Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Dolphinsiu on February 26, 2007, 12:47:37 PM
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I get stucked when I calculate these two values.
I am doing an experiment to determine the alkalinity of industrial waste water.
The titrant (solution in burette) = HCl is known to be 0.0202 M.
What I do is using phenolphthalein as first indicator while methyl orange as second indicator.
Here is my result.
Volume of waste water used for each titration trials (solution in conical flask) = 25 ml
Average volume of HCl added for 1st end pt = 8.40 ml
Average volume of HCl further added for 2nd end pt
(2nd end-pt volume - 1st end-pt volume) = 19.10 ml
Total volume of HCl added for 2nd end pt = 27.50 ml
I know
Phenolphthalein indicate CO32- + H+ --> HCO3-
while methyl orange indicate HCO3- --> CO2 + OH-
But
CaCO3 + 2HCl --> CaCl2 + H2O + CO2
How can I use one chemical equation to calculate phenolphthalein alkalinity and total alkalinity (CaCO3 mg/L) ?
No. of mole of CaCO3 (s) in waste water = 1/2 × No. of mole of HCl
= 1/2 × 0.0202 × 8.40/1000
= 8.48 × 10-5 mol
Concentration of CaCO3 (s) in waste water = 8.48 × 10-5 ÷ 25/1000
= 0.00339 M
Mass of CaCO3 (s) in waste water = 8.48 × 10-5 × 100.1 × 1000
= 8. 49 mg
Total Alkalinity = 8.49 ÷ 25/1000 = 340 mg/L
Do my calculation has problem? Why I cannot calculate the phenolphthalein alkalinity ? Please *delete me*
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You sre there is nothing more but CO32- in the solution? Perhaps some strong base like NaOH as well?
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Do my calculation has problem?
(See your result Total Alkalinity = 8.49 ÷ 25/1000 = 340 mg/L) Red = wrong
Why I cannot calculate the phenolphthalein alkalinity ? Please *delete me*
Yes, You did it. Your calculations were to phenolphthalein alkalinity (P alkalinity)
The phenolphthalein alkalinity (P alkalinity) Value is ~ 339,3 mg CaCO3/L
For Total Alkalinity you must to do the same calculation, using the Total Volume (including the volume of acid used for your 1st end-point). V=27.50 mL
The Total alkalinity (MO alkalinity) Value is ~ 1111 mg CaCO3/L
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Thank you!