Salinity

[Middleton5]

I have a salinity question: Seawater is 35 ppt, so when I make up my saltwater with Instant Ocean, I add 35 grams per liter of water (I work in a lab so I can easily weigh things out). This gives a specific gravity of 1.022 (per the instructions on the box). Now for the question...aren't the units of specific gravity "grams per liter"? If so, then shouldn't you add 22 grams of salt per liter of water? As you can see, I am not a chemist. If someone can help me understand this a little better, I would appreciate it.

Thanks!

[EdKruzel]

I can't answer your question, but I do know that 35ppt is equal to an SG of .026.

HTH...

[marduc]

no, the measurement is not based on mass. It is how many molecules of salts are dissolved in how many molecules of water.

So for 35 ppt.. you have 965 molecules of water, and 35 molecules of salts. since salt molecules and water molecules have different respective masses, your methodology will not be accurate. If you know the average weight of all the various salt molecules being added, then it could be calculated based on molecular weights, but that is too complicated for this application.

If you wanted to use units of specific gravity, then a corresponding 1.026 means that a set volume of salt water has 1.026 the mass of an equal volume of pure fresh water (provided they are the same temperature).

[kevin2000]

Salinity to SG comparison is impacted by temperature.

[corals b 4 bills]

<a href=showthread.php?s=&postid=10134218#post10134218 target=_blank>Originally posted</a> by marduc
no, the measurement is not based on mass. It is how many molecules of salts are dissolved in how many molecules of water.

So for 35 ppt.. you have 965 molecules of water, and 35 molecules of salts. since salt molecules and water molecules have different respective masses, your methodology will not be accurate. If you know the average weight of all the various salt molecules being added, then it could be calculated based on molecular weights, but that is too complicated for this application.

If you wanted to use units of specific gravity, then a corresponding 1.026 means that a set volume of salt water has 1.026 the mass of an equal volume of pure fresh water (provided they are the same temperature). NOW THAT"S ONE EDUCATED BARTENDER! dude you missed your calling.

[drummereef]

In general a weight measurement of salinity is not accurate. I agree it could be water content that is throwing off the measure. The use of a refractometer will yield better results.

[Aquarist007]

sometimes we can get too scientific, causing more work and stress then needed.
I have a 20 gal plastic pail I use for mixing up salt. If the mixture calls for 10 cups I will put 7 in, run a powerhead and heater in it overnight and remeasure with a refractometer.
Then add one cup remeasure, another etc etc until I get it around 1.024
The whole process is effortless--5min over two days.

[marduc]

After while you do start to get a feel for how much salt to use. I always have a set volume of water for my water changes (~10 gallons), I know that with my current batch of salt that if I add just over 6.5 cups of salt mix my SG will be just about where I want it, I still use my refractometer and test to make sure, and need to tweak slightly.

[Aquarist007]

yes you do get a feel but you don't want to get too sure of yourself and overshoot---I always add less then top up the next day before my water changes

[HPD Turbo]

<a href=showthread.php?s=&postid=10141589#post10141589 target=_blank>Originally posted</a> by capn_hylinur
yes you do get a feel but you don't want to get too sure of yourself and overshoot---I always add less then top up the next day before my water changes

What is the diference betew SG and Salinity?

[Whisperer]

Specific gravity has no unit (dimensionless), if I remember correctly. Grams/liter is density (mass per unit volume).

[HPD Turbo]

What is the Salinity the a reef mixed tank should have?, is 1.23 bad?

[HPD Turbo]

<a href=showthread.php?s=&postid=10141634#post10141634 target=_blank>Originally posted</a> by Whisperer
Specific gravity has no unit (dimensionless), if I remember correctly. Grams/liter is density (mass per unit volume).

But why measure both?

[Whisperer]

yes, that's too concentrated. maybe you meant 1.023.

[Whisperer]

<a href=showthread.php?s=&postid=10141659#post10141659 target=_blank>Originally posted</a> by HPD Turbo
But why measure both?
If I remember correctly, density is affected by temperature (mass/volume, volume of materials expand when heated), specific gravity is not affected by change in temp or volume <---not absolutely sure about this. :D

[HPD Turbo]

Ok, I get it now.
But is 1.023 bad for a reef tank?

[kevin2000]

<a href=showthread.php?s=&postid=10141737#post10141737 target=_blank>Originally posted</a> by HPD Turbo
Ok, I get it now.
But is 1.023 bad for a reef tank?

In general reefs have a salinity that would equate to an SG of about 1.026 ... as such reef critters may prefer a higher salinity. If you have a FOWL then your SG is fine ... if you have corals then you might consider raising it.

[marduc]

salinity is measured as parts per thousand, and yes specific gravity is variable with temperature.

However for our purposes since 99% of our aquariums are within a 10 degree range, a salinity of 35 ppt equates to a specific gravity of 1.026.

1.023 is not necessarily bad, but since we are emulating ocean environments, we use what is the typical average on reefs 1.026. Some reefs are a tad higher (red sea notably), some a tad lower, but this is a good average zone and an ideal target to strive for.

[HPD Turbo]

Ok, thanks.

[marduc]

just dug up an old article for those interested:



bonus reading (http://www.advancedaquarist.com/issues/jan2002/chemistry.htm)