Salinity + Temp and whats the link

[wonderphil-reef]

So this is just out of personal interest to help improve my knowledge, hopefully without having to have a science degree to understand it

I know I had been told any time I measure Salinity to make sure the water is at 25c(77f).

What I want to know is why?
What is the chemistry that causes the temp of the water to move the level of salt?

I know while trying to sort my temp out in my tanks, the salt levels always seems to follow the Temp levels:

 

[Randy Holmes-Farley Verified Community Expert]

There is no impact of temperature on actual salinity. Salinity is a measurement of the weight of salt in a specific weight of salt water. Neither of those numbers can change with temperature.

What can change with temperature are the functioning of the devices we use and the properties that we measure to try to get at the salinity.

Many of the devices we use try to automatically correct for these changes, but not all do.

For example, the refractive index of 35 ppt seawater changes with temperature. So does the density and the conductivity.

 

[wonderphil-reef]

oh that is interesting, Thanks!

 

[Jedi1199]

There is no impact of temperature on actual salinity. Salinity is a measurement of the weight of salt in a specific weight of salt water. Neither of those numbers can change with temperature.

What can change with temperature are the functioning of the devices we use and the properties that we measure to try to get at the salinity.

Many of the devices we use try to automatically correct for these changes, but not all do.

For example, the refractive index of 35 ppt seawater changes with temperature. So does the density and the conductivity.

So following this, and in answer to the OP statement about "Always test salinity at 77*F" it would seem to follow that "If your refractometer is calibrated to read true @77F, then test it at 77*. If you don't, you will be slightly off." ? Or, am I off track here?

 

[Randy Holmes-Farley Verified Community Expert]

I know while trying to sort my temp out in my tanks, the salt levels always seems to follow the Temp levels:

You should not be seeing such a large change in apparent salinity with temp, which leads me to believe that the temperature correction for your device is either turned off, or is set wrong.

Here's the section on temperature and conductivity from one of my old articles for the OP.

AquariumFish.com :: Aquarium Fish International :: The monthly magazine for both novice and expert aquarium hobbyists.

AQUARIUM FISH MAGAZINE is THE monthly magazine for both novice and expert aquarium hobbyists. Aquarium Fish Magazine provides information on choosing, caring for, and breeding freshwater and marine fish.

web.archive.org

One final complication is that the conductivity of ions in water depends upon temperature. There are a number of factors that cause this in seawater, but one big one is simply that the ions are naturally moving around faster as they get warmer. When the same number of ions are moving faster, the apparent conductivity is increased. The conductivity of seawater at 41 degrees Fahrenheit, for example, is a little over half of that at 56 degrees. For this reason, all conductivity meters simultaneously measure the conductivity and the temperature. The internal electronics then take the temperature into account, and normally provide a value that is “corrected” to what the conductivity would be at a standard temperature (47 degrees). Consequently, you can measure the salinity of water regardless of the temperature of the sample.

The exact dependence of the conductivity of seawater on temperature is well known, and some meters use this exact relationship. Other use a slightly different correction — that for simple aqueous solutions. Still others provide several temperature correction options. The closer that you are to 47 degrees, the smaller the correction is and the less important the nature of the correction used. Nevertheless, if you have a choice, select the correction used for seawater. If you don’t, you won’t be far off.

 

[Randy Holmes-Farley Verified Community Expert]

So following this, and in answer to the OP statement about "Always test salinity at 77*F" it would seem to follow that "If your refractometer is calibrated to read true @77F, then test it at 77*. If you don't, you will be slightly off." ? Or, am I off track here?

That's a complicated question.

For a glass hydrometer it is always true. For most conductivity meters, it is not true (they supposedly correct).

For an ATC (automatic temperature correction) refractometer, it should not be true, and any temp for calibration and any same or different temp for measurement that is within its ATC range should work OK.

I discuss such refractometer effects in great detail here:

Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com

 

[Jedi1199]

That's a complicated question.

For a glass hydrometer it is always true. For most conductivity meters, it is not true (they supposedly correct).

For an ATC (automatic temperature correction) refractometer, it should not be true, and any temp for calibration and any same or different temp for measurement that is within its ATC range should work OK.

I discuss such refractometer effects in great detail here:

Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com

I read through that article ages ago.. I should go through it again to refresh myself.

For those of us who use a simple refractometer, it still makes sense to me that once calibrated at a certain temp, you should use that temp to test.

For example, my mixing can is heated to the same 79* temp at all times. So when I mix and test, I am always testing at (or as near to +/- 0.5*) the same temp as I calibrated my refractometer.

 

[Randy Holmes-Farley Verified Community Expert]

I read through that article ages ago.. I should go through it again to refresh myself.

For those of us who use a simple refractometer, it still makes sense to me that once calibrated at a certain temp, you should use that temp to test.

For example, my mixing can is heated to the same 79* temp at all times. So when I mix and test, I am always testing at (or as near to +/- 0.5*) the same temp as I calibrated my refractometer.

There's nothing wrong with temp matching, certainly, and it reduces the concern about whether the ATC was actually made correctly.

But in theory it isn't needed for an ATC refractometer and is always needed for a non ATC refractometer.