ACID – BASE TITRATION LAB file=NaOH std lab r1.doc
LAB: Standardization of a Sodium Hydroxide Solution by Titration
PURPOSE: To determine the concentration of an unknown sodium hydroxide solution by acid-base titration.
INTRODUCTION:
In this experiment you will determine the concentration of a NaOH solution by acid-base titration. Finding the concentration is called standardizing a solution. Titration is the name given to the technique of carefully measuring the volume of a solution required to react with a known amount of another reagent.
A known quantity of a solid acid (potassium hydrogen phthalate) is dissolved in water in a flask, and phenolphthalein indicator is added. The NaOH solution is added from a buret into the flask containing the acid. The acid and the base react with one another according to the equation:
During the first stages of the titration, the NaOH will be completely neutralized, and an excess of acid will remain. However, eventually there will be a point at which the NaOH neutralizes the last portion of acid. At this point, the theoretical endpoint, the acid and the base have neutralized one another exactly, and no more base should be added to the flask. The endpoint volume of base, measured from the buret, is used to determine the concentration of the basic solution.
The phenolphthalein indicator is used to determine experimentally the point at which the base has neutralized all of the acid. This is called the experimental endpoint. Phenolphthalein is colorless in acid solution. It turns pink when the acid is completely neutralized and a slight excess of base is present. In this titration, a successful endpoint is achieved if one drop of base from the buret turns the solution in the flask from colorless to pink.
Since you know the mass of potassium hydrogen phthalate (KHC8H4O4), you can calculate the number of moles of acid. At this endpoint, the number of moles of acid equals the number of moles of base. Thus, you know the number of moles of base in the endpoint volume of base, and you can calculate the concentration of the NaOH solution using the equation:
x mol/L = y mol / z L
The apparatus used to measure the NaOH solution is called a buret. A buret is a graduated glass tube with a valve at the bottom. There are three types of valves, which can be used to regulate the flow of the NaOH solution through the buret tip. The most common valve is a stopcock. You can control the rate of flow by rotating the stopcock slowly until the solution starts to come out of the buret tip. Stopcocks can be made with glass or plastic.
A liquid placed in a buret forms a curved meniscus at its upper surface. In the case of water or aqueous solutions, the meniscus is concave, and buret readings are obtained by determining the position of the bottom of the meniscus. Furthermore, the eye must be on a level with the meniscus. Remember that a buret is read from top to bottom and not bottom to top. That is, the 0 mL mark is near the top of the buret and the 50 mL mark is near the bottom.
MATERIALS:
150 mL beaker
50 mL buret
buret clamp
ring stand
2 250 mL Erlenmeyer flasks
50 mL graduated cylinder
250 mL beaker
vial
NaOH solution
Phenolphthalein solution
Potassium hydrogen phthalate
PROCEDURE:
A. Prepare a data table as shown. Record all your experimental results in the table as soon as you obtain them.
DATA TABLEMEASUREMENTS / #1 / #2
Mass of acid / ______g / ______g
Mass of acid used / ______g / ______g
Initial reading of buret / ______mL / ______mL
Final reading of buret / ______mL / ______mL
Volume of NaOH used / ______mL / ______mL
B. Label two Erlenmeyer flasks 1 and 2.
C. Weigh two separate samples of potassium hydrogen phthalate. Each sample should be between 1.00 g and 1.50 g. Be sure to record the mass of each sample. (The samples and their masses will be provided to each group by the Teaching Assistant.)
D. Pour each sample of potassium hydrogen phthalate into its appropriate Erlenmeyer flask. Dissolve each sample in 50 mL of distilled water. Make sure that all of the acid dissolves.
E. Obtain about 120 mL of NaOH solution in a 150 mL beaker. Rinse the buret with about 10 mL of this solution, and let the liquid drain through the buret tip into an empty 250 mL "waste" beaker. Repeat this procedure twice more, using new 10 mL samples of the NaOH solution.
F. Refill the buret so that the meniscus is above the 0 mark, and fasten it to a ring stand with a buret clamp. Let some of the NaOH solution run rapidly from the buret to expel all air bubbles from the tip and to bring the level of the solution down to the calibrated region of the buret. If there is a drop of solution hanging on the buret tip, remove it by touching the drop to the inside wall of the 250 mL beaker.
G. Hold a piece of white paper behind the meniscus and read the initial volume of the NaOH solution to the nearest 0.02 mL.
H. Add three drops of phenolphthalein indicator to each of the Erlenmeyer flasks, and swirl the flasks to ensure mixing.
I. Place one of the Erlenmeyer flasks under the tip of the buret. A piece of white paper placed under the flask will make it easier to see the color changes.
J. While continuously swirling the flask to ensure thorough mixing, run in NaOH solution from the buret. Initially, a pink color appears at the point where the NaOH solution comes in contact with the solution in the flask; however, this color quickly disappears. As the endpoint is approached, the color disappears more slowly. The NaOH solution should then be added drop by drop until one drop turns the entire solution in the flask pink. The pink color should remain at least 15 seconds while the solution is being swirled. The pink color indicates that enough NaOH solution has been added to neutralize all of the acid.
K. Read the final volume of the NaOH solution from the buret.
L. Refill the buret and repeat the titration using the second Erlenmeyer flask.
QUESTIONS:
1. Determine the number of moles of acid used in each titration.
2. Determine the number of moles of base used in each titration using the number of moles of acid in each trial and the equation provided above.
3. Determine the molar concentration (mol/L) of the NaOH solution obtained from each titration. Then average the results.
4. What is the concentration of a NaOH solution if 32.47 mL of it are required to neutralize 1.27 g of potassium hydrogen phthalate?
5. What is the percentage of potassium hydrogen phthalate in a mixture if 2.81 g of the mixture requires 35.61 mL of 0.152 mol/L NaOH to neutralize it?
6. Why must the buret be rinsed with NaOH before it is filled?
7. Why must air bubbles be expelled from the buret tip?
8. Why does a pink color appear, and then disappear quickly, at the point where the NaOH solution comes in contact with the solution in the flask?
9. In these titrations, how does the experimental endpoint differ from the theoretical endpoint? Explain.
10. Potassium hydrogen phthalate is often used in the standardization of basic solutions. Occasionally, the purity of this compound when used as a standard acid is slightly greater than 100%. How is this possible?
Modified from:
Newsted, Bill. Frontenac Secondary School: Kingston.
Whitman, R.L. et al. Chemistry Today 1: Laboratory Manual. 1989. Prentice-Hall Canada Inc.,: Scarborough.
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