Sentence:  The precipitate was washed twice with an 80 % saturated solution of * * f , dissolved in a small quantity of 0.1 M neutral phosphate buffer , dialyzed against cold distilled water till free from <WSD>ammonium</WSD> ions , and lyophilized using liquid nitrogen . 
Meaning of the word marked with <WSD> tags:  the ion NH4 derived from ammonia; behaves in many respects like an alkali metal ion