An Experiment to Determine the Identity of a Unknown Acid with a Given Base

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This is a experiment to determine the identity of a unknown acid with a given base. To do this experiment an acid-base titration is required with the base being NaOH. This is a experiment that requires a burette to release allotted proportions of base into a beaker of unknown acid and H2O, which are recorded into a excel spread sheet. Once all data has been recorded it can be determined by the graph if the unknown was a diprotic or triproitc, in this case a diprotic was found. Its identity was glutaric acid.

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The pH meter was created in 1938, and it has been since that time that we have been able to make fundamental advances in science. The meter is used to determine the level of accidences of a substance, and it is used in today’s experiment to help determine the pKa of our acid-base titration. An acid-base titration is an experiment that is trying to neutralize either the acid or the base of known concentration, to determine the other. To help find this missing concentration, an indicator is used to help find the endpoint of the experiment, it does this by giving off a color change.

By determine the pH of an unknown sample a titration curve is able to be graphed. When seeing this graph and using calculations such as pH=pka + log (base/acid), the equivalence point can be determined. At half equivalence point the pka can be found, which can also determine your ka [H30+] =10-ph.


The chemicals being used in this experiment are the following:

  • Phenolphthalein 0.1% in alcohol
  • Deionized Water
  • NaOH
  • Unknown Acid

The equipment will consist of:

  • Three 150-250 ml beakers
  • One pH Meter
  • One Stir Plate
  • One Magnetic Stir Bar
  • One Burette

Experimental Methods and Procedure

Step one is to set up your magnetic stir plate. Place a 150-250 ml beaker onto the stir plate and add the appropriate mass of the unknown and 50-100ml of H20. Add a small magnetic stir bar into the beaker and allow the solid to dissolve completely, insert the pH meter carefully into the beaker. Make sure you do not allow the magnetic bar and the meter to touch. Record the initial pH.

Step two is to set the burette stand above the stir plate and beaker. For this you will need to fill your burette with NaOH until it is above the 0-mark line. Since this is an unknown acid we do not know how many mL of NaOH it will take to neutralize, begin the titration by 1.0 mL to the beaker and record the pH change. When there is a large jump in the pH limit aliquots to 0.2 mL of NaOH until the pH is over 10, do not forget to record every pH with the new addition of NaOH. After seeing that it has reached 10, resume 1.0 mL aliquots until succeeding in reaching a pH of 12 or slightly higher. Add three 5.0 mL aliquots to finish titration.

Step three is to repeat this process two additional times with new and clean beakers.

On your third set of calculations begin titration in increments of 0.2 mL instead of 1.0 ml, once reaching a pH of 12, increase mL to 1.0.

Once finished with the titration, make sure to empty all of excess chemicals and beakers in the waste container located in the vent hood. Wash all used glassware with deionized water. Clean the burette under hot water and detergent located at the sink. Once clean, rinse with deionized water before placing back on the buret rack with the stopcock open to dry.


Trial 1: With this trial we have a pka1 of 2.73 and pka2 of 2.09.

Trial 2: With this trial we have a pka1 of 2.76 and a pka2 of 2.08.

Trial 3: With this trail our pka1 is 2.83 and pka2 is 2.10.

Discussions and conclusions:

With the 3 trials over and the calculations finished it is possible to determine that the unknown was Glutaric acid, which is an organic compound with the formula of C3H6(COOH)2. This acid is water soluble and is naturally produced in the body during metabolism.

By determining the pka, it can then be plugged into our calculations to give us a ka1 and ka2 value. Looking at the data table below it can then be determined which acid we are dealing with. The results found on the calculations page are not 100% accurate to what is shown on the table, but to only a small degree. To almost each of the calculations, the same result of 1.70*10-4 and 8.31*10-7 was found.

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