The method used for carrying out this experiment is called a titration this is a technique where the exact endpoint of a reaction can be found. A burette is used to run, in this case the acid into the alkali, contained in a conical flask below the jet of the burette. An indicator, methyl orange is added to the alkali so colour change can be seen as the acid is added. When performing a titration a rough titration is first performed quickly so a general endpoint for the reaction can be found. By doing this the exact endpoint can be approached more slowly giving an exact amount of titre required for the reaction.
The indicator which will be used for this reaction is methyl orange. Which is commonly used as the indicator for titrations between weak alkalis and strong acids. This is because when using a strong acid and weak alkali the pH changes rapidly at the equivalence point (end point) from pH 3.5 to 7.0 or vice versa. Any indicator which works between pH 3.5 and 7.0 is suitable, methyl orange changes colour at pH3.5-4.5 so it is very suitable for this use. Methyl orange belongs to a group of organic chemicals called chromophores, which have an extended delocalised electron systems. It is coloured because of the –N N- unsaturated group. Electrons in the double bond are excited by lower amounts of energy than in single bonds, so they are able to absorb energy in the visible region. Methyl orange has an active range between pH 3.5 and 4.5. When in an acidic solution with a pH below 3.5 methyl orange is red. I saw this in a preliminary experiment, where I observed how and when methyl orange changed colour, when in an acid or alkali solution. In solutions above this pH methyl orange appears in a range of yellow colours. A H+ ion is bonded to one of the N atoms in the double bond changing the amount of energy absorbed and therefore the colour of the solution.
Amount of sodium carbonate
I have chosen to use a 0.1moldm-3 solution of sodium carbonate, this is because it is in the middle of the range for the concentration given in the aim. I have chosen to use 250cm3 of sodium carbonate because it is an easy to handle quantity, it allows me to do many titrations without wasting too much which isn’t used. To make a solution of this concentration I will have to dissolve an amount of sodium carbonate in a fixed amount of water here is my calculation for this:
Find the relative molecular mass (Mr) of sodium carbonate by adding the relative atomic mass (Ar) of everything that makes up sodium carbonate (Na2CO3) together.
Na, Ar = 23 C, Ar = 12 O, Ar = 16
23 + 23 + 12 + 16 + 16 + 16 = 106 Mr = 106
The mass of 1 mole of Na2CO3 is 106g
If 106g of Na2CO3 was added to 1dm3 of water this would produce a 1moldm-3 solution. I want 250cm3 of 0.1moldm-3 solution to find this I will apply the formula:
Concentration = number of moles
Volume we need to find the number of moles so by multiplying concentration by volume to give number of moles
To do this calculation the volume must be converted into dm3 from cm3 this is done by dividing the volume by 1000.
250 = 0.25dm3
1000
Concentration x volume = moles
0.1moldm-3 x 0.25dm3 = 0.025 moles
Multiplying the number of moles required by the mass of 1 mole gives the mass required to make the solution.
106 x 0.025 = 2.65g of Na2CO3 required to make 250cm3 of 0.1moldm-3 solution. So I will dissolve 2.65g of sodium carbonate in 250cm3 of distilled water to make 250cm3 of 0.1moldm-3 sodium carbonate solution.
Apparatus
Burette
250cm3 beaker
25cm3 glass pipette
Pipette filler
250cm3 volumetric flask
White tile
250cm3 conical flask
Methyl orange
?moldm-3 sulphuric acid solution
Anhydrous sodium carbonate
Distilled water
Precision balance
Glass rod
Spatula
Funnel
Diagram
Method – making the sodium carbonate solution
-
Place the 250cm3 on the balance and zero (using tare button). Measure out the mass of 2.65g of sodium carbonate into beaker using the balance to measure.
- Add small amounts of distilled water at a time to the beaker and stir using glass rod until all of the sodium carbonate is dissolved. Crush any large pieces of sodium carbonate with glass rod so they fully dissolve.
- Once all of the sodium carbonate has dissolved transfer all of the solution to the volumetric flask, then rinse out the flask using distilled water and add it to the volumetric flask.
-
Top the flask up to the 250cm3 with distilled water, so the meniscus of the solution reaches the 250cm3, then stopper then mix by gently rocking the flask. This prevents froth forming in the solution that may cause problems when measuring quantities later.
Method – titration
- Fill the burette with distilled water, then empty whilst turning the burette to clean.
- Half fill the burette with the sulphuric acid solution using a funnel, then run half of the solution out through the tap checking for any bubbles in the jet. If there are any bubbles in the jet tap the side of the jet to remove it.
- Pour the rest of the solution out through the top of the burette, whilst turning the burette to remove any water drops. Then repeat this step and step 2.
- Fill the burette to above the zero level, then remove the funnel and place a beaker below the jet of the burette and run the acid through until the meniscus is on zero.
-
Use a pipette and pipette filler to measure 25cm3 of sodium carbonate solution and transfer to a 250cm3 conical flask. Touch the end of the pipette on the conical flask to remove any drops of sodium carbonate from the pipette. Make sure the bottom of the meniscus is level with the 25cm3 line on the pipette.
- Add 3 drops of methyl orange to the solution in the conical flask.
- Add the acid from the burette to the conical flask swirling constantly, until the indicator in the solution changes colour. This is a rough end point that will help give an area for finding the more accurate endpoint. Record this result. Keep a white tile under the conical flask so the colour change can be seen easily.
- Repeat the titration several times, more accurately than the first adding the acid drop by drop when approaching the endpoint. Keep repeating until the results are consistent.
- The burette may need to be topped up after each titration use the same method in step 3 for this. Record the start and endpoint for each titration.
Accuracy
- Washing the glassware out with the solutions that they will contain removes the remains of previous solutions and water drops. This makes sure there are no contaminants that will affect the concentrations of the solutions used.
- Clearing the bubble from the jet of the burette prevents errors in volume that may occur.
- When measuring quantities of solutions filling so the bottom meniscus is at the level required. This gives an accurate measurement of all quantities and prevents small anomalies in results. Readings should be taken at eye level so that a parallax error does not occur.
- Mixing the sodium carbonate solution when it is diluted in the volumetric flask, ensures that all of the solution in the flask is the same concentration so the results are not affected.
- Using a white tile makes it easier to see a colour change so an accurate endpoint can be reached.
- Adding acid drop by drop when near the end point means that an exact endpoint can be reached.
- Using a precision balance means that the exact amount of sodium carbonate can be measured out. By having the exact amount of sodium carbonate added to the solution this helps make a solution of an exact concentration. A solution of an exact concentration is required to accurately find the concentration of the acid solution.
-
Repeating the titration until all results are within 0.1cm3 of each other ensures that the endpoint found is correct and accurate.
- The burette readings are to 2 decimal places because this gives precise readings for the amount of titre so when calculating the concentration of the acid it will be accurate.
- The acid is placed in the burette because it is the quantity of the sulphuric acid solution required to neutralise the sodium carbonate solution. By having a fixed amount of sodium carbonate and seeing how much acid is used it makes it easier, when calculations are done to find the concentration of the acid.
Safety
See risk assessment for safety precautions to be taken when handling chemicals.
Bibliography
Methyl orange diagram and information, page161, Salters Advanced Chemistry Chemical Ideas, Heinemann.
Acids and Alkalis, Ideas 8.1, Salters Advanced Chemistry Chemical Ideas, Heinemann.
