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Volumetric Analysis

01Volumetric Analysis

Making a standard solution

This section covers the accurate preparation of a solution of known concentration from a measured mass of solid.

Standard solutions

Volumetric analysis can determine the concentration of an unknown solution by using measured volumes and a solution whose concentration is already known. Titration is the main technique used.

Standard solution: a solution with an accurately known concentration.

The required amount of solid can be calculated using:

n=cVandm=nMn=cV \qquad \text{and} \qquad m=nM

Here, MM is the molar mass in g mol−1\mathrm{g\,mol^{-1}}. When concentration is given in mol dm−3\mathrm{mol\,dm^{-3}}, convert volumes in cm3\mathrm{cm^3} to dm3\mathrm{dm^3} before applying n=cVn=cV.

Preparing the solution

  1. Measure the solid accurately. Use a balance reading to at least two decimal places. One suitable method is to weigh the sample bottle containing the solid, transfer the solid, reweigh the bottle and obtain the transferred mass from the difference.
  2. Dissolve the solid completely. Transfer it to a beaker, add distilled or deionised water and stir with a glass rod. If necessary, the beaker and its contents can be warmed gently until dissolution is complete.
  3. Transfer the solution. Pour it through a funnel into a volumetric flask.
  4. Transfer the washings. Rinse the beaker, glass rod and funnel with distilled or deionised water and add the rinsings to the volumetric flask.
  5. Make up to the mark and mix. Add water until close to the calibration line, use a dropping pipette for the final additions, then stopper and invert the flask several times.
1 Weighaccurately2 Dissolveand stir3 Transferand rinse4 Make to markand mix

The bottom of the meniscus should sit on the calibration line. Do not heat a volumetric flask or make a hot solution up to its mark because expansion of the flask makes the final volume inaccurate.

Exam Tip: For a complete method, name the volumetric flask, specify distilled or deionised water, state that dissolution is complete, include all rinsings, make the liquid level up accurately and finish by mixing the solution thoroughly.

Worked example: Calculate the mass of NaOH\ce{NaOH} needed to prepare 250 cm3250\ \mathrm{cm^3} of a 0.200 mol dm−30.200\ \mathrm{mol\,dm^{-3}} solution.

1. Convert the volume and calculate moles.

V=0.250 dm3V=0.250\ \mathrm{dm^3}

n=cV=0.200×0.250=0.0500 moln=cV=0.200\times0.250=0.0500\ \mathrm{mol}

2. Calculate the molar mass.

M(NaOH)=23.0+16.0+1.0=40.0 g mol−1M(\ce{NaOH})=23.0+16.0+1.0=40.0\ \mathrm{g\,mol^{-1}}

3. Find the required mass.

m=nM=0.0500×40.0=2.00 gm=nM=0.0500\times40.0=\mathbf{2.00\ g}

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