sodium carbonate and HCl titration indicator?

[pasta8]

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what indicator do you use for sodium carbonate and HCl titration?

[gurmies]

Methyl orange

[Uncle]

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.

[jetblack2007]

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.

[pasta8]

but doesn't this titration have two equivalence points? i'm so confused

[gurmies]

In that case, phenolphthalein and methyl orange.

[pasta8]

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

and you mean you can use either?

[jetblack2007]

Hmmm... that seems quite weird

[gurmies]

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.

[brenton1987]

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.

[brenton1987]

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)

[Kaatie]

used universal in this prac in class, titrated with same chemicals as you

[pasta8]

Thanks you guys rock! for answering my question