied upon to produce accurate and reliable results.

Implementing: -

Analysing Evidence and Drawing Conclusions: -

Moles = concentration x (volume/1000)

What is the concentration of 250cm3 solution containing 2.65g of Na2CO3?

Na2CO3 (aq) + H2SO4 (aq) Na2SO4 (aq) + CO2 (g)

1 mole of Na2CO3 neutralises 1 mole of H2SO4

1:1

Number of moles of sodium carbonate = Mass/RFM

=2.65g/106

=0.025

Concentration of sodium carbonate = moles x (1000/volume)

=0.025 x (1000/250cm3)

=0.1M

Number of moles of sodium carbonate = concentration x (volume/1000)

= 0.1 x (25/1000)

= 0.0025

What is the concentration of 28.43ml of H2SO4?

Na2CO3 (aq) + H2SO4 (aq) Na2SO4 (aq) + CO2 (g)

1 mole of Na2CO3 neutralises 1 mole of H2SO4

1:1

I have already worked out the number of moles of sodium carbonate, and as there is a 1:1 ratio, sulphuric acid is the same number of moles.

Therefore, 0.0025 moles of sulphuric acid = concentration x (25/1000)

Concentration of sulphuric acid = 0.0025 x (1000/25)

= 0.1M

Conclusion:

I can see from my results that the sodium carbonate solution and the sulphuric acid solution have the same concentration of 0.1M. This is why only one mole of sodium carbonate solution is needed to neutralise one mole of sulphuric acid.

My first titration is an anomalous result because it did not require as much sulphuric acid to neutralise the sodium carbonate as the rest of my titrations did. This may have been because I did not add 3 drops exactly. This means the green colour of the indicator (methyl orange) was less concentrated and required less acid to make it become clear.

For my rough titration and my fourth titration, I used three drops. However, the bottle was extremely difficult to squeeze and the drops were quite big compared to my second and third titrations. That is why they required more acid, as the green colour was more concentrated, meaning more acid was needed to make it turn clear and transparent.

I think my second, third and fifth titrations are extremely accurate and precise as they are within 0.1cm3 of each other, and I must have followed my procedure extremely accurately. Both of those times, I did get the solution to turn clear at the exact point, and there was no purplish tinge to it.

Evaluating Evidence and Procedures: -

From looking at my results, I can identify a slightly anomalous result: 27.6cm3. Especially as I carried out that titration straight after my pilot experiment, to determine the approximate value for the volume of sulphuric acid I would need, which was 29.6cm3. However, I did put in less drops of methyl orange by accident, and that could have contributed to less acid being needed to neutralise the sodium carbonate. As the green was lighter, it was easier to make it turn clear, with less acid. Human error is a big factor contributing to the inaccuracy of my measurements. However, it was extremely difficult to put in exactly 3 drops of methyl orange, as the bottle was hard to squeeze. I do not know whether this affects the accuracy of the experiment or not, but from looking at my results, it is the most likely explanation.

My measurements were quite precise and reliable. When I weighed out the sodium carbonate using the electronic balance, I used a spatula to add or remove sodium carbonate, to ensure it was exactly 2.65g. When I was using the graduated volumetric flask, it is difficult to pour in distilled water and get it exactly on the graduation mark. To make my measurements more precise, I used a pipette as I got closer to the graduation mark, as it is easier to put in distilled water drop by drop. There is more accuracy in this procedure, and the graduated volumetric flask was at eye level, so that I knew when the meniscus was sitting on the graduation mark. I also used a pipette to make sure the sulphuric acid was exactly on zero in the burette. The pipette and pipette filler was extremely accurate and precise because it is designed to release exactly 25cm3 of liquid, so I know that there was exactly 25cm3 of sodium carbonate in the 250cm3 conical flask. My results are quite reliable because I repeated them many times, and three of the results are within 0.1ml of each other. The concentration of the sodium carbonate solution was accurate because I used distilled water to wash all of the powder from the watch glass into the beaker, and all of the solution from the funnel into the graduated volumetric flask. I can work out the percentage error for my measurements, using this formula:

Formula: True Value - Experimental Value x 100
True Value

I can work out the percentage error for the concentration of my sulphuric acid solution. We are already told it is in between 0.05 and 0.15 mol dm-3. I will take the value that is in between these two numbers, which is 0.08, to be the true value.

0.08 – 0.1 x 100 = 25%
0.08

I can also work out the percentage errors for measuring devices.

These percentage errors will not have a very significant effect on the outcome of my results, apart from the transferring of the sodium carbonate solution using the pipette. This means the sulphuric acid solution is not neutralising exactly 25cm3 and it means the concentration of the sulphuric acid solution will be inaccurate. The other measuring devices contain hardly any error, just below 1%. This is hardly high enough to have a detrimental effect on my results.

I kept the temperature constant to minimise errors, as heat will cause the liquid to expand and the concentration will not be accurate. I did this by not touching glassware where the liquid is present as heat from my hand will cause it to gain energy to expand.

There were many fixed errors in all the measuring devices, such as the balance, burette, beaker and pipette, but as they were fixed and present in the experiment throughout it, the experiment is fair as all titrations are subjected to the same errors. So the fixed errors do not affect my results too much. The main source of error in the procedure was the end point of the titration. It was extremely difficult to get the stopcock to let out the sulphuric acid drop by drop and record the end point as soon as it turned clear. Sometimes too many drops went in, and it turned slightly lilac. This means my titration results are not accurate enough, and there is actually too much acid being used to neutralise the sodium carbonate solution. Human error was also a major source, as reading off the burette may not have been accurate, due to it not being at eye level. This also affects the results and my ability to calculate the concentration of sulphuric acid. It is impossible to accurately weigh exactly 2.65g of sodium carbonate because the balance is only accurate to two decimal places. This means the acid was not neutralising one mole of sodium carbonate solution, as I may not have used exactly 2.65g. Some of the sodium carbonate powder may also have remained on the watch glass although I made sure all of it was washed into the beaker. The sodium carbonate solution may have remained in the beaker and the funnel when I was transferring it to the graduated volumetric flask, although this is also unlikely as I washed it with distilled water thoroughly to ensure it all got transferred. This affects the accuracy of the sodium carbonate concentration; therefore my results may not be accurate. When I was dissolving the powder in the distilled water, I may not have done it accurately, and some powder might have remained.

Overall, my evidence is quite accurate, as I did manage to get the sodium carbonate solution clear three times, and I have results that are within 0.1ml of each other. They are reliable results because I repeated them and the anomalies that occurred were re-investigated. Any errors present were either fixed or human errors, and there is little we can do about that, apart from use specialist equipment and be conscientious about performing the experiment and do it slowly and accurately. To solve the problem of getting the solution clear exactly, we will have to let the solution drip out of the tap drop by drop all the time. This is however, an extremely slow process.

To improve my procedure further, I think I should have checked the burette for air bubbles and leaks. Air bubbles in the burette could cause inaccuracy, as I will not be able to measure the exact amount of sulphuric acid going into the conical flask. If my burette leaks, then not all of the acid is going into the conical flask and I will not be able to work out the accurate concentration. To get rid of air bubbles in the tip of the burette, I have to let the sulphuric acid run through by turning the stopcock and then whacking the side of the burette top while the acid is flowing. This will prevent volume readings from being incorrect and I will be able to correctly work out the concentration of the sulphuric acid solution I used.

To accurately take readings, I could use a burette reading card with a black rectangle.

This makes it easier to take readings, and combined with the burette being at eye level, I should be able to take more accurate readings, providing more reliable results to work out the concentration of sulphuric acid.

To ensure that all of the sodium carbonate dissolves in the distilled water; I could use a magnetic stirrer, so that the concentration of the sodium carbonate solution is accurate, as all of it will be dissolved. I could also use the magnetic stirrer when I add the indicator and when the sulphuric acid solution is being delivered through the burette. This will help me determine the end point more accurately, if the solution is constantly being stirred. I can see exactly when the solution does not change back to the indicator colour (green) and remains clear. This will help me determine the exact amount of acid I used to neutralise the sodium carbonate solution, so that I can accurately work out its concentration.

To make my experiment even more accurate, I could use an analytical balance, which measures substances within 0.0001g, so they are good to use when a high degree of precision is needed. In this experiment I used a top loading balance, which I don’t think was adequate for the degree of accuracy required. This will ensure that my sodium carbonate solution is accurate, and will mean I can work out the concentration of sulphuric acid more accurately. However, this may be unnecessary for what I am doing, as the percentage error for the balance wasn’t that high, and did not have a very significant impact on my results.

A titration using a pH meter could be carried out, which is much more accurate, as the pH meter electrode is inserted in the solution, and I can read the pH and handle the burette with ease. It is better than a colour change because human error is not involved; it is all electronic and therefore more accurate. It will then be easier to work out the concentration of the sulphuric acid solution. I think I should have used a different bottle of methyl orange because the one I used was difficult to squeeze and the drops came out different sizes, leading to inaccuracies in the concentration of the green colour of my sodium carbonate solution. More acid is required to make a darker green turn clear, then a light green, so this is an extremely important change to improve my procedure.

Other than that, I think I had an extremely accurate and reliable procedure as I kept the temperature constant, therefore liquid expansion was prevented, and volume readings contained no errors. Fixed errors in the measuring devices were cancelled out by using the same instruments. Some of my glassware was extremely accurate, such as the volumetric flask, and I was able to make an accurate sodium carbonate solution. The pipette was also extremely accurate as it delivered exactly 25cm3 of sodium carbonate solution into the conical flask. I also washed out all my glassware to ensure there was no contamination, and so that all the sodium carbonate was transferred properly to get an accurate solution concentration. I also shook my sodium carbonate solution after the final dilution to mix it thoroughly. I took adequate safety precautions to ensure my procedure was safe as well as accurate and reliable.

Aisha Hussain Page