Measure and dilute without losing track

Explanation

Dilution adds solvent without changing the amount of solute transferred from the stock. If stock concentration is C1 and the transferred volume is V1, that amount equals the final concentration C2 multiplied by final volume V2. Use compatible concentration units and the same volume units on both sides.

To prepare 10 cm3 of 0.2 mol dm-3 solution from a 1.0 mol dm-3 stock, V1 = (0.2 x 10)/1.0 = 2.0 cm3. Transfer 2.0 cm3 stock and make the total volume up to 10.0 cm3 with solvent. In the simple volume-additive model used here, that is 8.0 cm3 solvent. Adding 10 cm3 solvent would give a different final volume.

A serial dilution applies a dilution factor repeatedly. Mixing one volume of a well-mixed solution with nine volumes of solvent reduces concentration tenfold. Two such steps give one hundredth of the original concentration, not one twentieth. Mix each stage before taking the next aliquot and use suitable clean tips or apparatus to limit carryover.

Choose apparatus whose range and graduations suit the volume. A micropipette should be used within its specified range with an appropriate tip; a measuring cylinder is not automatically suitable for a very small volume. Record measurements to the precision supported by the instrument, include units and avoid inventing extra decimal places.

A calibration or zero error can shift every result in the same direction. Repeating measurements may reveal random variation but will not correct that bias. Check calibration where appropriate. For microscopy, a graticule must be calibrated for the objective in use; changing magnification requires the correct calibration, not reuse of an arbitrary scale.