6.6 - Standard Solution

Meaning and Preparation from a Solid

• A standard solution is a solution whose concentration is accurately known.
• The solute mass and final solution volume must be measured accurately.

Preparing $250\mathrm{c}{\mathrm{m}}^3$ of $1.0\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ $\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3$:

1. Calculate the required moles using $n=MV/1000$, then calculate and accurately weigh the required mass.
2. Dissolve the solid completely in about $100\mathrm{c}{\mathrm{m}}^3$ distilled water in a beaker.
3. Transfer through a funnel into a $250\mathrm{c}{\mathrm{m}}^3$ volumetric flask.
4. Rinse the beaker, glass rod and funnel with distilled water; transfer every washing into the flask for quantitative transfer.
5. Add distilled water close to the calibration mark. Use a dropper until the bottom of the meniscus is at the mark at eye level.
6. Stopper and invert the flask several times to produce a homogeneous solution.

• Sodium hydroxide is unsuitable for preparing a standard solution directly because it is hygroscopic and absorbs $\mathrm{C}{\mathrm{O}}_2$ to form $\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3$, preventing an exact mass from being obtained.

• Solid oxalic acid, ${\mathrm{H}}_2{\mathrm{C}}_2{\mathrm{O}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}$, and sodium carbonate are suitable substances.

• Sodium carbonate solution is alkaline because carbonate ions react with water to produce hydroxide ions.

$\mathrm{C}{\mathrm{O}}_3^{2-}(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to \mathrm{HC}{\mathrm{O}}_3^{-}(\mathrm{aq})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{aq})$

Preparation by Dilution

• Dilution adds water to a stock solution to lower its concentration.
• The solute amount does not change:

$n_1=n_2$

$M_1{V}_1=M_2{V}_2$

Diluting sodium carbonate: prepare $100\mathrm{c}{\mathrm{m}}^3$ of $0.2\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ solution from $1.0\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ stock solution, using $V_1=(0.2\times 100)/1.0=20\mathrm{c}{\mathrm{m}}^3$.

Dilution procedure:

1. Calculate the stock-solution volume, $V_1$.
2. Rinse a pipette with stock solution, then pipette exactly $V_1$ into a volumetric flask.
3. Add distilled water until near the calibration mark, then add dropwise to the mark.
4. Stopper and invert several times until homogeneous.

• Rinse the pipette with stock solution so residual water does not dilute the measured portion.
• Do not blow out the final liquid remaining in a volumetric pipette; its calibration accounts for that residue.
• A beaker is unsuitable for setting the final volume because it is not calibrated precisely.