Experiment 14   Standardization of Sodium Hydroxide Solution

 

 

PRIOR READING       

Be sure to read and understand the material in chapter 4; stoichiometry and preparation of solutions

 

 

Introduction

Sometimes the exact concentration of a solution must be known.  Due to many factors it is very difficult to prepare a solution of exact concentration the way we did it in Experiment 13- Preparation of Solutions experiment.  Some of these factors include the difficulty of ensuring the sample’s purity.  This may be due to absorption of water by hygroscopic substances or other types of contamination.  In this experiment we will accurately determine the concentration of the Solution 3 prepared in Experiment 13.  

 

Volumetric analysis is one of the most useful analytical techniques to accurately determine the concentration of unknown substances.  It is fairly rapid and very good accuracy can be obtained.  A volumetric analysis is typically performed by conducting a titration.  In a titration, the substance to be tested or analyzed is reacted with a reagent of known concentration.

 

 

 

A titration must always involve the use of a standard solution, that is, a solution whose concentration is known exactly. A standard solution is prepared by dissolving an accurately weighed quantity of a highly pure material called a primary standard and diluting to an accurately known volume in a volumetric flask. 

 

A primary standard should fulfill the following requirements:

 

1)  It should be100.00% pure, although 0.01-0.02% impurity is tolerable.

2)     It should be stable to drying temperatures, and it should be stable indefinitely at room temperature. The primary standard is always dried before weighing.

3)     It should have a high formula weight. A high formula weight will reduce experimental error since a relatively large amount of it will have to be weighed in order to get enough moles to perform the titration and, the relative error in weighing a greater amount of material is smaller than for a small amount.

 

In order to perform a titration certain criteria must be met between the unknown solution and the primary standard.  The requirements of a titration are:

1)  The reaction between unknown and standard should be stoichiometric.  This means there should be a well defined and known reaction between the two.

2)  The reaction should be rapid.

3)  There should be no side reactions, and the reaction should be specific.  If there are interfering substances, they must be removed.

4)  There should be a marked change in some property of the solution when the reaction is complete.  This may be a change in the color of the solution or in some electrical or other physical property of the solution.  A color change is usually brought about by addition of an indicator whose color is dependent on the properties of the particular solution.

5)  The point at which stoichiometric amounts of each reagent have reacted is called the equivalence point.  The point at which the reaction is observed to be complete is called the end point.  The end point should coincide with the equivalence point.

6)  The reaction should be quantitative which means the reaction should proceed to completion.

 


 

 

Prelaboratory Questions 14 Standardization of Sodium Hydroxide Solution

 

The following questions involve the reaction of sodium hydroxide and potassium acid phthalate.  The balanced equation for the reaction is

 

NaOH(aq) + KHC8H4O4(aq) ® NaKC8H4O4(aq)  +  H2O(l)

 

1.  A student weighs 5.678 g of KHP and dissolves it in water in a 250. mL volumetric flask.  A

25.06 mL aliquot of this KHP solution is withdrawn and placed in a 250 mL Erlenmeyer flask to which 5 drops of the acid-base indicator phenolphthalein is added.  This solution is titrated with a sodium hydroxide solution of unknown molarity.  26.64mL of sodium hydroxide solution is required to reach the endpoint.  Calculate the molarity of the sodium hydroxide solution.

 

2.  A chemist is asked to standardize a sodium hydroxide solution whose molarity is known to be approximately 0.l M.  She has available a dry sample of potassium acid phthalate, a 250 mL volumetric flask, a 25.00 mL volumetric, a buret, and standard laboratory equipment.  Explain exactly how she will perform this standardization.  Your explanation should include the calculations showing how much KHP should be weighed out as well as a description of the procedure.  Assume that approximately 25ml of sodium hydroxide solution should be used.

 

 

 

 

Experimental Procedure

You will perform a titration to determine the concentration of a sodium hydroxide solution (Solution #3) prepared by one of your colleagues last week.  The primary standard you will use will be potassium acid phthalate, KHC8H4O4, commonly known as, KHP.  The indicator you will use will be phenolphthalein, an acid-base indicator which is colorless in acid solution (such as the KHP solution) and pink in basic solution (such as the NaOH solution).  The equivalence point and end point of the titration will be when the solution turns a faint pink color which last for 60 seconds.

 

You will prepare a solution of your primary standard, KHP, by weighing out an appropriate amount of KHP and dissolving it in water in a 250 ml volumetric flask.  For each titration you will withdraw aliquot of the KHP solution, add 5 drops of phenolphthalein, and titrate it with the sodium hydroxide solution.

 

You need to determine the amount of KHP you will use to make your solution.  Make sure that your initial mass of KHP is enough to perform about ten titrations.  In deciding the amount of KHP to weigh, assume you will be using approximately 25 ml of the sodium hydroxide solution of unknown concentration.  You need to perform at least three titrations in which the calculations of the molarity of the sodium hydroxide solution agree within + 0.002M.

 

 

 

Data and Analysis 14:  Standardization of Sodium Hydroxide Solution

 

Primary Standard (KHP) Preparation

In your notebook, record: 

 

Mass of KHP used                  

 

Molar mass of KHP                 

 

Moles of KHP                          

 

Volume of KHP solution made

 

Molarity of KHP                      

 

 

Unknown Solution #_______

                                               

Complete 3 titrations and record your data directly into your notebook: 

 

Volume of KHP aliquot used  

 

Moles of KHP used                 

 

Moles of NaOH                       

 

Initial buret reading              

 

Final buret reading               

 

Volume of NaOH used

to endpoint                             

 

Molarity of NaOH solution     

 

(If more titrations are performed add additional pages)

 

 

 

Questions Exp 14:  Standardization of Sodium Hydroxide Solution

 

1. In the standardization of an unknown sodium hydroxide solution, 0.256g of KHP react with exactly 22.62 ml of the unknown sodium hydroxide solution.  What is the molarity of the unknown solution?

 

2. If you wish to react 24.50ml of 0.62M NaOH with KHP, how many grams of KHP will be required?

 

3. If a sodium hydroxide solution is prepared to be 0.200M, can you be sure that it is exactly this concentration without standardizing it?  Explain your answer.