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My aim is to "To determine the concentration of citric acid in lemon juice using titration technique." and as following i need to standardise the NaOH in order to titrate it with validity. Is there a procedure and step to standardising NaOH. Thank you

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You should find it in any analytic chemistry textbook, considering volumetric methods. We did so in the high school.

In short, a solution of known molar concentration of well defined acidic substance is made, that get titrated by the $\ce{NaOH}$ solution.

Typically oxalic acid $\ce{(COOH)_2 . 2 H2O}$, weighted at good scales, preparing solution $\pu{0.05 mol / l}$.

When you Google "acidimetric titration sodium hydroxide oxalic acid" , you get tons of references, from illustrating videos to PDF documents.

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    $\begingroup$ You might have used oxalic acid in high school, but potassium hydrogen phthalate (KHP) is the go to primary standard. You dry it in an oven before use. // I can't imagine a hydrate being used for serious work. $\endgroup$ – MaxW Mar 27 at 4:25
  • $\begingroup$ It frequently used in universities and lab praxis as well.Potassium hydrogen phthalate is primary standard for pH. Hydrates are used often, if they are well defined. Another example of a hydrate standard is the Mohr salt $\ce{(NH4)_2Fe(SO4)2.6H20}$ for oxidimetry as a stable source of $\ce{Fe^{II}}$ $\endgroup$ – Poutnik Mar 27 at 4:38
  • $\begingroup$ I mentioned oxalic acid as even since high school ( chemical industry oriented), not only in high school. The acid is used since a dawn of volumetric. In fact, I do not remember, if we ever used KHP for that purpose. $\endgroup$ – Poutnik Mar 27 at 5:12
  • $\begingroup$ Additionally, KHP is less suitable for the particular $\ce{NaOH}$ case, due its rather high $\mathrm{p}K_{a2} = 5.51$, compared to the $\ce{(COOH)_2}$ $\mathrm{p}K_{a2} = 1.27$ and $\mathrm{p}K_{a2} = 4.27$. As it would interefere with $\ce{H2O + CO2 <=> H+ + HCO3^-}$ ( of $\ce{Na2CO3}$ origin ) with the apparent $\mathrm{p}K_{app} = 6.35$ $\endgroup$ – Poutnik Mar 27 at 17:19
  • $\begingroup$ Using KHP and phenolphthalein works fine to standardize NaOH. For both KHP and oxalic acid you'd need to boiled distilled water to remove dissolved CO2 from atmosphere. $\endgroup$ – MaxW Mar 27 at 17:38

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