Alternatively, heat for 30 min in an autoclave or pressure cooker at 98 to 137 kPa. Cool, add 0.05 mL(I drop) phenolphthalein indicator solution. and neutralize to a faint pink color with NaOH. Make up to 100 mL with distilled water. In some samples a precipitate may form at this stage, but do not filter. For any subsequent subdividing of the sample, shake well. The precipitate(which is possibly a calcium phosphate) redissolves under the acid conditions of the colorimetric reactive phosphorus test.