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In a titration of HCl against NH3OH, conductivity had increased in a linear fashion (think y=x), and before it approached the equivalence point, there was approximately 0.8mL of 'plateau'. Before logging (based on 'event' - mL of HCl titrated), I had ensured that the values were not fluctuating significantly for at least 10 seconds; the plateau appeared across 7 titrations. I can't find any literature which provides reason for this. TIA.

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    $\begingroup$ Are you adding NH4OH to an HCl solution, or HCl to NH4OH? There is some information on NH4OH being not being nearly as conductive as HCl solution, because it exists in equilibrium with NH3 + H2O. $\endgroup$ Commented Feb 28, 2023 at 14:03
  • $\begingroup$ HCL was added to NH4OH $\endgroup$
    – ja10
    Commented Feb 28, 2023 at 14:27
  • $\begingroup$ Add the graph please. $\endgroup$
    – ACR
    Commented Feb 28, 2023 at 14:43

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In an aqueous ammonia solution there is an equilibrium of

$\ce{NH3 + H2O << NH4+ + OH-}$

This solution has much lower conductivity than HCl titrant being added. HCl reacts with free OH-

$\ce{NH3 <-> NH4+ + OH- ->}$
$\ce{OH- + H+ Cl- -> H2O + Cl-}$

Net reaction:

$\ce{NH3 + H+ Cl- -> NH4+ + Cl-}$

Conductivity increases as HCl is added.

The "plateau" may be occurring in much the same manner as a weak acid titration because, essentially, ammonia buffers the reaction of HCl and OH -. The final neutralization of remaining ammonium hydroxide creates less of a net charge change

$\ce{NH4+ OH- + H+ Cl- -> NH4+ + Cl- + H2O}$

Notice there is no net charge change here.

One might conclude the "endpoint" is only there when conductivity resumes its rise as still more HCl is added.

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  • $\begingroup$ Thankyou for the response! I did find some literature which subtly suggested that would be the case, however, there were some inconsistencies between sources so I figure I would ask here. Have a great day! $\endgroup$
    – ja10
    Commented Mar 1, 2023 at 22:28

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