ACIDIMETRY

ACIDIMETRY

           

Acidimetry is the determination of acidic substances by titration. The term alkalimetry refers to the analogous basic process. Both processes are neutralization reactions, and they constitute an important part of volumetric analysis. In volumetric analysis, a solution of the substance being determined is treated with a solution of suitable reagent of exactly known concentration (standard). The standard reagent solution is added until it is equivalent to the amount of substance being determined. For the acid base reaction this equivalence is obtained when number of moles of H⁺ are equal to the number of moles of OH^-, since

H⁺   +   OH^- --------- > H₂O

            In a typical experiment, the buret is filled with the standard solution. An indicator is added to the unknown solution in the flask. The indicator will cause a visual change in the color of the solution to signal that the end point has been reached. This process is known as a titration.

            Many acid base indicators are available. They all have different pHs at which their color will change. For some acid base titrations like the ones to be performed in this experiment, phenolphthalein can be used. Phenolphthalein exhibits a pink color in basic solutions and is colorless in acidic solutions. In this experiment you will be adding base from the buret until a fraction of the drop of the base changes color from colorless to pale pink.

Steps in performing the lab

1)      Standardization of NaOH solution using Potassium Acid Phthalate (KHP)

2)      Using standardized NaOH from the first step to determine the molarity of an unknown acid

Materials:

2-     50 mL buret

1-     50 mL Erlenmeyer flask

1-     Buret card

1- Beaker (100 ml)

Phenolphthalein indicator solution

NaOH solution

Potassium acid phthalate

Unknown acid

SAFETY AND WASTE DISPOSAL

Goggles and Aprons must be worn at all times

Inform instructor immediately if any chemical gets in the eye

Waste can go down the drain

Step: 1

Procedure: Obtain 50 mL of NaOH solution from the instructor. Clean the buret thoroughly first with DI water and then with the NaOH solution. Fill the buret with NaOH solution and note down the initial reading of the buret. Obtain a clean dry 250 mL Erlenmeyer flask and weigh out about 0.1 g of KHP into it (use a weighing paper). Add about 50 ml of DI water to the flask and shake till the KHP completely dissolves. Add two drops of phenolphthalein indicator and titrate against NaOH solution. Note the volume of NaOH needed to obtain the end point.

Data Table                                          Run 1                         Run 2

Initial Reading of buret
Final reading of buret
Volume of NaOH used
Mass of KHP

Data Processing

1)      Calculate the moles of KHP used in each run. Molar mass of KHP is

2)      Using moles of acid equals moles of base, calculate the molarity of NaOH solution

3)      Take the average molarity of both runs.

Step 2

Determination of molarity of unknown acid

Obtain a beaker containing unknown acid from your instructor and note the number down in your lab notebook. Fill the acid buret with unknown acid and transfer 5.00 ml of unknown acid to an erlenmeyer flask. Add two drops of indicator solution and set aside. Titrate with NaOH solution and note down the volume of NaOH used to reach the end point. Repeat.

Data Table                                          Run 1                         Run 2

Initial Reading of buret
Final reading of buret
Volume of NaOH used
Volume of unknown acid

Data Processing

4)      Calculate the moles of NaOH used in each run. Molar mass of NaOH is 40.0 g/mol

5)      Using moles of base equals moles of acid and the volume of unknown acid, calculate the molarity of unknown acid

Take the molarity of both runs.

taken from
http://web.fuhsd.org/kavita_gupta/website%20chemH/acidimetry.htm