Quantitative Analysis of Soda Ash by Double Indicator Method

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Quantitative Analysis of Soda Ash by Double Indicator Method

Results and Discussion

The experiment focuses in the use of double indicator titration in quantitative analysis of soda ash sample. Titration refers to the process of measuring the volume of titrant required to reach the equivalnece point. This method of analysis, is based on the chemical reaction

aA + bB → products (1)

where a is the molecules of the unknown substances to be analyzed or the analyte. Analyte reacts with b molecules of B that is the titrant. The titrant is the standard solution which is added from a buret to react with the analyte until the amount of T added equals the amount of A, which is when equivalence point is reached. It is determined through a sudden change in physical property of the solution, such as indicator that changes color in response to appearance of excess titrant. That is however called the end point which is desirable to be as close as possible to the equivalence point.

In this experiment, the analyte was the soda ash sample and the titrant was the HCl which was standardized by a primary standard of known purity, the sodium carbonate. Standardization is the process in which the concentration of solution is accurately determined.

A sample of sodium carbonate with 99.9% purity was dissolved in boiled water and titrated with HCl.

Then, a sample of soda ash was dissolved in boiled water and then, titrated. In a this reaction of HCl with carbonate, the following reactions will take place:

CO32- + H+ → HCO3-(2)
HCO3- + H+ ← H2CO3 (3)

Carbonate ion in aquaeous solutions act as base and is able to accept a proton to form a bicarbonate ion. Bicarbonate ion, upon addition of more HCl then combine to another proton to form carbonic acid.

Solution mixture of the reaction (2) is alkaline while solution mixture in reaction (3) is acidic. In this case, the whole titration should have two breaks in the curve. This is where the use of double indicator is needed. Acid-Base indicators are dyes that are themselves weak acids and bases.  However, the conjugate acid-base forms of the dye have different colors. The Bronsted Lowry equation for indicator is:

HIn + H2O → H3O+  + In-(4)

At a low pH, an indicator is almost entirely in the HIn form. As the pH increases, the intensity of the colour of In- increases as the equilibrium shifts to the right. Different dyes will change color at different pH's as shown in the table below.

Table 1. Colors of indicators in different pH levels
Indicator
pH range
color
Phenolphthalein
pH > 10
Pink

8.3 > pH >10
Faint pink

pH < 8.3
Colorless
Methyl orange
pH > 4.5
Yellow

3.1< pH < 4.5
Near orange

pH < 3.1
Red

When conducting a titration, one must select the proper indicator so that its pH range will match the equivalence point of the titration. Also, you must use an indicator that changes color obviously, so that it can be detected easily.  This is why phenolphthalein and methyl orange were used in the experiment.

Graph 1. End points of HCl with CO32-and HCO3-

In this graph, it could be inferred that reaction (2) can be indicated by phenolphthalein while reaction (3) can be indicated by methyl orange.

On another note, boiling the water was necessary to remove dissolved carbon dioxide which is present in all water. CO2 gas dissolved in water forms small amounts of H2CO3, or carbonic acid. Carbonic acid will alter the pH of the water, making it more acidic and thus, it would affect the accuracy of titration process.

Table 2. Standardization of 0.050 M HCl
Trial
Weight (g)
VHCl(L)
1
0.1000±0.0002
0.4450±0.05
2
0.1002±0.0002
0.4500±0.05
3
0.1002±0.0002
0.4050±0.05

Table 2 shows the data obtained in the three trials of standardization of 0.050 M HCl. In reacting with the acid, the sodium bicarbonate neutralizes it, and the pH becomes less acid. When all the acid reacts with bicarbonate, the pH ought to be...
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