Melchizedek E. Damgo 2013 – 51946 October 1, 2014

Experiment 4 - Quantitative Analysis of Soda Ash by Double-Indicator Titration

 ANSWERS TO QUESTIONS:

1. 

Why there is a need to boil distilled water in the preparation of base solutions.

This was done in order to remove and vaporize soluble carbon dioxide present that may react

with the water and form carbonic acid which can cause a change in the pH of the solutions.

2. 

Why a mixture of NaOH and NaHCO3 incompatible.

The mixture of the two solutions is incompatible since the reaction between NaOH and NaHCO 3 

would just produce Na2CO3  and water. While these products react with the titrant, these cannot

suppress the reaction of the other components with the titrants. The amount of HCl needed for

titrating the two cannot be distinguished from one another.

3. 

Why there is a need to boil the solution before reaching the methyl orange endpoint.

This was done in order to remove the dissolved carbon dioxide and carbonic acid and to

destroy any buffer formation which might have formed in the solution. This would result to a

sharper endpoint and higher pH desired in titration processes.

4. 

The basic components of the unknown soda ash sample based on the volume relationship at

the phenolphthalein and methyl orange endpoints.

The soda ash sample can contain Na2CO3, NaHCO3, NaOH or a mixture of the three. In the

experiment Vph  was the volume of HCl used for the phenolphthalein endpoint while Vmo is the

volume of HCl needed to reach the methyl orange endpoint. The two volume readings can be used

to determine the components present in the soda ash sample. If V mo=0, then the sample only

contains NaOH. If Vph=0, it only contains NaHCO3. When the volume of the two is equal (V ph=Vmo),

only Na2CO3 is present since amount of hydroxide ions to be titrated for the phenolphthalein

indicator is the same for the methyl orange indicator. When V ph>Vmo, the sample is made up of

NaOH and Na2CO3. When Vmo>Vph, the soda ash is composed of Na2CO3 and NaHCO3.

5. 

The possibility of using NaOH as a primary standard for HCl and the properties of an ideal

primary standard.

The primary standard must be pure, must be atmospherically stable, must have a high formula

or molecular mass, and must only react in the titration system without any side-reactions. Since

NaOH has a lower molar mass and can be unstable if left for a very long time, it was not used as

primary standard.

 

 

6. 

The rationale behind not storing basic solutions in volumetric glassware.

Strong basic solutions react with the silicon present in the glass. This reaction results to the

lowering of the solution’s pH and basicity. Also, reaction between the base and a glass stopper

may cause the latter to “freeze” for a period of time. 

7.  Possible sources of errors and their effect on calculated parameters

Possible sources of errors may include but are not limited to the following: presence of

bubbles in the burette which could cause a larger volume of titrant to be recorded, the

possibility that the burette was not properly washed with the titrant which could result to an

incorrect molarity reading, and the absence of boiling the distilled water prior to the

experiment and the solution before the methyl orange endpoint.

8. 

The cause of carbonate error and its effect on the calculated values obtained in

standardization and sample analysis.

A carbonate error is a result of the contamination of the system with carbonic acid (formed

from dissolved carbon dioxide) which lowers the whole system’s pH value. The volume required for

the standardization was decreased because of the presence of more hydrogen ions. This would

result to a higher molarity of HCl recorded and a lower (more acidic) pH for the system.

REFERENCES

[1] Skoog, D.A., West, D.M., Holler, F.J., Crouch, S.R. Fundamentals of Analytical Chemistry   (8th ed.).

Brooks/Cole, Canada. 2004.