1. why is it necessary to use boiled and cooled water in the dilution of the carbonate sample?
2. an aliquot of the diluted sample is titrated with some standard HCl until near the equivalence point, then it is boiled for 2-3 minutes. why should we boil it?
3. any method to test the completeness of the precipitation of the sample with BaCl2?
1. Water is boiled to assure CO2 is not dissolved in it, then cooled to room temperature.
2. The water near the equivalence point is saturated with CO2 and this causes the end point to be somewhat obscured (at least it isn't very sharp.) By boiling, the CO2 is expelled the pH rises, the indicator returns to the basic color of the indicator, and the final few drops of HCl titrant gives a sharp end point.
3. I don't understand what you want with this part.
1. It is necessary to use boiled and cooled water in the dilution of the carbonate sample to ensure the removal of any dissolved gases, such as carbon dioxide (CO2). Boiling the water causes the dissolved gases to be released, as they have lower solubility in hot water. Cooling the water after boiling helps to prevent any further absorption of gases from the surrounding air. By using boiled and cooled water, we can eliminate the interference of dissolved gases in the subsequent analysis.
To achieve this, you can follow these steps:
a) Bring the water to a boil by heating it until bubbles start to form and rise to the surface.
b) Allow the water to boil for a few minutes to ensure complete gas release.
c) After boiling, let the water cool down to room temperature by leaving it undisturbed.
2. Boiling the aliquot of the diluted sample after titration with standard HCl helps to drive off any excess carbon dioxide, which may be present due to the reaction between HCl and carbonates. The boiling process facilitates the dissolution of excess CO2, ensuring that only the desired products, such as water and salts, remain. This step is crucial in accurately determining the amount of carbonate present in the sample.
To carry out this step:
a) Place the titrated aliquot containing the diluted sample in a heat-resistant container, such as a beaker or flask.
b) Heat the container with the sample over a heat source, such as a hot plate or Bunsen burner.
c) Allow the sample to boil for 2-3 minutes, ensuring continuous boiling throughout the process.
d) After boiling, cool the sample to room temperature before proceeding with further analysis.
3. One common method to test the completeness of the precipitation of the sample with BaCl2 is to add excess BaCl2 to the solution and check for the absence of any further precipitation. This method relies on the fact that barium carbonate (BaCO3) is insoluble in water, and any remaining carbonate ions will react with additional BaCl2 to form more precipitate. If no further precipitation occurs, it indicates that all carbonate ions have been completely precipitated as BaCO3.
To perform this test:
a) After the initial precipitation with BaCl2, add a few extra drops of BaCl2 solution to the sample.
b) Observe the solution for any signs of further precipitation, such as the formation of a white precipitate.
c) If no additional precipitate forms, it suggests that the precipitation of the carbonate sample with BaCl2 is complete.
It is important to note that this method provides an indication but not absolute proof of completeness. For more precise results, quantitative analysis techniques, such as gravimetric or titrimetric methods, may be employed.