c. Colorimetric equipment: One of the following is required
1) SPECTROPHOTOMETER, for use at 400 to 425 m/l, providing a light path of 1 cm or longer.
2) FILTER PHOTOMETER, providing a light path of 1 cm or longer and equipped with a violet filter having maximum transmittance at 400 to 425 mμ.
3) NESSLER TUBES, matched, 50 - ml tall form.