salinity calibration fluids (Magnesium Sulfate/epsom salt)

[Hermie]

Hi, I mixed up a calibration fluid (1.0266 sg) with Sodium chloride (I assume, morton's salt), and my hydrometer is showing 1.022 sg, so I now believe my hydrometer is wrong.
However, I want to be sure that the calibration fluid I mixed up is checked against something else, so I mixed up a calibration fluid using epsom salt (Magnesium Sulfate MgSO4).

4.301 grams Magnesium Sulfate to 161 grams H2O to get the 1.02660 sg (it's 1.02480 kg/L density).

However, when I compared my MgSO4 solution readings in the hydrometer, it was something around 1.011 specific gravity, about half of the Sodium Chloride fluid's readings.

Do you think my MgSO4 may have been tainted with absorbed water? If so, could I warm up the Epsom salt to evaporate water from it and then that "purified" salt could be used in the hydrometer testing fluid?

Sorry for the wordiness here, thank you for your expertise!!!

 

[Randy Holmes-Farley Verified Community Expert]

How did you determine how much Epsom Salt to use? Note that Epsom salt is about half water by weight.

You would need to look up in a table what the density of the solution you made is, and I don't think that material is likely a good choice anyway since you cannot know the exact moisture level when dry.

I picked sodium chloride because it does not readily form hydrated crystals. That isn't true of magnesium sulfate or chloride.