Early chemists defined an equivalent weight as the weight of one substance that would react with a second to form a third. As these chemists studied matter, they realized that reactions always occurred in set proportions. Many of their reactants seemed to contribute the same level of activity, regardless of the reaction involved.
Tables of equivalent weights, based on hydrogen reactions, were assembled in the late 18th century. Hydrogen was used as the standard, being the least massive element; however, it does not react readily with many elements. Easily purified and accessible metals form oxides readily and were often used as the experimental basis for determining the equivalent values.
The gain in mass of the metal was attributed to the oxygen content of the metal oxide. This weight was measured, divided by eight, and reported as the grams of equivalent weight of hydrogen for that metal. The weight was divided by eight because oxygen reacts with hydrogen by an eight-to-one weight ratio to form water. Oxygen was seen as the chemical opposite of hydrogen. Modern chemists would agree in that oxygen participates in oxidizing reactions and hydrogen in reducing reactions.
This procedure worked well as long as the reaction was not too complex. Many metals have different oxides, as they can achieve stable compounds in more than one valence configuration or oxidation state. As chemists learned more about the nature of the reactions they were conducting, the periodic chart replaced the earlier tables.
The calculations performed using an equivalent weight table have been succeeded by the use of molar masses. Molar refers to the number of atoms available to react. The extent of the reaction is based on this number, not the mass of the reactants. A mole of atoms has 6.023 x 10 23 atoms.
The use of the hydrogen standard illustrates the difference. Water is known to contain two atoms of hydrogen per one atom of oxygen. Since oxygen has a molar mass of 16 grams per mole, while the molar mass of hydrogen is 1 gram per mole, the mass ratio is eight to one, oxygen to hydrogen. The mole ratio is two to one, hydrogen to oxygen, which reflects the actual composition.
Certain fields of chemistry have continued to use the equivalent weight in limited contexts. In acid-base chemistry, an equivalent weight is the mass of a chemical species that reacts with 1 mole of hydronium (H3O+) ions or 1 mole of hydroxide ions (OH-). In reduction-oxidation reactions, an equivalent weight is the mass of a substance that accepts or donates one mole of electrons.
In the mining industry, equivalent weights were used to describe the concentration of ore in a sample. Silver, for example, will precipitate as silver chloride from a liquid solution. The equivalent weight is the mass of silver chloride that contains 1 gram of silver metal.
Polymer chemists react long molecules with active side groups to form tough cross-linked polymers. The activity or reaction affinity may be measured in equivalent weights. Equally weighted resins will produce the same degree of cross-linking within the same polymer families.