BRS 2 Part - by weight or by volume?

[bqq100]

So last month I started dosing 2 parted and started off with Randy's 2 part recipe and got everything dialed in so my Alk/Ca levels are holding steady. I ordered some BRS CaCl and Soda Ash and I am trying to figure out how much soda ash (by weight) I need to use to get the same results as Randy's Recipe #1. And here is why I get lost... :hmm5:

Using the BRS Calculator I set the water volume to 1000 gallons, current alk to 1ppm, and target alk to 95.65. This results in exactly 1 gallon of the soda ash solution.

When I select "Dry Soda Ash" I get 379.7 grams/13.3 oz/82.5 tsp. This is only 1.6-1.7 cups of dry soda ash, but the instructions call for 2 cups to mix the solution! A 15% difference!

Using this reef calculator I get the same 379.7 grams/13.3 oz/82.5 tsp result. I imagine most people use the BRS instructions, so I believe that the 2 cup volume measurement is probably the more accurate one. Is it possible both calculators have the weight calculation wrong? Or maybe the weight is correct, but the density is wrong?

 

[bertoni]

I'd use the volume measurement. I'm not sure how much the 2 cups of soda ash and gallon of water take up in volume after mixing, but that might be part of the issue.

 

[Randy Holmes-Farley]

Do you have a balance to measure weight?

 

[bqq100]

Do you have a balance to measure weight?

I have a digital kitchen scale and an inexpensive precision digital scale (max 100g, .01 resolution), but I don't have the soda ash yet. It should be delivered later today and I'll try measuring on both scales.

 

[Randy Holmes-Farley]

This shows the weight of the sodium carbonate to use:

http://www.reefcentral.com/forums/showthread.php?t=2416434

 

[bqq100]

This shows the weight of the sodium carbonate to use:

http://www.reefcentral.com/forums/showthread.php?t=2416434

Hmmm I think that just made me more confused :eek2:

Where did the 475 grams come from? Does that mean the 379 grams from the calculators is wrong?

 

[Randy Holmes-Farley]

Sorry.

BRS uses my DIY recipe, except they have substituted sodium carbonate for DIY baked baking soda.

The recipe is here:

An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System
http://reefkeeping.com/issues/2006-02/rhf/index.php

 

[bqq100]

I'm probably doing this wrong, but my calculation ends up @ 372 grams....

We need the equivalent of 1.86 molar sodium bicarbonate

Per your article the baking process yields the following:
2 NaHCO3 -> Na2CO3 + H2O + CO2

That means 2 molar sodium bicarbonate yields 1 molar sodium carbonate, so we need .93 molar sodium carbonate. And the molar mass of sodium carbonate is 106 g/mol.

.93 mol * 106g/mol * 3.78 L/gal = 372.6 g/gal

I appreciate your help, I haven't worked with chemistry formulas and molar mass in a very long time!

 

[Randy Holmes-Farley]

Oops, you are right. I messed up the calculation in that thread.

Should be 375 g.

 

[bqq100]

So I measured the bulk density of BRS Soda Ash and BRS CaCl

BRS Soda Ash: 0.934 g/cm3
BRS CaCl: 1.032 g/cm3

Doing some backwards calculations, the BRS calculator is correct, but based on a .93 molar solution. The mixing instructions from BRS (2 cups Soda Ash/2.5 cups CaCl) should give a 1.1 molar solution.

 

[Randy Holmes-Farley]

The BRS recipe is based off my recipe, so the weight and molar concentrations I give should be, by definition, correct. That recipe is 37,000 ppm calcium, or 0.93 M. Note that the BRS calcium chloride is likely the dihydrate (in my recipe it was), not anhydrous calcium chloride. Did you account for that?

That said, one can vary the concentrations as needed, but making the alk part more concentrated is a problem from a dissolution standpoint.

 

[bqq100]

The BRS recipe is based off my recipe, so the weight and molar concentrations I give should be, by definition, correct. That recipe is 37,000 ppm calcium, or 0.93 M. Note that the BRS calcium chloride is likely the dihydrate (in my recipe it was), not anhydrous calcium chloride. Did you account for that?

That said, one can vary the concentrations as needed, but making the alk part more concentrated is a problem from a dissolution standpoint.

Yes I did, the difference comes from the Dowflake being large flakes and having a bulk density of .84 g/cm3 while the BRS CaCl is powdered and has a bulk density of 1.03 g/cm3. Since we don't have anything else to go on, I assumed that it's pure CaCl dihydrate and is 147 g/mol.

2.5 cups CaCl/gal * 236.59 cm3/cup * 1.03 g/cm3 = 609.2g/gal
609.2 g/gal / 3.79 L/gal / 147g/mol = 1.09mol/L

This is all assuming the tablespoon I used to measure the bulk density was reasonably accurate! :rollface:

 

[Randy Holmes-Farley]

OK, BRS may not have adjusted for bulk density differences. In any case, if you have a balance, I'd use the weights that I propose regardless of the volumes BRS suggests.

 

[bqq100]

OK, BRS may not have adjusted for bulk density differences. In any case, if you have a balance, I'd use the weights that I propose regardless of the volumes BRS suggests.

Will do exactly that! Thanks for the help

 

[Randy Holmes-Farley]

:thumbsup:

Happy Reefing.