Nikki
Randy ran some test on Iodized Sea Salt, the dif is so nil it make no dif.
Bird
The same amount of salt in different temp. waters will give 2 different salinity readings.
No, on a refract or conductivity meter, they will be the same.
Salinity is salinity it is not affect by temp. 35 ppt =35 ppt no mater what the temp is. What is affected by temp is Specific Gravity and hydrometers measure SG. A refract or conductivity meter that does not have ATC (Automatic Temp Comp) will be affected by temp. However, I know of no one in this hobby that uses one without ATC for seawater mixing. So we are really only left with the hydrometer. However, some hydrometers, i.e SeaTest use thermal expansion plastic, so they too are ATC in a sense.
Lets expand on this, so no one is lost
If you want an S=35 standard for specific gravity, try 3.714 weight percent NaCl in water at 20 deg C
This means that at that temp and solution std the Salinity is = 35 ppt NSW. Now what does the hydrometer read ? That will be the number to use on the hydrometer when mixing at 20 C. If one wants, then raise the temp up to any temp you want and look at that number on the hydrometer and that number will = 35 ppt NSW
Like when you take water and add sugar, you get sweet water. But if you boil it, you can add more sugar as it wouldn't dissolve at the lower temp
That is a whole different issue and you are confusing people with this
You are talking about saturation, as in 100 % and we are at 3.5 %, we will say, no where near saturation. If you dissolve 3.5 grams of NaCl in water at 20 C there will be the same exact amount at 30 C or 15 C, as long as there is no evaporation or precip. And there won't be at these temp. Second, not ALL substances dissolve more/hold more in solution at warmer temp. For example, you can get more Ca(OH)2, aka Kalk, in solution at 20 C than yon can at 25 C. Lastly, there is the tricky thing called activity. The more of salt x you have in solution the more of salt y and z you can dissolve in solution, i.e. If you took a glass of FW water and dissolved NaCl to 100 % saturation and measured the amount and then took a glass of seawater and dissolved NaCl in it to saturation and then measured the total NaCl in both, the seawater glass would have more NaCl than the FW glass. This is why seawater can dissolve more CaCO3 than FW.
Activity is the wanting of a substance to go some place and do something. If it has a low activity it doesn't want to go anywhere, so it stays/likes to be in solution. Something with a high activity is not happay in solution, it wants to go some place and do some thing, so the only thing it can do is precip out of solution, to make itself be happy.
Here's another try at explaining activity
If you think of Mg(OH)2 dissolving, you are going from a solid where the Mg++ and OH- are very near to each other,
to dissolve into Mg++ and OH- that are farther apart. So there is an electrostatic energy of interaction between
the positive and negative charges that is lost when those ions are pulled apart.
In very dilute solution, that effect is countered by the energy that is given off when water surrounds each ion that
comes off. That is why salts dissolve at all.
So in the end, you reach a balance between the lost electrostatic energy, and the gained hydration energy (ignoring
things like entropy for the moment).
OK, so now look at the situation with a large amount of Na+ and Cl- ions also in solution.
In that case, the Mg++ and OH- ions that go out into solution are not so all alone. An Mg++ can have nearby Cl- ions,
and the OH- can have nearby Na+ ions (providing some electrostatic energy). So the electrostatic energy lost is not so
large. Since the hydration energy stays largely the same, the overall solubility goes up.
So most ionic salts are more soluble in NaCl solutions than in water. And most acids and bases that tend to ionize will
be more prone to be in their most highly charged form. This is one reason why pKa's sometimes shift considerably between
freshwater and seawater.
