Preposition: from

• acid: The ammonia molecule picks up a hydrogen ion from the acid and attaches it to the lone pair on the nitrogen.

Converse of object

• remove: An ammonia molecule removes a hydrogen ion from the -NH 3 + group in a reversible reaction.

• accept: Base: a substance that can accept hydrogen ions in solution.

• allow: At high hydrogen concentrations the microorganism would allow hydrogen ions to leak through the cell membrane.

Adjective modifier

• free: Urease breaks down urea to form ammonia and free hydrogen ions which raise the pH of the urine making it alkaline.

Modifies a noun

• concentration: Hydrogen ion concentration pH units 6.5 ( minimum ) 9.5 ( maximum ) 13.

• content: The pH scale is the negative logarithm of the hydrogen ion content of water.

• increase: With sustained muscle contraction hydrogen ion concentration increases and a general PH decreases ( Tesch et al, 1977 ).

Preposition: in

• solution: There are two components to total soil acidity: Active acidity due to hydrogen ions in solution in the soil.