Can any body tell me why NAOH cant considered as a primary standard? please provide an equation to the answers thanks

1 Answer
Aug 9, 2017

Answer:

For a start, #"sodium hydroxide"#, #"caustic soda"#, is highly #"deliquescent"#.

Explanation:

And WTF does #"deliquescent"# mean? It means it absorbs atmospheric moisture VERY readily. You can take some caustic out of the tin, and you can actually see the surface of the caustic become shiny due to adsorption of atmospheric moisture in real time......if you leave it long enuff (only approx.20-30 minutes), the salt will dissolve completely......

And thus clearly, if I measure out a sample of caustic soda, I measure (i) the mass of the reagent, (which I want to do), and ALSO (ii) the mass of the adsorbed water, the which I don't want to do. The result is that the mass I measure is not STOICHIOMETRIC in #NaOH# or #KOH#. Do you appreciate what I am saying?

And a second factor that militates AGAINST the use of caustic soda as a primary standard is its formula mass. #NaOH# and #KOH# have formula masses of #40.0*g*mol^-1#, and #56.1*g*mol^-1# respectively. A small mass, #100-200*mg# thus represents a fairly large molar quantity, which requires LARGE volumes of titrant.

Contrast this with a widely used primary standard, #"KHP"#, #"potassium hydrogen pthalate"#, #C_8H_5KO_4#, with a formula mass of #204.22*g*mol^-1#. As a bonus, the salt is air stable and not deliquescent. If I accurately weigh out a #250*mg#
mass I have a molar quantity of approx. #1*mmol#. Now I can titrate this with EITHER acid or base, and stoichiometric equivalents of #0.2-0.3*mol*L^-1# titrant represents about a volumes of #20-30*mL# titrant, which is just about the right volume for accuracy, precision, and convenience.

And the equation for #"KHP"# as an acid........

#1,2-C_6H_4CO_2^(-)CO_2H(aq) + NaOH(aq)rarr1,2-C_6H_4{CO_2^(-)}_2*Na^+ + H_2O(l)#

And #"KHP"# as a base.................

#1,2-C_6H_4CO_2^(-)Na^(+)CO_2H(aq) + HCl(aq)rarr1,2-C_6H_4{CO_2H}_2+NaCl(aq)#

Are you happy with this?