I made the standard from the article and my refractometer was reading about 3ppt higher than it should. Here's how to make it:
To make a 3.29 weight percent sodium chloride solution, dissolve 1 teaspoon (6.20 grams) of Morton's Iodized Salt in 182 mL (182 g) of freshwater (making a total volume of about 184 mL after dissolution of the salt). This solution can be scaled up as desired.
For a rougher measurement in the absence of an accurate water volume measurement:
1. Measure ? cup of Morton's Iodized Salt (about 73.1 g)
2. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g)
4. Add 3 tablespoons of purified freshwater (about 45 g)
5. Dissolve the total salt (73.1 g) in the total water volume (2149.4 g) to make an approximately 3.29 weight percent solution of NaCl. The volume of this solution is larger than the Coke bottle, so dissolve it in another container.
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Notice added post-publication: the standards described here that use Coke bottles are subject to variation in the volume of a 2-L Coke bottle. It has recently come to my attention that such 2-L bottles can vary in total volume, and that this can lead to at least a 1 ppt error in the salinity of the standards matched to seawater salinity of 35 ppt. Standards made with accurate measurements of salt and water should still accurately match 35 ppt.]]
I have a scale so I used the weighted amounts instead of the rough estimates based on volume.