Brian - Here is the paragraph from
Randy's article :
One suitable commercial standard is made by American Marine and sold under the brand name Pinpoint. It is sold as a 53 mS/cm calibration fluid for the company's electronic salinity probe (a conductivity probe),
but it also is suitable for use in a refractometer. NOTE that this is not necessarily true of all 53 mS/cm conductivity standards. The Pinpoint fluid happens to be made to match seawater in other respects, not just conductivity, but other brands, or do-it-yourself 53 mS/cm standards, may not be appropriate to use with a refractometer because, while they have the same conductivity as 35 ppt seawater, they may not have the same refractive index.
For example, standard seawater with S=35 (35 practical salinity units, or PSU) is defined as seawater with the same conductivity as a solution made from 3.24356 weight percent potassium chloride (KCl), and that conductivity is exactly 53 mS/cm (mS/cm, or milliSiemens per centimeter, is one of the units used for conductivity). That solution, however, has a refractive index of about 1.3371, matching seawater just below 26 ppt. So do not assume that all 53 mS/cm conductivity standards are suitable for refractometer calibration.
Salifert has a product called Refracto-Check that they often give away at meetings like MACNA. It is a 35 ppt seawater refractive index standard, but it is not widely available commercially.
Scott
