Salinity, Refractor, and Calbration

[oldschoolgoods]

Ok, here’s a tell of a possibly crappy product. The link above says it is 35 ppt and literally on both the label and the advertisement it says it is 53 us/cm. lol

Should be 53 mS/cm.

Having your actual product label (not just an ad typo) off by a factor of 1000 does not inspire any confidence whatsoever.

The bottle I have does say 53 ms/cm. But I do see in the Amazon pic it says us/cm. So that’s weird

 

[Randy Holmes-Farley Verified Community Expert]

I’m using this one. I am willing to upgrade. I see a lot of mixed reviews on digital so I would prefer old school

https://a.co/d/0bmOtdOz

I’d recommend a diy salinity standard for that one.

 

[oldschoolgoods]

I’d recommend a diy salinity standard for that one.

I can understand that. Is there a better refractor you can recommend. It’s not about money it’s about getting everything proper.

 

[Randy Holmes-Farley Verified Community Expert]

I can understand that. Is there a better refractor you can recommend. It’s not about money it’s about getting everything proper.

Which can be done easily with a proper standard. A “better” device is not needed.

Here’s the recipe:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/

Refractometer Standard

It is widely believed that only pure water is required to calibrate refractometers. That fact is true of some refractometers (known as true seawater refractometers; if it doesn’t say it is one, it likely is not), and it may be acceptable for routine calibration, but even for a true seawater refractometer, it assumes that they were manufactured correctly and have not been damaged since manufacturing. As refractometers used by aquarists become less and less expensive due to mass manufacture, there is every reason to believe that at some point they will no longer be accurate enough.

The only way to be sure that a given refractometer gives useful information is to check its accuracy in a solution similar to aquarium water. I believe that all refractometers should be calibrated or at least checked in this fashion when first purchased, and again any time there is a reason to be concerned. For example, an aquarist might be concerned if an aquarium that had been running for years at a salinity of 35 ppt suddenly reads 39 ppt.

In order to provide a standard for refractometers, a solution whose refractive index is similar to normal seawater is required. Seawater with S= 35 has a refractive index of 1.3394.1 Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252)4 has such a table. That table has entries for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater. Interpolating between these data points suggests that a solution of 3.65 weight percent sodium chloride has the same refractive index as S=35 seawater, and can be used as an appropriate standard(Table 2).

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified fresh water. That amount roughly corresponds to ¼ cup (73.1 g) of Morton's Iodized Salt dissolved into 2 liters (2000 g) of water (giving very slightly more than 2 L of total volume).