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Two most important solutions used in alkalimetric titrations are solution of hydrochloric acid and solution of sodium hydroxide. Both are used as titrants, and in some analytical procedures requiring back titration they can be both used as a first reagent added in excess.
Sodium hydroxide solution
Sodium hydroxide solutions are unstable - they tend to adsorb atmospheric carbon dioxide which changes their concentration. This is not always the problem, as sodium carbonate is a base strong enough to not interfere with neutralization of strong acids. If kept in sodium glass bottles sodium hydroxide solutions can dissolve glass and corrode ground-glass stoppers, so it is better to use plastic or rubber stoppers for the solution storage. For the same reason it is better to not use burettes with glass stopcocks.
Commonly used concentrations of sodium hydroxide are 0.1M and 0.2M.
Please note, that preparation of the sodium hydroxide solution free of carbonates requires additional actions. One of the suggested procedures calls for quick rinsing of NaOH granules with water, that dissolves carbonate usually present on the granules surface. Other procedure calls for addition of barium hydroxide or barium chloride, and subsequent filtration of BaCO3 precipitate.
To prepare the recipe for a needed volume of the solution use ChemBuddy concentration calculator. Download the sodium hydroxide solution preparation file. Open it with the free trial version of the concentration calculator. After opening the file enter solution volume and click on the button.
Hydrochloric acid solution
Hydrochloric acid solutions are stable and can be kept indefinitely. Solutions of known concentration can be used instead of solid standard substance for determination of NaOH concentration. Reverse procedure - that is use of the sodium hydroxide solution for hydrochloric acid standardization - although possible, is rarely used, as sodium hydroxide solutions are less stable.
Hydrochloric acid solutions are prepared by dilution of the concentrated stock acid. Stock acid - if prepared by distillation at constant temperature and known pressure - has a concentration that can be read from tables with ±0.01% accuracy. However, in normal lab practice it is much easier to start with just a stock solution which is ca 37% w/w and then to standardize solution against one of the standard substances used in alkalimetry. Commonly used concentrations of hydrochloric acid are 0.1M and 0.2M.
To prepare the recipe for a needed volume of the solution use ChemBuddy concentration calculator. Download the hydrochloric acid solution preparation file. Open it with the free trial version of the concentration calculator. After opening the file enter solution volume and click on the button.
Other solutions used in acid-base titrations are KOH, Ba(OH)2 and H2SO4, but they are much less popular. Ba(OH)2 solution is always carbonates free, but it has to be filtered and standardized before use.
Page was last modified on April 30 2009, 14:30:09.