Standard solutions of sodium and potassium hydroxide must be free from carbonates. Sodium hydroxide solution may be prepared sufficiently free from carbonates to be suitable for use with phenolphthalein by dissolving say 100g in an equal weight of water, allowing the sodium carbonate to settle out and pipetting off the clear solution before diluting for use. To standardise with this benzoic acid, weigh off a suitable quantity (e.g. 1.8318g*  30.0ml M/2 NaOH or 1g  55.15ml M/6.735 NaOH), dissolve in 20 – 30ml alcohol (industrial alcohol may be used if previously neutralised with M/2 NaOH using phenolphthalein indicator), add a few drops of 1% phenolphthalein in alcohol and titrate. For work of the highest accuracy the solution should be in a flask that has been swept free from carbon dioxide (see Hillebrand, Lundell, Bright and Hoffman, “Applied Inorganic Analysis”, 1953, p.181).

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