Acid-Base Titration
Introduction: A titration is a process of using a solution of known concentration to determine the concentration of an unknown solution. An indicator is a dye whose color is sensitive to pH. There are many different indicators that change color at various points on the pH scale. The indicator used in this lab will be bromothymol blue. Bromothymol blue is yellow in an acid, blue in a base and green in a neutral solution. Because the purpose of this lab is to neutralize a base, when the solution is green, the equivalence point - the point when there are equal amounts of moles of the acid and base - has been reached.
Safety: HCl and NaOH are corrosive. Avoid skin and eye contact. Neutralize spills and rinse with plenty of water. Goggles and aprons must be worn.
Objective: Students will determine the concentration of an unknown solution by performing a titration process with a standard HCl solution.
Materials:
2 burets
Erlenmeyer flask, 125 mL
buret clamp
HCl solution
wash bottle
NaOH solution
150 mL beakers (3)
bromothymol blue indicator
ring stand
disposable pipet
Procedure:
1. Record the concentration of the standard HCl solution.
2. Place approximately 60 mL each of hydrochloric acid and sodium hydroxide solutions in clean, dry labeled 150-mL beakers. Label the third beaker “discard.”
3. Fill one buret with sodium hydroxide solution above the zero mark. Withdraw enough solution to remove the air from the jet tip and bring the liquid level to the graduated region of the buret.
4. Repeat steps 3 using the acid and the other buret.
5. Record the initial reading of each buret in the Data Table, estimating to the nearest 0.01-mL. Release about 10-mL of HCl into an Erlenmeyer flask and add approximately 25-mL distilled water to increase the volume. Add 10 drops of bromothymol blue solution as an indicator.
6. Slowly add the NaOH from the buret to the Erlenmeyer flask, mixing constantly with a swirling motion. Wash down the sides of the flask occasionally with a little distilled water from the wash bottle.
7. When the color begins to change at the point of contact with the NaOH, add the sodium hydroxide drop by drop, swirling the flask after each addition. A sheet of white paper under the flask makes color changes easier to detect. Continue adding the NaOH until a faint green color remains after swirling for at least 30 seconds.
8. If the solution turns blue, add HCl one drop at a time until the solution turns back to green.
9. Read the final volume in each buret and record in the Data Table.
10. Clean and rinse all apparatus at the end of the experiment. Discard all excess solutions down the drain with plenty of water. Wash you hands with soap before leaving the laboratory.
Data:
DATA TABLEBuret Readings / (mL)
Hydrochloric / Acid / Sodium / Hydroxide
TRIAL / initial / final / initial / final
1
Known molarity of HCl ______
Calculations and Conclusions:
1. Write the balanced equation for the neutralization of sodium hydroxide and hydrochloric acid.
2. Calculate the number of mL of HCl used in the titration.
3. Calculate the number of mL of NaOH used in the titration.
4. Calculate the molarity of the NaOH used in the titration.
Ma∙ Va∙ #H’s = Mb∙ Vb ∙ #OH’s
5. Find the percent error of your titration.
6. Why may distilled water be added without disturbing the titration process?
7. IN YOUR OWN WORDS, define the following terms:
a. Titration
b. Indicator
c. Equivalence Point