ief
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Mixing sodium hydroxide and DL lactic acid
When we mix sodium hydroxide and Lactic acid, if the solution gets too hot (above 32 C) it causes the pH of the solution to go to pH 12.
Is there a method anyone has found to keep the heat cool enough to make the solution without causing the pH to skyrocket_
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nezza
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Lactic acid is a weak acid. Adding Sodium hydroxide will neutralise the acid and form sodium and lactate ions. The sodium salt of lactic acid will in
any care react alkaline because of hydrolysis and any excess sodium hydroxide will cause the pH to skyrocket. The heat evolved is basically due to the
neutralisation of the acid.
Lactic acid <> Lactate + H+ (equilibrium largely to the left)
Sodium Lactate + H20 <> Lactic acid (undissociated) + Na+ +OH-
Most organic acids react similarly.
If you're not part of the solution, you're part of the precipitate.
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Alice
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Your assumption is false. pH doesn't skyrocket from temperature. If you don't want the temperature to skyrocket, add NaOH slowly (and use the right
amount). Remind a solution of sodium lactate will have a basic pH, not neutral.
EDIT: Of course there are other ways reducing the temperature, like precooling both liquids and/or placing the lactic acid in an ice bath during
addition. Using more dilute solutions. Measure the temperature during addition, so you always know if you should add NaOH more slowly. For this
example it doesn't matter that much if it's getting hot, but for other cases it might be damaging to the chemicals if it becomes too hot.
[Edited on 23-5-2017 by Alice]
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gdflp
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Thread Moved
23-5-2017 at 06:02 |
LearnedAmateur
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pH measures the concentration of H+ ions in a logarithmic format, it is independent of temperature when dealing with strong acids/bases due to the
fact they completely dissolve. Of course, a higher temperature will drive the dissolution of weak acids (your lactic acid for example) thus decreasing
the pH, but in the presence of NaOH it will immediately neutralise.
All in all, your high pH is indicative of excess hydroxide - try the reaction in stoichiometric quantities assuming you have relatively pure reagents.
Like Alice said, a 1:1 ratio of lactic acid and NaOH will have a slightly basic pH (7-8), and if you're worried about temperature then adding the
hydroxide solution dropwise from a pipette and/or carrying out the reaction in an ice water bath will be perfect for such a simple reaction.
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DraconicAcid
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Also, if your lactic acid is relatively concentrated, a significant portion of it will be in the form of a cyclic diester, which reacts with base
quite slowly.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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