Single crystals or sulphamic acid were grown from aqueous solution. The occluded and adsorbed water of the crystals prepared was measured, and the purities of the single crystals were determined by precise coulometric titration. Large single crystals more than 10 g in weight were readily prepared. The adsorbed water on the surface amounted to ca. 0.0004%. The purities of the small and the medium crystals were not satisfactory for primary standard use. However, the purity of the pieces obtained by dividing a large crystal and polishing the surfaces, was about 99.98%, with a standard deviation of about 0.015%. These large single crystals of sulphamic acid are recommended as a primary standard.