O + CO
It is important to note that the stoichiometry between HCl and Na
is a 2 to 1 ratio.
Today, the indicator phenolphthalein (pH range: 8.3~10) and bromocresol green (BCG, pH
range: 3.8~5.5) will be used. BCG changes from blue to green, and then yellow. The color at the
end-point however is
. If you titrate to the yellow color, you have gone too far. When
reacts with HCl, carbon dioxide (CO
) is produced. This will give us an incorrect
endpoint. To correct this, we must remove the CO
by boiling the solution when it first starts to
turn green. The solution will turn back to blue and then can be titrated to the correct endpoint.
Standardization of HCl
: The TA will provide you a fresh known number of moles of
standardized HCl solution.
Preparation of soda ash unknown
: Soda ash unknown has been dried in the oven for at
least two hours before the lab starts. Bring one
clean and dry
weighing bottle to obtain
soda ash unknown from TA, and keep in the desiccator.