KH reference info and calculations - The Planted Tank Forum
 
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post #1 of 11 (permalink) Old 05-21-2011, 09:24 PM Thread Starter
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KH reference info and calculations

First, I meant to post this when Loop's "Simple DIY Drop Checker" post was current, but life intervened and I forgot about it until I needed to change my CO2 bottle. Second, I apologize if this post is a little long, but I want to include enough detail that others can avoid encountering some of the problems I have. If you are confident that your DIY KH reference solution is more accurate than your test kit, you can ignore this post; or better yet proof read it and make sure I haven't made any mistakes or miscalculations. The last thing I want to do is add more confusion to this subject.

When I first saw the DIY drop checker thread, I didn't even know what one was. I knew about the pH, KH, CO2 relationship, but had long ago discovered that it was useless for my tank due to the high GH, pH, and dissolved Calcium Carbonate in Kansas tap water. Once I read about how they worked, and saw how easy a drop checker was to build; I immediately began hunting for suitable materials to build one. I quickly put together a prototype to test using an old salt shaker, some tubing, a suction cup and a zip tie. Now all I needed was a known KH solution.

I searched the web expecting to find a commercial product that I could purchase. Instead I found dozens of links on how to make your own using baking soda and water. My first thought was "Great, now I don't have to pay shipping and wait a week." Then I began reading through the various instructions. I can't ever recall encountering so much confusing, conflicting, missing, and just plain incorrect information; even for the internet. I felt like I was reading recipes for Gumbo or Vegetable Soup as opposed to a technical procedure. Even different threads from this site had conflicting information regarding the ratio of baking soda to water, which was rather surprising.

Unfortunately, most of the instructions didn't include any technical info, assumptions, or example calculations, so I had no way to know which recipe to trust. I knew some of the instructions had to be correct, but which ones? To make matters even more frustrating, most of the instructions required weighing out minute amounts of baking soda accurate to within milligrams. While that is fine for some people (chemist, pharmacists, people who reload their own ammo, meth dealers, etc.); most people don't have a properly calibrated lab grade scale sitting on their kitchen counter. I really wanted to start with a known, easy to measure VOLUME of baking soda; say 1/4 or 1/8 teaspoon; and then calculate the necessary amount of water to add for a given KH. It's pretty easy to measure a volume of water with standard kitchen items.

Most good Pyrex measuring cups have graduations of 25 ml. Combine that with a syringe, marinade needle, turkey baster, or even measuring spoons and it's not too difficult to get within +/- 5-10ml. I decided the best course of action was to start with the basic chemistry, grind through the calculations, and then compare my results with some of the more detailed threads on the internet. Since I wasn't concerned about too much CO2, I decided to start with a 30dKH solution and dilute that down to 3dKH for the drop checker (30dKH also easily dilutes to 6,5, or 2 dKH, and 4dKH isn't much more work).

*Note: remember that to dilute 30dkH to 3dKH you need a 9:1 ratio of distilled water to KH solution, NOT a 10:1 ratio (just remember that 9+1=10, and 30/10 = 3).

Naturally, my first batch was a complete failure. I ended up with a solution that tested 14dKH as opposed to 30dKH (it was actually closer to 12dKH). In the process of trying to figure out where I had screwed up I learned a few very useful pieces of information.

First, as most of you already know, test kits expire quite rapidly compared to how often you normally use them. I knew the reagents would degrade over time, but I figured the box would contain an expiration/use by date if the product was likely to go bad before being used. While trying to find the expiration date of my KH test kit I stumbled upon an e-mail from API regarding this exact subject. The reason they don't put a use by date on the box is because the majority of their large customers (distributors and large retailers) specifically request that this information NOT be printed on the packaging. Essentially, they don't want customers to know the test kit they just purchased is only good for another 3-6 months. For example, not only was my test kit well past the suggested use by date, the GH test kit that came packaged with it was already over 2 years old when I bought the kit (which explains why I could never get it to change colors).

To add insult to injury, the nearest pet store(s) likely to stock a KH test kit were 90 miles away; which meant it was going to be at least a week before I could get a new one. Even with an expired test kit, I knew that something else had gone wrong. I was almost certain it had to due with my mass to volume conversion for the baking soda, but all of my math checked out fine. I decided to purchase a fresh, unopened box of baking soda and a new jug of distilled water; and then try again when I got a new test kit. When I purchased the baking soda I made sure it said "Active Ingredient: Sodium Bicarbonate 100%." Later I began to wonder if there was perhaps an inactive ingredient in baking soda from the grocery store. There's not, but the last line of the fine print read: "616mg Sodium per 1/2 teaspoon of baking soda."

I realized I could use that to calculate an exact density of Arm & Hammer baking soda. I ended up with a density of 913.9mg/cm^3; which was no where near the listed density for sodium bicarbonate (2.196g/cm^3). At this point I realized my monumentally stupid mistake. Chemically, baking soda is 100% sodium bicarbonate, but physically it is a powder, not a solid; meaning it contains a significant amount of air. The exact amount will depend upon how fine the powder is ground, and how much it has been compacted. Luckily backing soda doesn't compact much under its own weight unless you're dealing with huge quantities. Once I substituted this value into my calculations I got a theoretical value of 12.5 dKH for my original batch; which matched the results of my new test kit.

Anyway, after all of my blabbering, here is a sample calculation along with necessary chemical information and conversion factors.

Basic Info:
Chemical formula for Sodium Bicarbonate (baking soda) --> NaHCO3
Sodium content of Arm & Hammer (A&H) baking soda --> 616mg/.5 tsp

Molecular weights:
NaHCO3 --> 84.01 g/mole
CO3 --> 60.01 g/mole (this is the carbonate ion)
Na --> 22.99g/mole

Conversion Factors:
1dKH = 17.857ppm (part per million; by weight)
1ppm = 1mg/L of H20 (this will vary somewhat with temperature)
1cm^3 (cc) = .203 tsp (teaspoons, NOT tablespoons)

Keep in mind that KH is a measure of the concentration of carbonate ions (CO3), so the sodium and hydrogen atoms present in baking soda do not contribute to the KH value.

Sample Calculation for a 30dKH solution:

17.857ppm/1dKH X 30dKH = 535.71ppm --> 535.71mg CO3/L of H20

535.71mg CO3/(L of H20) X (84.01 g/mole NaHCO3)/(60.01 g/mole CO3) = 749.96 mg NaHCO3/(L of H2O)

For those of you wanting a 40dKH solution this equals 1.000g baking soda per L of water.

Density of A&H baking soda:

616mg Na/.5 tsp X (.203tsp/cm^3) X (84.01 g/mole NaHco3/22.99 g/mole Na) = 913.9mg/cm^3

Final Calculations:

749.96mg NaHCo3/(L of H20) X (1 cm^3/913.9mg) X (.203 tsp/cm^3) = .166 tsp NaHCO3/(L of H20)

.166 tsp/(1 L of H2O) = .125 tsp/(? L of H20); ? = .7504 L of H20

Therefore 1/8 tsp of A&H baking soda dissolved into 750.4 ml of H20 yields a 30dKH solution. (For a 40dKH solution this works out to 1/4 tsp per 1.126 L of H20).

I mixed up a batch using this ratio and tested it with a fresh KH test kit. At 29 drops the solution turned from blue to green. At 30 drops the solution turned bright yellow. Adding additional drops didn't have any noticeable affect on the color. Considering that reading a drop checker relies on color matching, this formula should be more than accurate enough. Personally, I plan on using my 30dKH solution to verify my test kit as it gets older.

I hope this information is useful to some of you out there, and that my calculations made sense. I'll gladly answer any questions to the best of my ability, but I'm a mechanical engineer NOT a chemical engineer.
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post #2 of 11 (permalink) Old 05-21-2011, 10:57 PM
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The major problem with weighing baking soda, or measuring it by volume, to determine how much carbonate you are adding, is that sodium bicarbonate is very hygroscopic. Once you expose it to air, it quickly starts adding H2O to its chemical formula, and it doesn't take many molecules of H2O to change the percent of bicarbonate in a teaspoon significantly. You can heat the sodium bicarbonate to drive off the attached H2O molecules, but that also converts the sodium bicarbonate to sodium carbonate, which has a much different percentage of carbonate in it.

To do as you have done, you just have to accept that it will not be very accurate. It will be much more accurate with a freshly opened box of baking soda than with one in the kitchen for a week, but it will still be inaccurate.

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post #3 of 11 (permalink) Old 05-22-2011, 04:12 AM Thread Starter
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That was the reason for using a fresh, unopened box. I then sealed up the 30dKH solution in a canning jar. Luckily the air in western Kansas is usually very dry so hopefully the baking soda contained very little water. Can you remove the water with a desiccant of some sort? Other than relying on a fresh KH test kit how else are you supposed to make it? I couldn't find any KH reference solution for sale, but I didn't think to try looking for calibration fluid for electronic water testers at the time. I don't know if any of them test for KH or not. I live in a small town and thanks to Wal-Mart the local pet store went belly up so my options are limited (I refuse to give money to Wal-Mart unless it's an emergency or absolute necessity).
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post #4 of 11 (permalink) Old 05-22-2011, 04:42 AM
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My attitude towards WalMart is the same as yours.

Usually you can buy 4 dKH water on the Swap n Shop forum. Some people make it with lab standard carbonate solutions, with known very high ppm of carbonate. Others use baking soda and weigh out the amounts. The first method will necessarily be most accurate. But, for a drop checker, any KH between around 3.5 and 4.5 dKH will work fine anyway. I have done both ways, and I still keep a small flask of 40 dKH (as I recall) water, made from a lab grade solution. I checked a KH test kit I had, using that, and it read 3 dKH vs the real 4 dKH hardness. So, now I buy a new KH test kit, looking for the most recent manufacturing date I can find, when I want to play with fixed KH water.

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post #5 of 11 (permalink) Old 05-22-2011, 04:48 AM
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Quote:
Originally Posted by Hoppy View Post
The major problem with weighing baking soda, or measuring it by volume, to determine how much carbonate you are adding, is that sodium bicarbonate is very hygroscopic.
I'm not claiming that sodium bicarbonate is not hygroscopic, but none of the chemical information that I'm finding about it indicates that it is. Sodium carbonate is listed as being such, but nothing of that sort is readily available for NaHCO3. This includes MSDS, general information pages, heck, even google can't find anything worthwhile in the first 3 pages for "sodium bicarbonate hygroscopic." At the very least it does not appear to be strongly hygroscopic. It also doesn't come packaged with an anti-caking agent, which would be required for most hygroscopic powders (baking powder, for instance.)

If I had a good scale handy I'd weigh some fresh from the box then leave it open but covered for a few days and try again. Unfortunately I don't...
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post #6 of 11 (permalink) Old 05-22-2011, 06:01 PM
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I'm not a chemist either. "Hydrated sodium bicarbonate involves adding additional water molecules, thereby hydrating the sodium carbonate." from http://www.ehow.com/about_5497318_so...e-hydrate.html

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post #7 of 11 (permalink) Old 05-22-2011, 10:07 PM
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I'm not a practicing chemist, but I do have a chem degree and taught it for many years before switching to physics. The page you linked has very little to do with sodium bicarbonate, or baking soda, instead it's about sodium carbonate, or washing soda, which is very hygroscopic (though I believe washing soda is sold fully hydrated, could be very wrong there.) The author is describing the various hydrates of sodium carbonate, one of which is formed when it captures both a water molecule and a carbon dioxide and forms two molecules of sodium bicarbonate. It's a mix of sodium carbonate and bicarbonate, basically. Looking for more info on "hydrated sodium bicarbonate" makes it clear that the stuff is properly named hydrated sodium bicarbonate carbonate, indicating the mixture of the two compounds. It's pretty interesting stuff, both geologically and historically.

The stuff sold in stores is 100% bicarbonate though, and there's no way to hydrate it further chemically (it would just form carbonic acid and sodium hydroxide, which is a super unstable mixture.) Again, it's entirely possible that baking soda is hygroscopic to some degree, it wouldn't surprise me in the slightest, but it does not appear to be strongly so.
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post #8 of 11 (permalink) Old 05-22-2011, 10:08 PM
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Actually, if sodium carbonate is sold fully hydrated, it would solve the hygroscopic problem completely... Washing soda for the win? Where's my milligram balance when I need it? Teach me to take a few years off to raise the kids.
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post #9 of 11 (permalink) Old 05-23-2011, 02:57 AM
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Quote:
Originally Posted by jasonpatterson View Post
I'm not a practicing chemist, but I do have a chem degree and taught it for many years before switching to physics. The page you linked has very little to do with sodium bicarbonate, or baking soda, instead it's about sodium carbonate, or washing soda, which is very hygroscopic (though I believe washing soda is sold fully hydrated, could be very wrong there.) The author is describing the various hydrates of sodium carbonate, one of which is formed when it captures both a water molecule and a carbon dioxide and forms two molecules of sodium bicarbonate. It's a mix of sodium carbonate and bicarbonate, basically. Looking for more info on "hydrated sodium bicarbonate" makes it clear that the stuff is properly named hydrated sodium bicarbonate carbonate, indicating the mixture of the two compounds. It's pretty interesting stuff, both geologically and historically.

The stuff sold in stores is 100% bicarbonate though, and there's no way to hydrate it further chemically (it would just form carbonic acid and sodium hydroxide, which is a super unstable mixture.) Again, it's entirely possible that baking soda is hygroscopic to some degree, it wouldn't surprise me in the slightest, but it does not appear to be strongly so.
I bow to your better knowledge! Back when I first tried making my own KH solutions it was widely said that bicarbonate of soda would have varying amounts of hydration, so you couldn't rely on measuring it by mass to give you a specific amount of carbonates. That, apparently, was just one more of the many myths that circulate among planted tank keepers. (I always wondered how a cardboard box could prevent the hydration, but "they" said it, so I believed it.)

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post #10 of 11 (permalink) Old 05-24-2011, 06:53 PM Thread Starter
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I won't get a chance to try this until next week, but I believe I have an experiment that will determine how much baking soda expands with moisture. The basic idea is to combine a reusable desiccant pad with a vacuum bagging system to remove as much moisture as possible from the baking soda. I'll then take the "dry" baking soda, fill a small glass container (shot glass or toothpick holder), and level it off. Finally, I'll take the container of baking soda and put it in an air tight pyrex container along with a small cup of near boiling water. I'll then watch the baking soda for a day or two as it absorbs moisture. If it expands, I'll scrape the excess into a small syringe via a funnel to measure the "extra" amount of baking soda. By comparing the two amounts I should be able to get a reasonable estimate of the percentage of expansion. Any ideas or suggestions for the experiment?
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post #11 of 11 (permalink) Old 06-04-2011, 10:49 PM Thread Starter
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It may not have been the most rigorously scientific experiment, but I didn't get any measurable change in the volume of baking soda with varying humidity. I left the sample of baking soda in the vacuum bag with the desiccant for 48 hours, to get it as dry as possible. I then leveled off a small cup and placed it in an air tight glass container with another small cup of boiling water. I left it there for 48 hours in 100% humidity to allow it to absorb as much moisture as possible. There was no expansion of the baking soda, and any contraction was limited to a tiny crack around the rim at the top of the cup. Any change in volume was far too small to measure.

I don't have an accurate scale, so I can't attest to any changes in weight. I do know that it's possible to add quite a bit of liquid water to a level table spoon full of baking soda and not see any measurable change in volume. Whether any of that water gets absorbed, or will simply evaporate I can't say. If someone with a scale can perform a similar experiment I'd be interested to see if the weight changes.
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