20/10/2010
Module : Principles of Chemistry
Student no :
MARK: 63
MARK: 63
Standardise Hydrochloric Acid
Titration
1. Aim
The purpose of this lab is to find, by titration, the concentration of dilute hydrochloric acid using
a standard solution of sodium carbonate.
2. Introduction
By the definition of both Thomas Lowry (England) and J.N. Brønsted (Denmark) working independently in 1923, an acid is a material that donates a proton and a base is a material that can accept a proton.
Acids neutralize bases in a neutralization reaction. An acid and a base combine to make a salt and water. A salt is any ionic compound that could be made with the anion of an acid and the cation of a base.
Bases neutralize acids in a neutralization reaction. The word - reaction is: Acid plus base makes water plus a salt.
Where 'Y' is the anion of acid 'HY,' and 'X' is the cation of base 'XOH,' and 'XY' is the salt in the product, the reaction is: HY + XOH
Where 'Y' is the anion of acid 'HY,' and 'X' is the cation of base 'XOH,' and 'XY' is the salt in the product, the reaction is: HY + XOH
Titration is a method of analysis that determine the precise endpoint of a reaction and therefore the precise quantity of reactant in the burette. A burette is used to deliver the reactant to the flask containing the standard solution and an indicator. When the indicator changes colour, this is often described as the end point of the titration. The colour change happen when the two solutions are mixed together in exactly equation proportions. That particular mixture is known as the equivalence point.
The overall equation for the reaction between sodium carbonate solution and dilute hydrochloric acid is:Na2CO3(aq) + 2HCl(aq) --à 2NaCl(aq) + CO2(g) + H2O(l)
The reaction between sodium carbonate and hydrochloric acid takes place in two stages as follows :
A. Na2CO3(aq) + HCl(aq) --à NaHCO3(aq) + NaCl(aq)
B. NaHCO3(aq) + HCl(aq) --à NaCl(aq) + CO2(g) +H2O(l)
The endpoint of the titration is usually shown by the changing colour of the indicator. A pH indicator is a material, usually an organic dye, that is one colour above a characteristic pH and another colour below that pH.
Indicators used in this experiment are phenolphthalein and methyl orange.
Phenolphthalein often used in titrations, it turns pink-fuchsia in basic solutions and colorless in acidic solutions.
When added into a clear solution of sodium carbonate the phenolphthalein is in a basic solution and the colour of the solution is pink-fuchsia.The pH of sodium carbonate solution (Na2CO3 and water) 0.05M at 250C, the pH is 11.37 ( Mallinckrodt Baker Inc. Canada)
Phenolphthalein (pH indicator) | ||
below pH 8.3 | above pH 10.0 | |
colorless | ⇌ | |
During titration it so happens that the phenolphthalein has finished its colour change from pink-fuchsia to colourless at exactly the pH of the equivalence point of the first stage of the reaction in which sodium hydrogencarbonate is produced. The pH of NaHCO3 is 8.3 (0.1M aq.sol.) (Fisher Scientific)
Methyl orange is a pH indicator frequently used in titrations because of its clear colour change. Unlike a universal indicator, methyl orange does not have a full spectrum of colour change, but has a sharper end point. In an acid it is reddish and in alkali it is yellow.When added into the clear solution of sodium hydrogencarbonate the methyl orange is in a basic solution and the colour of the solution is yellow. The pH of NaHCO3 is 8.3 (0.1M aq.sol.) (Fisher Scientific)
Methyl orange (pH indicator) | ||
below pH 3.1 | above pH 4.4 | |
3.1 | ↔ | 4.4 |
During the process of titration the methyl orange will respond to the pH change associated with the final formation of sodium chloride, NaCl.The pH of hydrochloric acid is lower than 2.(Fisher Scientific)
3. Apparatus and Methodology :
Apparatus:
Goggles
Lab coat
Bench mat
100cm3 beaker
250cm3 beaker
250cm3 conical flask
25cm3 bulb pipette
Pipette filler
Burette 50cm3
Burette stand and 2 holders
Plastic filter funnel
White tile
Teat pipette
standard sodium carbonate- solution prepared by student
dilute hydrochloric acid to standardise
phenolphthalein indicator solution
methyl orange indicator solution
de-ionised water
A buret is used to deliver from a rapid stream to drop-by-drop solution in precisely-measured, variable volumes. Buretts are used primarily for titration, to deliver one reactant until the precise end point of the reaction is reached.The reading of the burett is taken at eye level to the bottom of the meniscus (curve in the liquid) and comparing the bottom of the meniscus to the marks on the glass. A reading of the buret is taken before and at the end of the titration. The amount of know - concentration liquid used is the difference of the beginning and ending buret reading.
Flasks and beakers are used for mixing, transporting, and reacting, but not for accurate measurements. The volumes stamped on the sides are approximate and accurate to within about 5%.
A volumetric flask is used to make up the standard solution of fixed volume very accurately. This volumetric flask measures 500 mL ± 0.2 mL. This is a relative uncertainty of 4 x 10-4 or 400 parts per million.
Hazards:
Anex 1 : Hazard Awareness form and COSHH form
Hydrochloric acid might cause burns to the skin if in contact and is corrosive to the metal surfaces.
Safety goggles must be used.
Methodology
1. The student was putting on the lab coat and the goggles.
2. All the Apparatus were brought to the bench and prepared for the experiment.The standard solution was prepared by the student in a previous experiment and stored in the lab cupboard into a volumetric flask(250cm3).About 100 cm3 of diluted hydrochloric acid to standardise was taken to the bench in a 250 cm3 beaker and placed in a secured place.
3. The burette stand was installed and the burette holders were attached to the stand.
4. The burette was gently placed into the holders and checked to be vertical and stable .
5. Underneath the burette was placed a beaker(100cm3) and the tap of the burette was closed.
6. The burette was rinsed with some de-ionised water from the top of the burette.
7. The tap of the burette was opened and the de-ionised water was drained out into the beaker underneath then the tap was closed.
8. Steps 6 to 7 were then repeated.
9. Using the funnel some of the diluted hydrochloric acid was poured into the top of the burette and then by opening the tap the solution was drained through into the beaker. After completion the tap was closed.
10. Step 9 was repeated.
11. Using the funnel, the burette was filled with the dilute hydrochloric acid to just above 9.00 cm3 mark. The funnel was lifted up slightly, to allow the solution to flow in freely.
12. The funnel was removed and the student checked the presence of air bubbles inside the burette. Slowly opening the tap the regent level in the burette was set up on the 9.00 cm3 mark of the burette. The tap was closed.
13. The beaker was removed and the white tile was placed on the support under the burette.
14. The conical flask was placed under the burette and by moving the burette up/down the height adjustment was made that the tip of the burette was just above the lip of the conical flask.
15. The pipette filler was attached to the bulb pipette(25 cm3 capacity) end.
16. By rotating the wheel of the pipette filler the solution of sodium carbonate was aspirated from the volumetric flask into the pipette to just above the mark of 25 cm3.By gently turning the wheel with gradual release of solution the reading was done at 25cm3 mark at eye level. Fast release of the solution into the conical flask was executed by pressing air inlet valve button.
17. Few drops of phenolphthalein were added to the solution into the conical flask 250 cm3 capacity.
18. Titrate with the hydrochloric acid. Initial volume of acid was noted in the notebook before start. The acid was delivered to the titration flask by turning the tap on. The solution was delivered quickly until a couple of cm3 from the endpoint. While swirl the conical flask the endpoint was approached slowly, a drop at a time. When the solution colour changed from pink to colourless and this was persistent for at least 15 seconds the titration was stoped and the volume of acid used was noted in the notebook.
19. A few drops of methyl orange were added.
20. Titrate with hydrochloric acid. Initial volume of acid was noted in the notebook before start. The acid was delivered to the titration flask by turning the tap on. The solution was delivered quickly until a couple of cm3 from the endpoint. While swirl the conical flask the endpoint was approached slowly, a drop at a time. When the solution colour changed from yellow to red and this was persistent for at least 15 seconds the titration was stoped and the volume of acid used was noted in the notebook.
21. Step 16- 20 were repeated 3 times.
22. The titrations volumes and calculations were recorded in the notebook.
23. The experience site was cleaned up and the calculations required for the experience were completed.
4. Results
A.Phenolphthalein indicator (first part of each run)
Burette reagent | Diluted hydrochloric acid to be standardised |
Conical flask reagent | Standard sodium carbonate solution 0.05M |
Indicator | Phenolphthalein |
Final volume (cm3) a | Run 1 27 | Run 2 42 | Run 3 18.1 | Run 4 35.5 |
Initial volume (cm3) b | 9 | 26 | 1 | 18.1 |
Titre (cm3) (a-b) | 18 | 16 | 17.1 | 17.4 |
Mean titre (cm3) | 17.1 | |||
Run 1 - overshoot
B.Methyl orange indicator (second part of each run)
Burette reagent | Diluted hydrochloric acid to be standardised |
Conical flask reagent | Standard sodium carbonate solution 0.05M |
Indicator | Methyl orange |
Final volume (cm3) a | Run 1 41.5 | Run 2 21.5 | Run 3 20.5 | Run 4 40.5 |
Initial volume (cm3) b | 20.5 | 1 | 0 | 20.5 |
Titre (cm3) (a-b) | 21 | 20 | 20.5 | 20 |
Mean titre (cm3) | 20.5 | |||
Run 1 - overshoot
NOTES:
Run 1 was overshoot, after the endpoint of the titration one drop more of acid was added to the reaction in order to get a comparison in colour for the next runs.
5. Observations
When phenolphthalein is added into a clear solution of sodium carbonate the colour of the solution turns from clear to pink-fuchsia.The endpoint of titration in the first stage of reaction was marked by the change of the colour from pink-fuchsia to colourless.
When added into the clear solution of sodium hydrogencarbonate and sodium chloride ( the compounds obtain after the first stage of reaction) the methyl orange changes the colour of the solution in yellow. The endpoint of titration in the second stage of reaction was marked by the change of the colour from yellow to red
.
6. Calculations
1. Calculate the Mr (relative mass) of Na2CO3 .
Ar (Na) = 23 Ar (C) = 12 Ar (O) = 16
Mr = (2x23) + 12 + (3x 16)=106 g
2. Calculate the number of moles of Na2CO3 that were dissolved in 250cm3 of water during the laste experience(Volumetric Analysis 1).
3. Calculate the number of moles of Na2CO3 in the 25cm3 transferred to the conical flask.
Stage 1 Phenolphthalein results
4. The equation for the first stage of the reaction between sodium carbonate and hydrochloric acid is shown below :
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)
From the equation, is shown that 1 mole of Na2CO3 will react with 1 mole of HCl.
How many moles of HCl will react with the number of moles of Na2CO3 calculated in step 3?
5. Calculate the concentration of HCl in mol dm−3 used in the first stage of titration
Stage 2 Methyl orange results
6. The equation for the second stage of the reaction between sodium carbonate and hydrochloric acid is shown below :
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
From the equation, it is shown that 1 mole of NaHCO3 will react with 1 mole of HCl.
The number of moles of NaHCO3 is equal to the number of moles of Na2CO3.
How many moles of HCl will react with the number of moles of NaHCO3 calculated in step 3?
7. Calculation of the concentration of HCl.
7. Discussion
The purpose of this lab is to find the concentration of dilute hydrochloric acid using a standard solution of sodium carbonate.
The experiment was successful and by completing the calculations the concentration of the HCl solution was 0.07 mol dm-3.
Titration was used in the experiment as a method of analysis to determine the precise endpoint of the reaction and therefore the precise quantity of acid used from the burette. The burette was used to deliver the reactant to the flask containing the standard solution and an indicator.
In the first stage of the reaction the endpoint of the titration was shown by the changing colour of the indicator from pink to colourless.When added into a clear solution of sodium carbonate the phenolphthalein was in a basic solution and the colour of the solution was pink-fuchsia(Diagram 1)..The pH of sodium carbonate solution (Na2CO3 and water) 0.05M at 250C, the pH is 11.37 ( Mallinckrodt Baker Inc. Canada).
The phenolphthalein has finished its colour change from pink-fuchsia to colourless at exactly the pH of the equivalence point of the first stage of the reaction in which sodium hydrogencarbonate was produced. The pH of NaHCO3 is 8.3 (0.1M aq.sol.) (Fisher Scientific).
In the second stage of the reaction methyl orange was used in the titration as an indicator. When added into the clear solution of sodium hydrogencarbonate the methyl orange was in a basic solution and the colour of the solution was yellow(Diagram 1). The pH of NaHCO3 is 8.3 (0.1M aq.sol.) (Fisher Scientific)
During the process of titration the methyl orange wasl responding to the pH change associated with the final formation of sodium chloride, NaCl.The pH of hydrochloric acid is lower than 2.(Fisher Scientific)
Diagram 1. Colour change of indicators depending on pH value.
8. Conclusion
The purpose of this lab is to find, by titration, the concentration of dilute hydrochloric acid using a standard solution of sodium carbonate.The experiment was successful and the concentration of the solution of hydrochloric acid was 0.07 mol dm-3.The expected result was 0.07 mol dm-3.
9. References
- Acids and Bases
Chemtutor
http://www.chemtutor.com/acid.htm#titr
(visited on )
- Acids and Bases
Titration
http://www.lakelandschools.us/lh/lburris/pages/acid-base.htm#titration
(visited on )
- Creative Chemistry
The Burette
http://www.creative-chemistry.org.uk/alevel/module1/documents/N-ch1-23.pdf
(visited on )
- Creative Chemistry
Volumetric Analysis 2
http://www.creative-chemistry.org.uk/alevel/module1/documents/N-ch1-36.pdf
(visited on )
- Dartmouth College
ChemLab
http://www.dartmouth.edu/~chemlab/techniques/titration.html
(visited on )
- Fisher
S/4200/63 - Sodium hydrogen carbonate 99+% SpeciFied
Material Safety Data Sheet
https://extranet.fisher.co.uk/chemicalProductData_uk/wercs?itemCode=S/4200/63
(visited on )
- Fisher
Material Safety Data Sheet
Hydrochloric acid, 0.1 N standard solution
https://securesearch.acros.com/msds?for=acros&sup=acros&lang=UK&search=124200010
(visited on )
- Material Safety data Sheet pH of Na2CO3 Standard Solution.
Mallinckrodt Baker Inc. Canada
http://www.jtbaker.com/msds/englishhtml/s3247.htm
(visited on )
- Socorex
Labware
http://www.socorex.com/manual-pipette-fillers-profiller-en-1-1-13-140.html
(visited on )
· Titration
http://www.ehow.com/about_5065624_definition-titration.html
(visited on )
- Titration
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
Jim Clark 2002 (visited on )
- Wikipedia
Methyl Orange
http://en.wikipedia.org/wiki/Methyl_orange
(visited on )
- Wikipedia
Phenolphthalein
http://en.wikipedia.org/wiki/Phenolphthalein
(visited on )
