Why does Temperature Effect Salinity?

[Dom]

I understand and accept that temperature impacts salinity.

Salinity is a ratio of salt to water.

If a 10 gallon tank has a salinity of 35ppt at 68 degrees, and then you raise the temperature to 74 degrees, the ratio of salt to water has not changed.

So why the change in salinity?

 

[robbyg]

Because the water loses Density as it gets hotter.

 

[Dom]

Because the water loses Density as it gets hotter.

Ok... I agree with that.

But the physical ratio of salt to water molecules doesn't change.

So are you saying that salinity goes up as water gets cooler and goes down as water gets warmer?

 

[CCauthers]

The specific gravity changes, not salinity in ppt (I believe)

 

[Casual_reefkeeping]

The specific gravity changes, not salinity in ppt (I believe)

Exactly. Specific gravity is a measure of density compared to that of pure water. So as the density of the water changes with temperature, the specific gravity will change even though the ratio of salt to water hadn't.

 

[Nick428]

Basically everything is getting less dense even the water so the specific gravity will decrease.

 

[robbyg]

Ok... I agree with that.

But the physical ratio of salt to water molecules doesn't change.

So are you saying that salinity goes up as water gets cooler and goes down as water gets warmer?

In any given sample SW you have an amount of FW and the Sodium (NaCl) and all the rest that make it salty. As you heat the water the H2O molecules spread apart, if you keep heating them they spread apart until you get steam rising. Meanwhile the Salts remain the same, so as the water density goes up the salt remains the same but it is diluted more in this water that is expanding. When the water is frozen the H2O molecules contract and shrink and as the water is getting colder the Salinity goes up because the density of the water has increased and therefore the ratios of salt to water mass has increased. I am not a chemist but that's the layman's jist of how I concluded it works.

 

[Biglew11]

Salinity is a measure of ppt, if you heat 1000 particles of saltwater you still have 35 of them as salt. The salinity does not change.

However the tools we use to measure our saltwater measure either specific gravity, or the refractive index of saltwater. both of these measurements are affected by temperature.
I've never used conductivity for saltwater so I'm not sure if it is affected by the temperature of saltwater.

 

[Randy Holmes-Farley]

As noted above, temperature has NO EFFECT WHATSOEVER on salinity (in units of, say, ppt).

What temperature does impact is some of the ways of measuring salinity. Those various ways may need to be corrected, if they do not automatically correct for how temperature impacts that device.

 

[Randy Holmes-Farley]

Basically everything is getting less dense even the water so the specific gravity will decrease.

This is a bit misleading.

Yes, both seawater and freshwater get less dense as temperature rises.

However, specific gravity is ratio of the density of seawater to the density of freshwater at the same temperature.

Thus, to explain why there is a very small effect of temperature on specific gravity,a nd to explain what direction it is in, requires explaining why the density of seawater changes DIFFERENTLY than does freshwater to the same temperature change.

There is such an effect, but I think it is beyond a simplistic "this is why".

 

[Crashjack]

Think of specific gravity as measuring buoyancy. It has nothing to do with salt per se (I used to be a home winemaker and we used s.g. to measure sugar). A floating hydrometer used to be the instrument of choice to measure s.g. Hydrometers are designed to measure a specific range, and it is that range that defines whether the hydrometer is to be used for a saltwater aquarium or for making wine or for something else. The hydrometer itself is going to act the same no matter what liquid you put it in and since temperature alters the density of liquids, it also has an effect on the resulting s.g. This is also why hydrometers are calibrated for specific temperatures... so the user can do the math to determine the s.g., correcting for the temperature of the sample.

 

[LadyTang2]

As noted above, temperature has NO EFFECT WHATSOEVER on salinity (in units of, say, ppt).

What temperature does impact is some of the ways of measuring salinity. Those various ways may need to be corrected, if they do not automatically correct for how temperature impacts that device.

What ways are not effected? Is a hydrometer? Does an electronic hannah check temp and correct for it?

If not what would 35ppt at 77 look like at 81, or whats the easy way to remember which way things appear incorrect (in which direction?) w a hydrometer or refractometer roughly?

 

[bigdog7]

That’s why cargo ships have buoyancy loading lines!
Freshwater on top and usually above equated salt water on bottom...

the summer salt water and tropic salt water line is in between fw and sw....

Learned this in my master captains class!

now my question is this....
My mixing station is in the garage.. i live
In Florida... my garage is 96degrees...
I mix water to 1026 and the tank is 1026 at 78 inside....

If I had a boat ready to float! .... it would sit lower in the garage salt
Tank than in the display tank.....

But outside of the density and less buoyancy in the warmer water...

It will not effect the parameters of the tank correct? As the water cools it still reads 1026...

 

[Biglew11]

The time consuming scientific method of actually counting the particles would be the only method not affected by temperature. Temperature affects specific gravity, refractive index used by refractometers digital and optical, and conductivity. Our test equipment is also affected by temperature. All while the actual salinity in ppt stays the same. Our optical and digital refractometers usually have automatic temperature compensation.

 

[Homebrewer]

What ways are not effected? Is a hydrometer? Does an electronic hannah check temp and correct for it?

If not what would 35ppt at 77 look like at 81, or whats the easy way to remember which way things appear incorrect (in which direction?) w a hydrometer or refractometer roughly?

It depends on what temperature the device is calibrated at. Don’t own a Hanna for salinity so cant answer that question, but if you know at what temperature your hydrometer is calibrated at, you can use this calculator to correct: https://www.brewersfriend.com/hydrometer-temp/

 

[Randy Holmes-Farley]

What ways are not effected? Is a hydrometer? Does an electronic hannah check temp and correct for it?

If not what would 35ppt at 77 look like at 81, or whats the easy way to remember which way things appear incorrect (in which direction?) w a hydrometer or refractometer roughly?

The original way of measuring salinity by drying out the sample and weighing the dry salts is obviously not impacted by temperature.

Conductivity (the preferred method for oceanographers to measure salinity) is always internally corrected for temperature changes, so the users sees no changes (assuming it is done properly).

Salinity measurements using a refractometer via refractive index may or may not be automatically temperature corrected by the device involved.

Hydrometers to measure salinity by density may or may not be impacted by temperature changes. Floating hydrometers definitely are, swing arm types are generally designed to not be.

 

[Randy Holmes-Farley]

In this article I go into exhaustive detail about the relationship between specific gravity and temperature, which is more complicated than most folks imagine:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

 

[LadyTang2]

The original way of measuring salinity by drying out the sample and weighing the dry salts is obviously not impacted by temperature.

Conductivity (the preferred method for oceanographers to measure salinity) is always internally corrected for temperature changes, so the users sees no changes (assuming it is done properly).

Salinity measurements using a refractometer via refractive index may or may not be automatically temperature corrected by the device involved.

Hydrometers to measure salinity by density may or may not be impacted by temperature changes. Floating hydrometers definitely are, swing arm types are generally designed to not be.

Thanks, for a floating one hydrometer, if the water had the same weight salt in it, but temp went up from 76 to 80, how would those look different on hydrometer? How would 76 look and 80? By roughly what % and in what direction? Thanks!

 

[Biglew11]

warmer water is less dense, a hydrometer will float lower in the water. Cold water will be more dense it will float higher in the water.

 

[Randy Holmes-Farley]

Thanks, for a floating one hydrometer, if the water had the same weight salt in it, but temp went up from 76 to 80, how would those look different on hydrometer? How would 76 look and 80? By roughly what % and in what direction? Thanks!

Water (salt or fresh) expands on warming (at least in the range of reef tanks; it shrinks in certain other temp ranges), and importantly, it expands more than a floating glass hydrometer. Thus the water effectively becomes less dense and the sg will read lower at higher temps.

There are tables of corrections for floating glass hydrometers, but you need to know the reference temperature for the hydrometer you are using.

Here's a calculator that may help:

Hydrometer Calculator | MoreBeer

In need of hydrometer calculation? This hydrometer calculator will help you master specific gravity temperature correction.

homebrewsupply.com