Titration: Volumetric Analysis Technique

Introduction
Titration is a volumetric analysis technique used to find the concentration of an unknown substance. There are several types of titration but the one used for this experiment is called acid-base titration as the substance we are titrating is acidic. Acid-base titrations are based on the neutralisation reaction between the analyte and the acidic or basic titrant. The analyte is the solution of unknown concentration and the titrant is the solution of accurately known concentration used to determine the concentrations of other solutions. Titration is used in the industry for medicine, wastewater analysis and acid rain analysis.
In an acid-base titration, a suitable indicator is added to the analyte. Titrant is added to the solution until the analyte permanently changes colour. The point at which the indicator just changes colour due to an excess amount of base or acid is known as the end point. This differs from the equivalence point where the equivalence point is the point at which the analyte and the titrant is stoichoimetrically equal in the number of moles. This can also be referred to as the point at which all of the analyte has reacted with the titrant. The equivalence point and the end point usually lie very closely together on a titration curve.
This is an example of a weak acid-strong base titration curve. As we add more base to the acid, the pH slowly increases. The steep curve indicates the reaction reaching completion. As you can see from the graph, there’s a very small range between the equivalence point and the end point. As such reaching the equivalence point is very difficult to achieve.
In this experiment we are titrating “no frills” vinegar with a standardized solution of sodium hydroxide. It is essential that all glassware is properly cleaned and sterilize to acquire accurate results as any contamination may result in inaccurate results. The burette is washed with the titrate as to ensure the concentration of the titrate is unaffected. If we were to wash it with distilled water, there would be a change in concentration which affects the accuracy and validity of the experiment. The conical flask, however, can be washed with distilled water as the number of moles in the analyte is unaffected regardless. The pipette
The reaction that will occur is CH3COOH (aq) + NaOH (aq) NaCH3COO (aq) + H2O (l) As we reach the end point, there will be an excess in sodium hydroxide. Hence a suitable basic indicator is needed to detect the end point and also must detect the change in pH without the need of having too much excess sodium hydroxide. Therefore phenolphthalein is used as the pH range is approximately 8-9.8. As the solution goes from slightly acidic to basic, the indicator colour changes from colourless to light pink.
A primary standard is the solution used to determine the concentration of other solutions. A primary standard needs to have certain properties to acquire a very accurate concentration of the primary standard. These properties include: * Relatively stable or available in a pure form so that it won’t react with the moisture in the air * Unhydrogenic so that it doesn’t absorb moisture from the air and change its mass during weighing * Soluble in water * Relatively large molecular mass
In this experiment we are using a secondary standard (sodium hydroxide). A secondary standard is made by titrating it against a primary standard to determine its own concentration. Sodium hydroxide is anhydrous and hence absorbs moisture from the air. Hence it needs to be titrated to acquire accurate concentrations.
Aim: To calculate the ethanoic acid concentration in “No Frills” vinegar using acid-base titration.
Equipment:
Risks: - Acids and bases can be dangerous. Be careful not to spill any acid or base used. Contact with an acid or base can result irritation and/or damage your skin. TO minimise the risk wear safety glasses and a lab coat when conducting the experiment. In the case of accidental contact, wash thoroughly with water. * Handle all glassware with care as there may be sharp shards when it breaks.
Method
[Draw Diagram here] [Apparatus] [Eye level meniscus]
Rinse the burette with the standardized solution of sodium hydroxide, ensuring all areas of the burette are rinsed. Discard any washing used to clean equipment in this experiment. Clamp the burette with the burette clamp to the retort stand. Fill the burette with sodium hydroxide using a clean glass funnel until the meniscus of the solution is on the engraved 0mL mark. Ensure the valve is tight before you fill the burette.
Rinse the volumetric flask and conical flask using distilled water. Rinse one of the pipettes using the vinegar. Transfer 25ml of vinegar into the volumetric flask using a pipette and dilute it by a factor of ten using distilled water. Place a lid on the volumetric flask and shake a few times. Rinse the other pipette with the diluted solution of vinegar. Use the pipette to transfer 25ml of the diluted vinegar into the conical flask. Place 3 drops of phenolphthalein indicator into the conical flask. Place the conical flask underneath the burette.
Using your dominant hand to continuously swirl the conical flask, loosen the valve with the other hand and allow it to run until within 10ml of the expected end point. Avoid splashing the solution and the titrant. Tighten the tap slightly to reduce the rate at which the titrant is flowing and observe any permanent change in colour. Immediately tighten the valve when this occurs as this indicates we have reached or passed the end point. It should turn from colourless to light pink. Allowing the conical flask to rest may be necessary in the event that the colour of the solution may revert to its initial colour. If this occurs, add a drop of the titrant and swirl. If the colour still doesn’t permanently change, repeat the previous step until the colour change is permanent. Record the amount of titrant used as indicated by your burette.
The first titration is only a rough estimate and is not included in the calculation of the average.
Rinse the conical flask with distilled water. Repeat the experiment but approach the end point slowly. Adding only a few drops at a time as you approach the end point improves your accuracy.
Results

CH3COOH (aq) + NaOH (aq) ---> NaCH3COO(aq) + H20
Acetic acid + sodium hydroxide - sodium acetate and water