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When constant successive portions of Sodium Hydroxide are added to Acetic Acid; how do the changing amounts of Sodium hydroxide mixed with Acetic Acid in the conical flask affect the pKa of Acetic Acid?
A weak acid is defined as being an acid that does not donate all of its hydrogen ions in a solution (Neuss, 2007) A weak acid represented by HA will always be in equilibrium with its ions in an aqueous solution, for example: HA (aq) – H+(aq) + A-(aq)
The equilibrium constant will thus be given as products over reactants by:
Ka is most commonly known as the ‘acid dissociation constant’. The pKa is just the pH of the Ka i.e. pKa = -logKa and is used as a quantitative measure the strength of a weak acid in solution. Acetic acid (CH3COOH) is a weak acid and Sodium Hydroxide (NaOH), on the other hand, is a strong base and reacts with Acetic acid (CH3COOH) to produce water (H2O) and a salt (NaCH3OO) as follows:
CH3COOH + NaOH –> H2O + NaCH3OO
A method called a ‘Titration’ provides information about the behavior of acids through the pH scale. In a titration, base is gradually added until the acid reaches an endpoint or equivalence point. When the equivalence point is reached, the pH of the solution will change rapidly, because all the acid has reacted with the added base. A pH meter can be used to determine the pH of the acid throughout the titration, and can be used to determine the equivalence point. When carefully measured volumes of strong base are added to a solution of weak acid and the pH is noted, a graph can be drawn with pH on the y axis and the volume on the x axis. The graph is known as the Titration/pH curve and the theoretical pH curve of the neutralization reaction between Acetic acid and Sodium Hydroxide is provided below:
Figure 1: Theoretical Titration curve of Acetic acid and Sodium Hydroxide.
This picture was obtained through www.google.com/images/titrationcurve
The flat portion of the titration curve before the end point (refer to figure 1) is called the buffer region. In this part of the pH scale, that the Acetic acid and Sodium hydroxide are both preset in significant concentrations and the solution resists changes in pH. In the middle of the buffer region lies the half equivalence point. Here the volume of base added is half that required to reach the equivalence point. We can determine the pKa or Ka of an acid by finding the pH when half way to the endpoint of the titration since pKa = -log Ka (refer to figure 1).
A smaller Ka value suggests a larger pKa value. The larger the pKa value the weaker the acid. Base solution is added until the equivalence point is reached. Thus, to determine the pKa of Acetic acid, the amounts of base mixed with acid will vary, the pH will be measured at regular intervals which will ultimately allow the pKa to be determined.
In this experiment, the amount of Sodium Hydroxide released from the burette will never change. What changes, however, is how much base is in the conical flask. To begin with there will be no base but when 1cm3 of Sodium Hydroxide is added then there is 1mL of Sodium Hydroxide in the conical flask. Then when another 1 cm3 of Sodium Hydroxide is added there will be 2mL of Sodium Hydroxide in the conical flask. So what is changed is the amount of base in the conical flask and it is this amount of base that changes the pKa.
Thus, I hypothesize that as the amount of Sodium Hydroxide increases, pH will gradually increase until it reaches the end point where there will be a sudden increase. Afterwards, the amount of base will overtake the Acetic acid and this would result in a plateau in the pH curve.
Defining Independent and Dependent Variables
Table 1: Dependent and Independent Variables
The amount of base (Sodium hydroxide) mixed with acid. In other words, the volume of NaOH dropped into the acid is controlled but what is changed is the amount of base in the conical flask.
(1 cm3 will always be followed with 1 cm3, there will be no change in how much you put in each time; what changes is how much acid there is in the beaker)
The pH of the solution
Table 2: Variables and Methods of Control
Method of Control
The volume of the Acetic acid will be controlled
20 cm3 of the Acetic acid will be measured out using a Mohr pipette and then carefully released into the conical flask
The pressure under which the experiment is carried out.
All experiments will be conducted standard laboratory conditions, which means at 1atm pressure
The Temperature under which each experiment is conducted.
All experiments will be conducted in the same room within quick succession.
The amount of base added to conical flask each time
Successive portions of 1 cm3 of NaOH will be added to the conical flask until it reaches the endpoint
Human judgment errors
The person conducting the experiment must read from the burette in which holds the NaOH. By using the same person for each experiment- the error of judgment will be kept constant.
Materials and Equipment List (enough for 1 titration including the rough titration)
– 1 x 50 +/- 0.05 cm3 Burette
– 1 x Burette Stand and clamp
– 1 x funnel
– 1 x 100 +/-0.050 cm3 Conical flask
– GLX pH probe
– 1L Distilled Water
– 1 x 20 +/- 0.020 cm3 Mohr Pipette
– 50 cm3 of Acetic Acid concentration of approximately 0.1mol dm -3
– 100 cm3 Sodium Hydroxide solution 0. 1mol dm -3
– Safety Glasses
– White Paper
– Graph Paper
– Pencil, for drawing the Graph
– Rubber Gloves (in case of glass breakage)
– 1 x White tile
– 1 x Standard Bulb
– 50 cm3 bottle of Phenolphthalein indicator (only 4 drops are required)
– Paper towels (for cleaning)
Figure 2: Conical Flask Figure 3: Burette Figure 4: Mohr Pipette
These pictures were all obtained from www.google.com/images
1. Before commencing this experiment it was made sure that all involved in the experiment were wearing the safety goggles and a lab coat to avoid risk of injury (refer to table 3)
2. The Titration was set up as follows, with the clamp holding the burette and the funnel at the top of the burette. The conical flask should be placed on a white tile underneath the burette. The burette should be clamped so that its tip is within the conical flask but above the surface of the solution.
Figure 5: Titration set-up
3. The burette was then rinsed with distilled water to ensure that it is clean and to avoid errors
4. Usually experimenters cannot reach the top of the burette when its placed on a lab bench, so as a result, the burette and the burette stand were placed on the ground and Sodium Hydroxide was carefully poured to the first digit on the top of the burette (usually 0) via the funnel. The initial reading of the burette was then recorded in the results table, refer to table 3. Afterwards, the burette was returned to its original position on the top of the bench
5. The Mohr pipette was then checked for chips and cracks and was rinsed several times to ensure accurate volume measurements
6. Afterwards, the Mohr pipette was used to collect 20 cm3 of acid (Please refer to the ‘Using Mohr pipette method’) and the acid was then poured into the conical flask
7. Immediately following, the GLX pH probe, refer to GLX probe method, was adjusted and the head was placed in the acid, the pH of the initial acid was recorded in the results table, refer to table 3
8. 2 drops of Phenolphthalein was then added to the acid. The Phenolphthalein is an indicator which means it turns pink when base is added, an indication of endpoint would be that the liquid in the conical flask turns pink
9. To begin with, a rough Titration was be done to estimate the endpoint
10. In a rough Titration the tap was opened, and the base was simultaneously released into the conical flask until the liquid turned pink in which the tap was closed. When the liquid turned pink the final pH was recorded in the results table. Also record the final reading of the burette. The Rough Titration is only an indication and so should not be used in data analysis.
11. After the Rough Titration, the solution in the conical flask was discarded in the sink. Since the products were neutralized solutions of common salts they can safely be disposed of down the sink.
12. The conical flask was then washed with distilled water and 20 cm3 of Acetic acid was then poured into it via the Mohr pipette. And immediately following this, the GLX pH probe was placed inside the acid. Sodium Hydroxide was also then poured into the burette, using the method discussed in point 4, to top it up
13. The titration was then initiated, and successive portions of 1 cm3 of Sodium Hydroxide were added to the conical flask. After each addition, the burette and pH readings were obtained and recorded. If the person doing the experiment is right handed, then their right hand should be used to open and close the tap to allow Sodium Hydroxide to drip out and their left hand should be used to swirl the solutions in the conical flask. Swirling is important as the pH may drift until a completely homogenous solution is achieved. The same principle applies to left handers, except they would of course use their left hand instead of their right hand to release the NaOH.
14. The Titration was continued until the pH reached 12, the final burette reading was also recorded
15. This titration process (steps 11-14) was repeated another two times, allowing more reliable results.
16. After the Titration the benches were wiped down using paper towels, the solution was discarded in the sink and the glassware was placed in the designated container. Hands were washed before leaving the laboratory.
17. By the results gathered, a Titration curve can be drawn. The pH would simple be on the y axis while the volume of the base would be on the x axis. The pKa can be determined by finding the pH halfway to the endpoint of the Titration curve.
GLX pH probe method:
1. The probe was first turned on and then pH probe section was plugged in at the top.
2. The pH probe section was then placed into the Acetic acid
3. Automatically, a pH reading appeared on the screen
4. When enough data was collected the recorded was stopped and the probe was unplugged
5. The GLX pH probe was then plugged into a laptop to export the data to the computer’s hard drive
Using Mohr Pipette method:
1. The standard bulb was attached to the end of the pipette by carefully fitting the bulb at the end of the pipette
2. The pipette tip was then placed below the surface of the liquid and the bulb was squeezed to draw the liquid up.
3. The volume of the Acetic acid in the pipette was determined by reading the meniscus of the pipette
4. The pipette was then held above the conical flask, and the bulb was released to release all the liquid into the conical flask
Table 3: The results table
Volume of Sodium Hydroxide added
Amount of base in the conical flask (burette reading)
Initial burette reading (cm3)
Final burette reading (cm3)
Volume of Titration (cm3)
Note: The purple shaded region is the rough Titration. The volume of Sodium Hydroxide added each time will always be 1 cm3 but the Amount of base in the conical flask will change. Also, the patterns in this table should be continued until the Amount of base in the conical flask is at least 30 cm3. The pH will be recorded via the GLX data probe.
Table 4 : The risks involved in the experiment and safety precautions to reduce the risks
Can cause serious burns, however the 0.1 concentration of Sodium Hydroxide cannot cause harmful burns unless excessive amounts of it are poured on the skin. Harmful by ingestion and skin contact.
Safety Glasses are needed to reduce the risk of injury if a spill occurs. Avoid skin contact with Sodium Hydroxide as well as ingestion.
Causes burns, however the 0.1 concentration of Acetic acid cannot cause harmful burns unless excessive amounts of it are poured on the skin. Harmful by ingestion and skin contact.
Safety Glasses should be worn the entire time to reduce the risk of injury. Gloves and Lab coat should be worn to avoid skin irritation. Avoid skin contact with Acetic acid.
Breakages can cause cuts and serious chemical spills
Wearing rubber gloves make sure that all broken glass is disposed of appropriately. The burette should also be handled very carefully as it is very fragile.
Spillage of Sodium Hydroxide or Acetic acid
Can cause serious burns
Wash any spills copiously with water
* “Volumetric analysis.” 4 Feb 2009 <http://www.uwplatt.edu/chemep/chem/chemscape/LABDOCS/CATOFP/measurea/volume/pipet/pipet4.htm>.
* “Acid-base titrations.” Wikipedia. 4 Feb 2009 <http://en.wikipedia.org/wiki/Acid-base_titration>.
* “pKa and LogP Measurements.” 4 Feb 2009 <http://www.raell.demon.co.uk/chem/logp/logppka.htm>.
* “pKa of a weak acid.” 4 Feb 2009 <http://188.8.131.52/search?q=cache:E6yRz3RiHlwJ:www.profpaz.com/Files/chem102/Exp_10.pdf+%22volume+at+equivalence+point%22&hl=en&ct=clnk&cd=1&gl=au>.
* Neuss, Geoffrey. Chemistry . London: Oxford, 2007.
* Neuss, Geoffrey. Chemistry For the IB diploma. London: Oxford, 2007.
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