- Insert the stopper and shake the mixture several times. Not mixing the solution very well can cause common faults.
- Pipette 10 cm of the sodium carbonate solution to a clean conical flask.
-
Add three drops of the methyl orange indicator using a small dropping pipette into the solution in the conical flask. This ensures that the same amount of indicator is used for each solution, this then gives a constant environment and ensures that all titration end points are based on the same amount of indicator. Methyl Orange produces a yellowish colour in alkalis and a distinctly red colour in acids. This indicator is very useful as the change in colour could be easily identified when the titrations are taking place.
The following shows the equipment to be used in the actual titration:
- Tripod stand
- White Tile
- 2 Clamps
- Conical Flask
- Burette
- Distilled Water
- Dropping Pipette
- Sulphuric Acid
- Filter Funnel
- Set up the stand and burette which should be held firmly in positions by the clamp.
-
A funnel will then be placed at the top of the burette and the valve set so that no solution can flow through. The burette and funnel should be set up so that the funnel is below eye level for safety and practical reasons. Fill up the 50 cm burette with sulphuric acid. Air bubbles should be avoided. Ensure that the bottom of the meniscus line reaches the 0 mark at eye level. Remove the funnel after use as otherwise more solution may drip into the burette from the funnel and an accurate volume would not be recorded. The burette must be filled with 50 cm of sulphuric acid to the 0 cm mark so that the meniscus is just touching the graduation mark.
- The conical flask should be placed on a white tile. This makes it clearer to see the colour change of the indicator as the colour of the laboratory bench may interfere with the colour of the indicator and the exact end-point would be difficult to determine.
- Perform a rough titration by running the sulphuric acid whilst also swirling the flask of alkali until the indicator turns a pink colour. Record the rough value of volume of acid used to neutralize the acid and alkali with each other as this is the end point.
- Perform a second titration, this time run the acid until it reaches 2 cm less than the rough value. From this time add only drops, keeping a close eye on the colour of the solution.
- Continue to do the titration until two readings are received within 1 cm of each other. Work out the averages and record this in the table.
The Table that you will record the results on should look similar to the one below. Using this form of table should allow me to provide a clear and concise way of representing my results and should aid me in my analyzing and evaluating section of this investigation.
Risk Assessment
Sodium Carbonate
The sodium carbonate is very dilute, so there will be little harm. Normal laboratory precautions need to be applied. Avoid contact with eyes, skin and clothing. It is also irritating to the respiratory system. Make sure all spillages are attended to.
Methyl Orange Indicator
Methyl orange is also dilute therefore it will not be harmful. Use the same laboratory precautions as the sodium carbonate.
Sulphuric Acid
This is colourless and viscous, reacting vigorously with water. Higher concentrations are more reactive. The concentration I am using is between 0.05 mol dm and 0.15 mol dm, therefore it is not very harmful. Prevent contact with skin and eyes. Clean up any spillages. The dilute acid irritates eyes and may cause burns.
General Safety
Care must be taken at all times. Safety Goggles and overalls must always be worn.
If any substance is swallowed medical attention should be sought as soon as possible. If any substance splashes in the eye the eye should be rinsed for 10 minutes and medical attention should be sought. If spilt on skin wash area with excess water and remove contaminated clothing. If a large area is affected or blistering occurs seek medical attention.
Explanations
The reason I am using the following equipment are:
Volumetric Flask: This is useful as it measures the solution to a high degree of accuracy. It can be easily shaken when the stopper is placed on to the mouth of flask. On the other a hand, a beaker cannot measure solutions accurately and when shaken, there is a likeliness, the solution may spill.
White Tile: The white tile is used so that we can identify the change of colour clearly.
Dropping Pipette: This is used I order to obtain a correct measurement of the solution by adding minor drops to reach the bottom of the meniscus line.
Distilled Water: As the conical flask has a certain number of moles measured into it rinsing it with the solution would increase the number of moles in the solution. Therefore distilled water must be used.
My plan for the titration should provide precise and reliable results if all chemicals are weighed and measured to the correct mass or volume. The experiment will be repeated until 3 titres that agree to within 0.1cm3 of each other are recorded. The average of these 3 results will then be taken for use in later calculations. This will provide an accurate result and will rule out any anomalous results that might have affected the experiment.
Reference:
Chemical Ideas – Advanced Salters Chemistry
Hazcards – School science services (card 98)
Hazcards – School science services (card 95)
Making Standard Solution - Margaret Ferguson
Finding out how much acid there is in a solution – Analysing evidence and drawing conclusions assessment
Results
Using only the concordant results (0.1cm³ apart), I found the average titre:
39.25 +
39.30 +
39.20 = 117.75
3
= 39.25
Average titre = 39.25cm³
(a) Calculation of the concentration of sodium carbonate solution used
2.65g of solid anhydrous sodium carbonate was used.
To work out the concentration, we use the following equation.
No of moles = concentration × volume (equation 1)
This shows the relationship between concentration (c) in mol dm-³, the amount in moles (n) and the volume of solution (v) in dm³.
In order to work out the number of moles, we use the following equation.
No of moles = mass (equation 2)
Molar mass
The molar mass of Na2CO3 = (2 × 3) + (12) + (3 × 16)
= 106g
No of moles = 2.65g
106g
= 0.0250 moles (3sf)
This is combined with 250 cm³ water, we can convert this to dm³ as 1000cm³ = 1dm³.
So,
250cm³ = 0.250dm³ (3sf)
1000
We then rearrange equation 1 as shown below and plot in the known amounts to receive the concentration.
No of moles = Concentration
Volume (dm³)
So, the concentration in moldm-³ is:
0.0250 = 0.100 moldm-³ (3sf)
0.250
Concentration of sodium carbonate = 0.100 moldm-³ (3sf)
(b) Concentration of acid solution
We are already aware that the concentration of the acid solution is between 0.05 and 0.15 mol/dm³. The equation for the neutralization of sulphuric acid with sodium carbonate is as follows:
H2SO4 (aq) + Na2CO3 (aq) Na2SO3 (aq) + H2O (l) + CO2 (g)
This shows that the mole ratio of sulphuric acid : sodium carbonate is 1 : 1. Therefore, we can say that
The average concordant results of sulphuric acid used are:
39.25 +
39.30 +
39.20 = 117.75
3
= 39.25
If the concentration of Na2CO3 is 0.100 dm³, we can now work out the moles of H2SO4 using equation 1. This is because the ratio of Na2CO3 and H2SO4 is 1:1, therefore they both have equal volumes of 0.1 dm³, and hence, we can now use this in equation 1 to find the number of moles of H2SO4.
Moles of H2SO4 = 0.100 dm³
0.025
Moles of H2SO4 = 0.100 × 0.025
= 0.0025 moles.
We convert the average titre to dm³ before inserting it into the equation:
39.25 = 0.03925
1000
Using equation 1 once again, we divide the number of moles by the volume to receive the concentration of acid solution in mol/dm³.
0.0025
0.03925
= 0.06370 moldm³ (3sf)
Concentration of sulphuric acid = 0.06370 moldm³ (3sf)
Evaluation
From the results in the titration, the calculations suggest that the concentration of sulphuric acid is 0.0637mol/dm³. Despite the value not being precisely accurate, it seems to be in agreement with the initial statement that the acid is thought to have a concentration between ‘0.05 and 0.15moldm³’. This clearly suggests that the investigation results are quite satisfactory, as they are in the expected range. I carried out the experiment according to the method and I resulted with six different titres of which three were concordant (within 0.1cm³ apart). I also included the rough titre in my average as I believed that the way I did the experiment for the rough titre and the result found was similar to the actual titres. Also, the more experiments I did, the more reliable my outcomes would be. However, as with most investigations, there are bound to be some errors during the experiment.
Precision and accuracy are very important when performing experiments such as finding the concentration of a substance. Precision is how accurately something is measured and accuracy is how close to the real value the outcome is. An experiment is never 100% precise, the features that would affect precision are precise readings, equipment errors (glassware causes the most errors), etc. On the hand accuracy is mainly due to human errors, for example when reading scales or measuring the correct amount of the substance.
My results show that I have three anomalous results as they are not 0.1cm³ apart. The three remaining results are concordant as they have 0.05cm³ differences between them. The anomalous results will be due to the certain amount of error that has occurred during the experiment. The most difficult part of the experiment was judging the colour change of the indicator. Even though the colour change was fairly obvious, I had to ensure that I stopped the titration at the right time and colour. This was rather tricky as analysing the different shades of pink and ensuring that the colour was the same for each experiment was only done with my natural sight through my goggles. This could have been the main reason for the anomalies.
Percentage Errors
The percentage errors can be calculated by:
Precision error x 100
Actual reading
Anhydrous Sodium Carbonate – Weighing Balance (2.65g)
The percentage error (±0.005g) =
0.005g × 100 = 0.189% (3sf)
2.65g
Volumetric flask (250cm3):
This is used to make standard solution of a particular volume. If a 250cm3 volumetric flask is filled correctly i.e. the bottom of the meniscus rests on the calibration line, the error is 0.2cm3.
The percentage error =
0.2 cm3 × 100 = 0.0800% (3sf)
250cm3
The percentage error is low and therefore the flask measures the volume to a high degree of accuracy.
Volumetric pipette (25cm3):
This was used because of its high degree of accuracy in quantitative analysis. If it is used correctly i.e. it is allowed to drain and retain the last drop, the error is 0.06 cm3
The percentage error =
0.06 cm3 × 100 = 0.240% (3sf)
25cm3
Despite the percentage error being higher than that of the volumetric flask, it is still very low and therefore measures to a high degree of accuracy.
Burette:
One drop from a burette has a volume of approximately 0.05cm3. All the burette readings should include 2 decimal places in which the second figure is either 0 or 5. An error of one drop in a volume of 25.00cm3 gives the following percentage error:
The percentage error =
0.05 cm3 × 100 = 0.20% (3sf)
25cm3
The percentage error is similar to that of the volumetric pipette; therefore it does measure to a suitable degree of accuracy.
Total percentage error = 0.709% (3sf)
The total percentage error can effect the concentration of acid. I can now work out how much it affects the acid in the following way:
In my analysis, I already calculated the concentration of acid, which came to a total of 0.0637mol dm-3.
I can now divide this figure by 100 and then multiply by the total percentage error:
0.0637mol dm-3 × 0.709% = 0.000451633% (3sf)
100
If the errors were that only of the above equipment, then the experiment would be approximately 99.9995484% correct. This means that the solutions in the investigation would be measured to a very high degree of accuracy. However, this is not the only errors in this case, as accuracy and human error also needs to be taken into consideration. Most human errors are unavoidable; therefore, it may not have been possible to improve the experiment further. On the other hand, some errors could have been enhanced by being more careful and attentive to certain matters.
Reasons for using particular methods or equipment
Methyl Orange Indicator
The methyl orange indicator was used in the experiment because it is a suitable indicator for a titration between a strong acid and weak alkali which was appropriate in this case. The colour change is also easy to identify between the orange and pink.
Pipette (22cm3)
The volume of sodium carbonate used in the titration (25cm3) was appropriate because using a 25cm3 pipette gives more accurate and reliable result than using a pipette with a greater volume capacity.
Sodium Carbonate Solution (250cm3)
Making the solution of sodium carbonate to a volume of 250cm3 in a volumetric flask was the right amount because there was enough solution to go beyond 6 different titrations. If I needed to do any repeats, I would have enough solution to use instead of producing more and hence, wasting time.
Beakers:
The beakers are convenient for measuring large volumes of a solution. This is not very accurate but can give a fairly accurate volume (taking the glass errors into consideration). I used it to measure the volume of water because the amount of water needed was approximate.
Conical flask:
Using a volumetric flask is more suitable for this titration than a normal beaker because the shape of the flask makes it easier to hold and swirl the flask so that no splashes occur and also so that the burette can fit easily into the mouth of the flask when the titrations occur.
Errors caused by method
Some of the errors that may have occurred during the experiment are:
- I may not have washed the equipment thoroughly with distilled water; this may have led to some traces of the previous solution still remaining on the equipment, thus affecting the solution within my experiment.
- Too much or too little of the methyl orange indicator may have been added each time as I may have miscounted the drops placed in the flask, also taking into consideration the errors of the dropping pipette used. Also the size of the drops varied, this could have also affected the final results.
- The meniscus line may have not been properly interpreted or I may have not maintained the same meniscus line throughout the experiment.
- The solution in the volumetric flask may not be properly blended. Not mixing the solution in the volumetric flask thoroughly would make the solution have uneven concentration through out. Some areas would be more concentrated than others. Also there may be a precipitate left at the bottom of the flask affecting the concentration of the solution.
- The end point may not be accurate if the solution from the burette is not added drop by drop with continuous swirling.
- When taking the reading of the burette, I could have misread the value or not taken the reading from the bottom of the meniscus therefore recording an inaccurate result.
- Different bottles of sulphuric acid were used, this could have affected the results as they may have been variance in the concentrations of sulphuric acid, thus, affecting the final result.
-
When transferring 25cm3 of sodium carbonate solution into the conical flask using the pipette filler, all of the solution may not have been transferred properly, therefore leaving a few drops in the pipette filler. There also could have been air bubbles in the pipette. This leads to different amounts of sodium carbonate solution in the flask, hence, affecting the final result.
- More drops of the acid could have been added to the solution than needed, resulting in different sulphuric acid concentrations. As I was adding the sulphuric acid by hand into the flask, I could have added more drops than needed to neutralise the solution. This would therefore affect the tire amount clearly.
- There was a set time to finish the experiment completely, therefore, this would have caused slight pressure, resulting in some shortcuts during the experiment, thus affecting the results.
Ways to improve method
If the experiment was repeated a further number of times, it would limit the effect of human error and may alter the conclusions I came to. The main improvements would be to reduce human error as much as possible. This could be done in a number of ways:
I could have used a pH metre instead of a pH indicator to identify exactly when neutralisation occurs. This would have removed the most significant source of error in this investigation. The methyl orange indicator changed from a yellow to a pink colour when neutralisation had occurred. On the other hand a pH probe would give the exact point of neutralisation.
A white piece of paper would have been useful as a background for the burette when taking the reading. This is because the solution in the burette and the burette itself are both transparent; therefore it would have been difficult to read the amount of solution used. Hence, this will reduce the degree of accuracy.
When transferring the sodium carbonate solid from the watch glass to the beaker, I would have to ensure that the entire solid is transferred by washing the contents on the watch glass into the beaker using distilled water so that exactly all of the 2.65g of sodium carbonate is transferred into the beaker.
I would also have to make certain that the burette is correctly placed so that my eyes are level to the 0 mark, where the sulphuric acid is filled up to. Also, the bottom of the meniscus should touch the 0 line for accurate results.
When the burette is filled with sulphuric acid, the funnel should be removed as soon as it reaches the 0 mark. This is to make sure that no other drops from the funnel will enter the burette. If this did happen the reading on the burette would be inaccurate.
When the titration has started, I would have make certain that I have full control over the burette tap and also take the rough titre into consideration so that I could slow down the tap once the figure gets closer to the rough titre. This is to ensure that I can immediately stop the titration once the solution changes colour.
Swirling the flask is very important; this is to ensure that all of the sodium carbonate solution and sulphuric acid is fully blended. It is also valuable because by mixing the solution together, I would be able to identify the change of colour. If the flask was not swirled, the colour change would not be clear and therefore the titre would be undoubtedly inaccurate.
When starting a new titration I would have to fully wash the volumetric flask that contains the sodium carbonate solution so that it does not affect the new experiment and hence modify the results.
Other equipment that could have been used in order to identify the change in colour for neutralisation is a light detector. A beam of light could have been shone through the volumetric flask. This would reduce human error and therefore make the results more reliable.
Practising using the equipment for the titration would assist to reduce human error such as practising using the tap on the burette, reaching the meniscus line, reading the burette etc.
Environmental control should have also been considered, such as evaporation of the liquid or temperature fluctuations. In order to overcome this, I would need a thermometer to ensure all liquids are about the same temperature. This is because temperature can affect the rate of reaction, and thus speed up or slow down reactions more than others.
On the whole, the results that I obtained fitted within the specification of between .05 and 0.15moldm³. From my investigation, I found the concentration of the acid solution to be 0.0637mol dm-3, this shows that this amount of concentration of sulphuric acid was needed to neutralise the sodium carbonate solution. These results obtained are to a certain extent reliable and accurate. However, if I were to repeat the experiment overall, I would use the techniques as mentioned in my evaluation.
By Farshea Saail
