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Topic: Calculations in phenolphthalein alkalinity and total alkalinity (CaCO3 mg/L)  (Read 28107 times)

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Offline Dolphinsiu

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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*
« Last Edit: February 26, 2007, 12:54:25 PM by Dolphinsiu »

Offline Borek

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You sre there is nothing more but CO32- in the solution? Perhaps some strong base like NaOH as well?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline chiralic

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Quote
Do my calculation has problem?
(See your result Total Alkalinity = 8.49 ÷ 25/1000 = 340 mg/L) Red = wrong

Quote
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

Offline Dolphinsiu

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Thank you!

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