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Old 25 Sep 2009, 3:56 PM   #1 (permalink)
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sodium carbonate and HCl titration indicator?

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what indicator do you use for sodium carbonate and HCl titration?
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Old 25 Sep 2009, 4:00 PM   #2 (permalink)
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Re: sodium carbonate and HCl titration indicator?

Methyl orange
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Old 25 Sep 2009, 4:01 PM   #3 (permalink)
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Re: sodium carbonate and HCl titration indicator?

Hydrochloric Acid is a strong Acid but Sodium Carbonate is a weak (?) base, so I would use an indicator that lies within the range of roughly 3 to 6 in pH, say methyl red, methyl orange or bromothymol blue, in order of increasing pH.
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Old 25 Sep 2009, 4:05 PM   #4 (permalink)
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Re: sodium carbonate and HCl titration indicator?

But it's an acid on a carbonate, giving the equation

2HCl + Na2CO3 -> 2NaCl + CO2 + H2O

Meaning that a neutral salt is created. The CO2 might affect the pH, but I would recommend Bromothymol blue since NaCl is neutral.
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Old 25 Sep 2009, 4:12 PM   #5 (permalink)
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Re: sodium carbonate and HCl titration indicator?

but doesn't this titration have two equivalence points? i'm so confused
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Old 25 Sep 2009, 4:15 PM   #6 (permalink)
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Re: sodium carbonate and HCl titration indicator?

In that case, phenolphthalein and methyl orange.
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Old 25 Sep 2009, 4:19 PM   #7 (permalink)
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Re: sodium carbonate and HCl titration indicator?

so don't use "Bromothymol blue since NaCl is neutral."?

and you mean you can use either?
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Old 25 Sep 2009, 4:19 PM   #8 (permalink)
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Re: sodium carbonate and HCl titration indicator?

Hmmm... that seems quite weird
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Old 25 Sep 2009, 6:12 PM   #9 (permalink)
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Re: sodium carbonate and HCl titration indicator?

The longest inflexion is the one to consider. This occurs in the range of the methyl orange indicator. I suppose the acidity can be attributed to CO2 (an acidic oxide) as jetblack has pointed out.
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Old 25 Sep 2009, 8:19 PM   #10 (permalink)
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Re: sodium carbonate and HCl titration indicator?

First Step
Na2CO3 + HCl --> NaCl + NaHCO3
Phenolphthalein (pink --> colourless)

Second Step
NaHCO3 + HCl --> NaCl + CO2 + H2O
Methyl Orange (yellow --> orange)

Add the methyl orange after the solution has turned colourless.

---

This is why pH titrations are super brilliant.
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Old 25 Sep 2009, 8:32 PM   #11 (permalink)
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Re: sodium carbonate and HCl titration indicator?

If you did use a pH probe the equivalence points will be at ~8.3 and ~3.7

And if you wanted to run the titration the other way you would get

First Step
HCl + Na2CO3 --> NaCl + NaHCO3
Methyl Orange (orange --> yellow)

Second Step
HCl + NaHCO3 --> NaCl + CO2 + H2O
Phenolphthalein (colourless --> pink)
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Old 25 Sep 2009, 10:40 PM   #12 (permalink)
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Re: sodium carbonate and HCl titration indicator?

used universal in this prac in class, titrated with same chemicals as you
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Old 26 Sep 2009, 12:42 AM   #13 (permalink)
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Re: sodium carbonate and HCl titration indicator?

Thanks you guys rock! for answering my question
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