<strong>(<em>c</em> Preparation of 0.1 M Carbonate-free Sodium
Hydroxide</strong>
Boil 1 liter of deionized water for approximately 5 minutes in a Florence flask. Do not
fill the flask to the neck or it may boil over violently; place a stirring rod or a thermometer in the flask to prevent "bumping." When boiling is
completed, let the water cool for 10 minutes, then seal the flask with a <strong>rubber</strong> stopper. Let cool (use an ice bath to
facilitate cooling, or let sit overnight); then add about 7 ml of <strong>saturated NaOH (17 M)</strong> stored in the base hood to the
cooled H<sub>2</sub>O. Saturated NaOH is used to make a dilute solution of NaOH—<strong>not solid NaOH
pellets.</strong> This is because solid NaOH always contains some Na<sub>2</sub>CO<sub>3</sub>. We do not want
Na<sub>2</sub>CO<sub>3</sub> in our NaOH titrant solution because the equivalent molecular weight of
Na<sub>2</sub>CO<sub>3</sub> is different from that of NaOH, i.e., if a sample contains equal weights of NaOH and
Na<sub>2</sub>CO<sub>3</sub> then the amount of HCl needed to neutralize NaOH will be different from the amount of HCl used to
neutralize Na<sub>2</sub>CO<sub>3</sub>. Fortunately, nature is kind. Na<sub>2</sub>CO<sub>3</sub>
is essentially insoluble in a saturated solution of NaOH (ca. 17 M), so that the saturated liquid contains very little
Na<sub>2</sub>CO<sub>3</sub>.
After adding the saturated NaOH, <strong>shake the solution
thoroughly</strong> and allow it to cool to room temperature before standardization. The flask should be capped with a <strong>rubber
stopper</strong> when not in use to minimize the absorption of CO<sub>2</sub>. We do not use a glass stopper or store NaOH in a
volumetric flask because any stray base etches glass (dissolves silica!). As a result, a ground glass stopper can become welded to the flask, thus
ruining the flask. |