Calibration Standards Calculation

YankeeTankee

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First off thank you for writing the piece Randy: http://www.reefkeeping.com/issues/2004-06/rhf/index.php

Looking at the specific gravity standard, your recipe says to make a 3.714 weight % NaCl solution, dissolve 1 tsp (6.2 grams) of iodized Salt in 161 mL (makes 163 mL).
I am assuming by weight percentage you mean weight in volume or grams per 100mL. I will cross multiply 6.2 / 163 = x / 100 gives 3.8 (3.85 when using 161). So it looks to me like you are making a 3.8 w/v% solution which is 35.8 PSU according to the table and not a 3.714 w/v% which is 35 PSU.

Looking at the refractive index standard I also get a slightly different number. The instructions say a 3.65 weight % NaCl solution can be made by dissolving 3.65 grams NaCl in 96.35 grams (mL) of purified freshwater. 3.65 / 96.35 = x / 100 gives 3.79 and if I account for the volume added which I calculated to be 1.17 then it's 3.65 / 97.53 = x / 100 which is 3.74. So again it looks like I am making a 3.74 % solution and not a 3.65 % solution.

I am off by basically the same amount on both recipes, my math shows me making solutions just over 2% stronger than advertised. I'm sure there is something you accounted for that I've misunderstood, what is it?
 
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YankeeTankee

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By definition, weight percent always means the weight of the solute (NaCl) divided by the total weight (water plus salt) times 100.

100 mL of the standard weighs more than 100 grams.
Thanks so this is different from (w/v %)

This calculator: https://www.ou.edu/research/electron/bmz5364/calc-percent.html
Says to make a 3.714 w/v in 163mL
"Amount of chemical needed: 6.0538 grams (g)
Add liquid to make a total volume of 163 ml."

EDIT: I think you are using W/W% and I was using W/V%.

For what it's worth I've learned that when doing W/V% (the one your not doing) that it's not per 100mL of water but per volume of water plus volume solute added or total vol. Right?
 
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YankeeTankee

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When I see solutions for sale with a %, should I assume it's w/w? It often does not say on bottle. I believed a solutions strength was usually in w/v (grams per 100mL) and not w/w but I could totally be wrong. If you think it's w/w, do you ever see it as w/v and when?

If it is w/w, say I have a 5% solution, this means that for every 100grams of solution I will get 5 grams of the salt. I can either weight out 100 grams of solution or convert this to w/v, how would I do that, with the density? Or should I just weigh out the solution and not bother?
 
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Randy Holmes-Farley

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The convention may depend on the solution and what the purpose is.

If it says wt %, I would assume it is w/w unless otherwise specified.

For low concentration solutions of a solid in water, w:w and w:v are nearly the same as the density is close to that of pure water.

For a high concentration solution of a solid in water, w:w is strongly preferred as the user may not know the density to understand the amount present. Yes, if you want to convert to a volume based concentration, you will need to know the density.

For two liquids, it might be w:w or V:V but W:V would be a poor choice.
 
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The convention may depend on the solution and what the purpose is.

If it says wt %, I would assume it is w/w unless otherwise specified.

For low concentration solutions of a solid in water, w:w and w:v are nearly the same as the density is close to that of pure water.

For a high concentration solution of a solid in water, w:w is strongly preferred as the user may not know the density to understand the amount present. Yes, if you want to convert to a volume based concentration, you will need to know the density.

For two liquids, it might be w:w or V:V but W:V would be a poor choice.
To confirm, if I dont wanna bother with the density calculation I can weight out 100 grams of a 5% w/w I will have exactly 5 grams of the salt right?
 

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To confirm, if I dont wanna bother with the density calculation I can weight out 100 grams of a 5% w/w I will have exactly 5 grams of the salt right?

Yes, but what are we talking about?

Some materials may have water in the solid crystal. For example, 5 grams of Epsom salt does not contain 5 grams of magnesium sulfate. So 100 grams of 5 wt% epsom salt in water will not contain 5 grams of magnesium sulfate.
 
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Yes, but what are we talking about?

Some materials may have water in the solid crystal. For example, 5 grams of Epsom salt does not contain 5 grams of magnesium sulfate. So 100 grams of 5 wt% epsom salt in water will not contain 5 grams of magnesium sulfate.
You mean if the dry form has like a hydrate in the crystal? Then I minus off the % of the MW which is the hydrate right, is this what you mean or are the other examples?
 
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Randy Holmes-Farley

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You mean if the dry form has like a hydrate in the crystal? Then I minus off the % of the MW which is the hydrate right, is this what you mean or are the other examples?

Yes, that's what I mean. :)
 
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Lol well now I'm curious about how to use the density to convert from w/w to w/v, this was the conversion you can do with the density right? So let's say its 1% potassium iodide (pretending it was stated in w/w to start) with a density of 3.12 per google, or is that a bad example because it's so dilute the difference will be tiny? In which case can you think of a better example and explain how to do this?
 
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is the difference between w/w and w/v similair to the difference between ppm and mg/L? I am trying to connect the dots and am wondering how to use density to convert between those as well. Thanks
 

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Lol well now I'm curious about how to use the density to convert from w/w to w/v, this was the conversion you can do with the density right? So let's say its 1% potassium iodide (pretending it was stated in w/w to start) with a density of 3.12 per google, or is that a bad example because it's so dilute the difference will be tiny? In which case can you think of a better example and explain how to do this?

No, you do not use the density of the solid, you need to know the density of the actual solution, which varies with concentration.

Let's suppose that you have a 10 wt/wt percent solution of sodium hydroxide in water.

The density is about 1.11 g/mL (I looked it up, you cannot calculate it).

If you have 1 liter, it will thus weigh 1,110 grams.

Ten percent of that 1,110 grams is sodium hydroxide, so it contains 0.1 x 1,110 g = 111 grams per liter.

Thus, 10 wt/wt percent sodium hydroxide is about 11.1% wt/volume (which, IMO, is not a usefgul way to think of it. Much better to just write 111 g/L.
 

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is the difference between w/w and w/v similair to the difference between ppm and mg/L? I am trying to connect the dots and am wondering how to use density to convert between those as well. Thanks

yes, but both ppm and mg/L are absolutely clear what they mean and are a far preferred unit of measure compared to w/v.
 
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