Molarity of reef water

[Mr_slinky_dragon]

Just a little tip bit of info i found in regards to the mass of salt mix. This gets a bit sciencey but bare with.

 

Common salt (NaCl, ignoring the trace elements in salt mix) has a molecular mass of 58.44 grams, so if you dissolve that in 1 litre of water you will get a solution of 1 mol/cm3(litre). 

 

if you use the following equation; M = m/v x 1/58.44 (this number is constant) where M is the final concentration, m is the mass of the solute and v is the volume, a solution of 0.6 mol in 10 litres of water requires 350 grams of salt mix. 

 

350 ÷ 10 × the constant = 0.5989 mol (rounding up to 0.6) 

 

I checked this against an online salinity calculator (marine depot) and after converting litres to US gallons and then pounds to grams the results was a difference of 1 gram, however the amount of steps needed you can easily get lost in the translation. I also checked the water with a refractometer and it turns out to be pretty much 1.026 sg 

 

This works regardless of volume or mass for instance 40 litres would need 750g of salt and so on

 

Hope this helps people out 🙂

[Mr_slinky_dragon]

Here are a few more examples.

 

These are to 1 litre of water 

0.1 mol: 5.8g (estimate)

0.5 mol: 29.2g 

 

By using these values one should be able to calculate the amount of salt required for any size tank, simply swap the numbers around.

[seabass]

Interesting.  Out of curiosity, do you measure how much salt you use by weighing it?  I have to confess, I just kind of count out a bunch of rough scoops, then adjust it after it dissolves enough.

[Mr_slinky_dragon]

Yup i weigh it out 🙂 kitchen scales will do, if they have 1 decimal place thats better, if they have 2 thats even better still! 

[Mr_slinky_dragon]

If you do weigh it out remember to zero the scales before adding the salt to the bowl!

[ValerieR]

I don't weigh mine, I just wish I had a Cups/Tablespoon measurement per gallon of RO. Each manufacturer should know how much volume a gram of salt takes.

[Mr_slinky_dragon]

Well a uk gallon is 4.5 litres and a us gallon is 3.8 litres. 

 

To work out the mass you have to multiply the original mass by 4.5 or 3.8 and then put the value for gallons in the volume part.

 

Eg: (157.5÷4.5)×(1÷58.44)=0.5989

(133÷3.8)×(1÷58.44)=0.5989

 

Uk gallon needs 157.5g 

Us gallon needs 133g

 

I have been doing water changes by 10litres as its easier to work out the salt 🙂

[mcarroll]

I'm in favor of using a scale for measuring ingredients whenever possible.  In the kitchen especially.

 

Hygroscopic salts seem like they could be problematic to weigh....how much is water weight?  You'd have to weigh .

 

I went through a phase using scales, but on a practical level salt is very amenable to scooping and leveling, unlike some other powders like baking soda and flour.  For larger amounts of either one, I weigh by preference.

 

Further, when I found out that the basic lab salt mix formula is used at 1/2 cup per gallon....and that this measure also worked for Reef Crystals when I tried it....I realized that it's just too easy and simple to measure salt by volume. 

 

It's too dusty to pour salt mixes, so you have to scoop even if you weigh...so make your scooper one that can measure 1/2 cup and you're all set...no further equipment required, not even a refractometer (once you're comfortable doing it).  At 1/2 cup per gallon....a 60 gallon drum get's 30 cups...a 5 gallon bucket get's 2.5 cups...etc.  Too simple!  (You can always scoop it onto a scale to be measured for redundancy if you want....easier for 5 gallons than 60 gallons though. )

 

I'm curious if there are any salt mixes this 1/2 cup per gallon doesn't work for....shouldn't be any in theory, but I haven't experimented with other salts.

[lizzyann]

I misread and clicked on this thread expecting to read a debate about the moRaLity of making saltwater with RODI and tips on using the leftover water 🤣 This is fun too I guess!

[Mr_slinky_dragon]

I realised something the other week... 35 grams per litre is 35ppt... so if all the maths is scary just remember i am a twit lol

[mcarroll]

It was a worthy exercise!!!

[xthunt]

On 8/1/2019 at 3:33 PM, mcarroll said:

I'm in favor of using a scale for measuring ingredients whenever possible.  In the kitchen especially.

 

Hygroscopic salts seem like they could be problematic to weigh....how much is water weight?  You'd have to weigh .

 

I went through a phase using scales, but on a practical level salt is very amenable to scooping and leveling, unlike some other powders like baking soda and flour.  For larger amounts of either one, I weigh by preference.

 

Further, when I found out that the basic lab salt mix formula is used at 1/2 cup per gallon....and that this measure also worked for Reef Crystals when I tried it....I realized that it's just too easy and simple to measure salt by volume. 

 

It's too dusty to pour salt mixes, so you have to scoop even if you weigh...so make your scooper one that can measure 1/2 cup and you're all set...no further equipment required, not even a refractometer (once you're comfortable doing it).  At 1/2 cup per gallon....a 60 gallon drum get's 30 cups...a 5 gallon bucket get's 2.5 cups...etc.  Too simple!  (You can always scoop it onto a scale to be measured for redundancy if you want....easier for 5 gallons than 60 gallons though. )

 

I'm curious if there are any salt mixes this 1/2 cup per gallon doesn't work for....shouldn't be any in theory, but I haven't experimented with other salts.

I use a 1/2 cup and 1/2 tbsp scoops, and an old plastic card to skim the top to level it. Almost all salts Ive used takes a bit more than 1/2 cup to get to 1.025. For example, Fritz RPM is what I have now, takes a leveled 1/2 cup scoop + 2 leveled 1/2 tbsp scoops per gallon to get to 1.025. And you’re right, as long as Im positive I counted my scoops right, I hardly check the salinity of my mix. I also have my buckets marked in gallon increments, so that stays constant as well. Always comes out right.