Standardization is the process of determining the exact concentration (molarity) of a solution. Titration is one type of analytical procedure often used in standardization. In a titration, an exact volume of one substance is reacted with a known amount of another substance.

The point at which the reaction is complete in a titration is referred to as the endpoint. A chemical substance known as an indicator is used to indicate (signal) the endpoint. The indicator used in this experiment is phenolphthalein. Phenolphthalein, an organic compound, is colorless in acidic solution and pink in basic solution.

This experiment involves two separate acid-base standardization procedures. In the first standardization the molarity of a sodium hydroxide solution (NaOH) will be determined by titrating a sample of potassium acid phthalate (KHP; HKC₈H₄O₄) with the NaOH. In the second procedure the standardized NaOH will be used to determine the molarity of a hydrochloric solution (HCl).

Example 1 A 0.128 g sample of KHP (HKC8H4O4) required 28.54 mL of NaOH solution to reach a phenolphthalein endpoint. Calculate the molarity of the NaOH. HKC8H4O4 + NaOH -----> NaKC8H4O4 + H2O (0.128 g KHP)(1 mol / 204.23 g KHP ) = 6.267 x 10-4 mol KHP (6.267 x 10-4 mol KHP)(1 mol NaOH / 1 mol KHP) = 6.267 x 10-4 mol NaOH 6.267 x 10-4 mol NaOH / 0.02854 L NaOH = 0.0220 M NaOH

Example 2 A 20.00 mL sample of HCl was titrated with the NaOH solution from Example1. To reach the endpoint required 23.72 mL of the NaOH. Calculate the molarity of the HCl. HCl + NaOH -----> NaCl + H2O (0.02372 L NaOH)(0.0220 mol NaOH / 1 L NaOH) = 5.218 x 10-4 mol NaOH (5.218 x 10-4 mol NaOH)(1 mol HCl / 1 mol NaOH) = 5.218 x 10-4 mol HCl 5.218 x 10-4 mol HCl / 0.02000 L HCl = 0.0261 M HCl

Part I. Standardization of NaOH with KHP
1.    Clean a buret by rinsing with several portions (about 10 mL) of tap water. (The buret is clean enough when water droplets do not cling to the inner surface.) This rinse water can be poured down the drain.

2.    Obtain about 150 mL of the NaOH solution in a clean, dry 400 mL beaker. Cover with a watch glass.

3.    Rinse the buret with three portions (about 5 mL) of the NaOH solution. Drain each NaOH rinse and discard into the waste container located under the hood.

4.    Fill the buret with NaOH to slightly above the zero mark and clamp the buret up vertically.

5.    Remove the air bubbles from the tip of the buret by draining the NaOH into a small beaker. Read the NaOH level to within + 0.02 mL and record this value in the initial base reading on the data sheet. (It is not necessary to have the NaOH level at exactly the 0.00 mL mark. Anywhere below 0.00 mL will suffice. What is important, is to record this initial NaOH reading to two decimal places.)

6.    Weigh a clean 250 mL beaker to + 0.01 g. Add 0.6 to 0.7 g of KHP (potassium acid phthalate; HKC₈H₄O₄). Re-weigh the beaker and contents to + 0.01 g. Record the mass on the data sheet.

7.    Dissolve all the KHP in water by adding about 30 mL of distilled water to the beaker and stirring with a glass rod. (If necessary, you can warm the solution to dissolve all the solid acid.) Transfer this solution into a clean 250 mL Erlenmeyer flask. Rinse the beaker twice with about 5 mL of distilled water to make sure all of the acid has been transferred to the flask.

8.    Add 3 to 4 drops of phenolphthalein indicator to the KHP solution in the Erlenmeyer flask.

9.    Place the flask containing the acid solution and indicator under the buret. Add NaOH from the buret to the flask with swirling until the color of the solution in the flask is a faint pink. This faint pink color should last only 45 to 60 seconds. There should be a one-drop difference between when the solution is colorless and when it is pink. If too much base is added (that is, if you "over-shoot" the endpoint), discard the solution and repeat the titration. A white piece of paper placed under the flask will aid in the color detection.

10.    When the proper end point is reached, read and record the final NaOH volume to within + 0.02 mL.

11.    Discard the contents of the Erlenmeyer flask into the waste container located under the hood.

12.    Repeat the titration procedure a second time by following steps 4 - 11.

13.    Calculate the molarity of the NaOH from the two titrations. If the calculated base concentrations from the first and second titration vary by more than 0.005 M, perform a third titration.

Part II. Standardization of HCl with NaOH
1.    Refill and zero the NaOH buret. Read the NaOH level to within + 0.02 mL and record this value in the initial base reading on the data sheet.

2.    Obtain a clean, but not necessarily dry, 250 mL Erlenmeyer flask , and using the buret labeled "HCl" (located at the back of the room), transfer approximately 25.00 mL of the HCl acid solution into the flask. Read and record the initial and final acid buret readings to + 0.02 mL. It is not necessary to deliver exactly 25.00 mL of HCl into the flask. What is important, is that the volume of HCl delivered into the flask is known to two decimal places.

3.    Add 3 to 4 drops of phenolphthalein indicator to the HCl solution in the Erlenmeyer flask.

4.    Place the flask containing the acid solution and indicator under the buret and add NaOH from the buret to the flask with swirling until a phenolphthalein endpoint is reached. There should be a one-drop difference between when the solution is colorless and when it is pink. If too much base is added (that is, if you "over-shoot" the endpoint), discard the solution and repeat the titration.

5.    When the proper end point is reached, read and record the final NaOH volume to within + 0.02 mL.

6.    Discard the contents of the Erlenmeyer flask into the waste container located under the hood. Repeat the titration procedure a second time by following steps 1 - 5.

7.    Calculate the molarity of the HCl (M_HCl) acid for each titration. If the calculated acid concentrations from the first and second titrations vary by more than 0.005 M, perform a third titration. Calculate an average acid molarity using the two closest values.

8.    Before returning the buret to the lab bench, please rinse it out with a couple of water rinses.

Name________________________________________Hood No.______ Date_____________

Part I. Standardization of NaOH with KHP

	Titration 1	Titration 2	Titration 3
wt. of beaker	______________g	______________g	______________g
wt. of KHP & beaker	______________g	______________g	______________g
wt. of KHP	______________g	______________g	______________g
Initial NaOH buret reading	______________mL	______________mL	______________mL
Final NaOH buret reading	______________mL	______________mL	______________mL
mL NaOH	______________mL	______________mL	______________mL
Molarity NaOH	______________M	______________M	______________M

Average Molarity of NaOH _________________
 

Part II. Standardization of HCl with NaOH

	Titration 1	Titration 2	Titration 3
Initial NaOH buret reading	______________mL	______________mL	______________mL
Final NaOH buret reading	______________mL	______________mL	______________mL
mL NaOH	______________mL	______________mL	______________mL
Initial HCl buret reading	______________mL	______________mL	______________mL
Final HCl buret reading	______________mL	______________mL	______________mL
mL HCl	______________mL	______________mL	______________mL
Molarity HCl	______________M	______________M	______________M

Average Molarity of HCl _________________