Standardization of NaOH and determination of Molarity

Objectives

1. Preparation and standardization of a 0.1M NaOH solution
2. To learn the technique of titration
3. Determination of the concentration of an unknown diprotic acid.

Introduction

Titration can be traced to the origins of volumetric analysis, which began in the late eighteenth century. Study of analytical chemistry began in France and the first burette was made by Francois Antoine Henri Descroizilles. The field began to spread to neighboring countries and eventually to the whole world. Titration can be defined as the gradual addition of one solution to another until the chemical amount of one reactant being added matches stoichiometrically the amount of another reactant in the solution initially present. Learning the right technique of Titration is essential because of its wide applications in all areas of science, it is used in laboratory medicine to determine unknown concentrations of chemicals of interest in blood and urine.

Pharmacists also use titration in the development of new pharmaceuticals. Titration may also be used to determine the amount of a certain chemical in food. Often, titration is used to determine fat content, water content, and concentrations of vitamins. It is also used to tell if cheeses and wines have aged enough for distribution to supermarkets and shops. Scientists of different disciplines use titrations for a variety of different reason. [1] This experiment demonstrates the most common method for obtaining standard solutions for titrimetric analysis using the very essential technique of titration.

NaOH was standardized using a primary standard KHP, by titration, this is an excellent method to prepare a secondary standard because of its high level of accuracy. KHP was used because it is of high purity, high molar mass which yields lower massing errors and reacts well with the solution it is titrated with. NaOH was the secondary standard, which was further titrated with the unknown to determine the molarity.

An indicator was used in both the titration to indicate the end point; they actually determine the degree of acidity of the solution, or state of a chemical reaction occurring in a solution. Phenolphthalein was used as an indicator in both the titrations because, it shows a very sharp and clear colour change from colorless (in acidic) to pink (in basic solutions). Moreover, its pH range is 8-10 which is fit for the experiment, thymol blue could be used as an alternative, because it has a similar range about 8.0-9.6, it is yellow in acidic and blue in basic.

Experimental Procedure
Please refer to the Laboratory Manual

Discussion

In this experiment we standardized 0.1M solution of NaOH using KHP by titration, NaOH was then titrated against H2SO4 to determine its concentration which was unknown. NaOH reacting with KHP

NaOH reacting with H2SO4
2 NaOH(aq) + H2SO4 (aq) —— Na2SO4 + 2H2O
If HCl was used the equation would have been slightly different. The reaction between HCl and NaOH, HCl + NaOH → NaCl + H2O
The different between the reactions is the ionization of the acid. HCl is monoprotic acid, whileH2SO4 is diprotic acid.A diprotic acid is an acid that yields two H+ ions per acid molecule. A diprotic acid dissociates in water in twostages:

(1) H2X(aq) —– H+(aq)+ HX-(aq)

(2) HX-(aq) —- H+(aq)+ X2-(aq)

A monoprotic acid is an acid that yields only one H+ ions per acid molecule. Which means that when HCl reacts with NaOH,it needs one mole to neutralize the NaOH, while the H2SO4 needs only ½ a mole to neutralize the same.

(It was ensured that the water used in preparation and titration was carbon- di- oxide free, because CO2 when dissolved in water would form carbonic acid. Which would result in greater acidity of the solution hence requiring more base to neutralize.) The experiment was successful since all the objectives were well achieved, and the results seem to be reliable since the titrant volumes were extremely close, and even identical in some runs, and the fade pink color (indication of the endpoint) disappeared within 30seconds for most of the runs. The molarity of sulphuric acid was determined to be 0.0525M. Conclusion

The molarity of the Sulphuric Acid was determined to be 0.0525M

Bibliography
Titration. (n.d.). Retrieved from http://chemteacher.chemeddl.org: http://chemteacher.chemeddl.org/services/chemteacher/index.php?option=com_content&view=article&id=56