Campbell OBS-5+ Operator's Manual page 48

Appendix A. Turbidity Standards
A-2
In the USA, formazin is a primary standard for the calibration of
turbidimeters. The median particle size of formazin is 1.5 μm; the standard
deviation of size is 0.6 μm (see size distribution graph); and as shown by the
SEM images below, formazin particles have many different shapes. The
preparation, storage, and handling of formazin will affect its accuracy and
stability. Recommended formazin storage times are listed in the accompanying
table. Working standards are prepared by volumetric dilution of 4000-NTU
stock formazin with distilled water. For example, a 2000 NTU calibration
standard is made by mixing equal volumes of stock formazin and distilled
water.
Turbidity Maximum
(NTU) Storage Time
1 – 10 1 day
2 – 20 1 day
10 – 40 1 day
20 – 400 1 month
> 400 1 year
Besides being the primary standard, formazin has two other advantages. It is
available from several chemical and scientific suppliers (www.vwrsp.com,
www.ColePalmer.com, www.riccachemical.com, and www.labchem.net) and it
is the least-expensive, commercially available standard. Formazin also has a
couple of disadvantages: 1) it has a MSDS health-hazard rating of 2;
2) turbidity can vary by ± 2% from the lot to lot; 3) the size, shape, and
aggregation of formazin particles change with temperature, time, and
concentration; 4) it settles in storage and must be mixed immediately prior to
use; and 5) dilute formazin standards have a storage life as short as one hour.