We are all plagued with uncertainty concerning the amount of CaCl2 in those little 'prills' that we buy from the LHBS. Are they dihydrate (CaCl2.2H20 which is what I assumed for years) or 78 - 85% (as some flake products are specified to be) and what are the effects of storage exposed to humid air? If you have ever left any out for a day or so when you came back the powder was gone and there was a puddle of syrup in its place. There is a rather elaborate procedure for assaying calcium chloride which contains lots of low level impurities such as sodium, magnesium and hydroxyl ions (or put another way it contains some lime and lye) but the main 'impurity' from our perspective is water of hydration. Anhydrous CaCl2 is obviously 100% calcium chloride. If a recipe calls for 10 grams we weigh out 10 grams (but we had better be quick about it because if the humidity is at all high we can see the scale increment in front of our eyes as the salt takes up that water.
The monohydrate is 86% calcium chloride (assuming no impurities other than water), the dihidrate 75.5% calcium chloride and the hexahydrate only 50.7%. Correspondingly greater amounts of these salts are needed to supply 10 grams of actual CaCl2. A product with 80% calcium chloride content, assuming again that the rest is mostly water, apparently is a mix of mono and di hydrates.
It turns out that we may be able to tell approximately what is in a strong solution of calcium chloride from its specific gravity. To do this we would take a fairly large quantity (a tsp or 2) of the product in question and add it to a small volume of water - small but large enough to float your hydrometer. After dissolving the salt allow the water to cool and then measure its specific gravity with your hydrometer (narrow range hydrometers will give more accurate results). The strength of the solution, in grams per liter CaCl2, is approximately:
g/L = -684.57 + 175.12*SG + 509.45*SG*SG
where SG is the true (the difference between true and apparent can not be seen on a hydrometer) specific gravity 20/20 °C. If you determine that your solution is, for example, 68 g/L, and you need 10 grams for a brew then you must use 10/68 = 0.147 L (147 mL) of this solution.
If you have carefully measured out the salt and the volume then you can determine the concentration of CaCl2 in your powder. For example, I opened a new jar of USP anhydrous CaCl2 and weighed out 7.413 g (I just shoveled it into a 100 mL volumetric flask until it looked like about a tsp) and then added water to the mark. Thus I have 74.13 grams of powder per liter of this solution. It's specific gravity measured 1.0557 which corresponds to a calcium chloride content of 68.14 grams per liter. I can thus guestimate that my powder's calcium chloride is about 68.14/74.13 = 91.9% CaCl2. This corresponds approximately to CaCl2•1/2H2O
On the other hand if I take an old jar of the dihydrate that I first opened years and years ago and chisel the stuff in it apart I find the CaCl2 content to be only 64.5% corresponding to CaCl2•3.5H2O. It looks as if each molecule has picked up over the years an average of 1.5 molecules of water.
[Edit]A new bottle of the prills from LD Carlson assays 96.2% CaCl2 (corresponding to CaCl2•0.24H20) whereas an apparently unopened bottle of the same product that is a couple of years old gave 76% CaCl2 corresponding to CaCl2•1.94H2O.
These results are probably good enough to be useful to home brewers. One supposes that the impurities in ACS, USP or food grades of CaCl2 are not likely to be large amounts of the salts of bismuth and arsenic and that therefore, the assumption that the principal 'impurity' is water may be a fairly good one. That leaves the basic sg vs strength data (which is based on measurements dating back to the 19th century) and the model into which they have been inserted as the principal error source.
Edit: Some people might be interested in knowing the SG of solutions of CaCl2 as a function of their strengths in grams/L. This is
SG = 1 + 0.00083641*gpl -2.7043e-07*gpl*gpl
where gpl are the grams per liter.
The monohydrate is 86% calcium chloride (assuming no impurities other than water), the dihidrate 75.5% calcium chloride and the hexahydrate only 50.7%. Correspondingly greater amounts of these salts are needed to supply 10 grams of actual CaCl2. A product with 80% calcium chloride content, assuming again that the rest is mostly water, apparently is a mix of mono and di hydrates.
It turns out that we may be able to tell approximately what is in a strong solution of calcium chloride from its specific gravity. To do this we would take a fairly large quantity (a tsp or 2) of the product in question and add it to a small volume of water - small but large enough to float your hydrometer. After dissolving the salt allow the water to cool and then measure its specific gravity with your hydrometer (narrow range hydrometers will give more accurate results). The strength of the solution, in grams per liter CaCl2, is approximately:
g/L = -684.57 + 175.12*SG + 509.45*SG*SG
where SG is the true (the difference between true and apparent can not be seen on a hydrometer) specific gravity 20/20 °C. If you determine that your solution is, for example, 68 g/L, and you need 10 grams for a brew then you must use 10/68 = 0.147 L (147 mL) of this solution.
If you have carefully measured out the salt and the volume then you can determine the concentration of CaCl2 in your powder. For example, I opened a new jar of USP anhydrous CaCl2 and weighed out 7.413 g (I just shoveled it into a 100 mL volumetric flask until it looked like about a tsp) and then added water to the mark. Thus I have 74.13 grams of powder per liter of this solution. It's specific gravity measured 1.0557 which corresponds to a calcium chloride content of 68.14 grams per liter. I can thus guestimate that my powder's calcium chloride is about 68.14/74.13 = 91.9% CaCl2. This corresponds approximately to CaCl2•1/2H2O
On the other hand if I take an old jar of the dihydrate that I first opened years and years ago and chisel the stuff in it apart I find the CaCl2 content to be only 64.5% corresponding to CaCl2•3.5H2O. It looks as if each molecule has picked up over the years an average of 1.5 molecules of water.
[Edit]A new bottle of the prills from LD Carlson assays 96.2% CaCl2 (corresponding to CaCl2•0.24H20) whereas an apparently unopened bottle of the same product that is a couple of years old gave 76% CaCl2 corresponding to CaCl2•1.94H2O.
These results are probably good enough to be useful to home brewers. One supposes that the impurities in ACS, USP or food grades of CaCl2 are not likely to be large amounts of the salts of bismuth and arsenic and that therefore, the assumption that the principal 'impurity' is water may be a fairly good one. That leaves the basic sg vs strength data (which is based on measurements dating back to the 19th century) and the model into which they have been inserted as the principal error source.
Edit: Some people might be interested in knowing the SG of solutions of CaCl2 as a function of their strengths in grams/L. This is
SG = 1 + 0.00083641*gpl -2.7043e-07*gpl*gpl
where gpl are the grams per liter.
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