DIY 10 dKH Alkalinity Standard

[Origami]

I recently picked up a new Hanna Checker for alkalinity. I love it, but one of the things it revealed was that my regular API alkalinity test was reading way high (it was telling me that I had more alkalinity in the tank than there was). So I decided to make a 10 dKH alkalinity standard to validate the Checker. It's easy to make. Here's the recipe.

 

First, you'll need some specialized equipment that is readily available from EBay or a lab supply store. This includes:

 

A precision scale that measures with 0.01 grams (commonly called a jewelry scale) - can be had on Ebay for <$20

A graduated cylinder, 500 ml (plastic OK)*

A graduated cylinder, 10 ml, OR a 10 ml plastic syringe

2-500 ml water bottles (empty and rinsed with RO/DI water)

 

*You can also use an empty 500ml water bottle. Much cheaper, but accuracy may suffer just a little (1-2% or so).

 

Chemicals required:

7.5 grams Baking Soda (sodium bicarbonate)

RO/DI water

 

Procedure:

Rinse the 500 ml graduated cylinder out with RO/DI water. Shake it to clear out the excess water. Weigh out 7.50 grams of baking soda and pour it into a 500 ml graduated cylinder. Add RO/DI water to the 500 ml mark, stirring to fully dissolve all of the baking soda. Pour the solution into an empty water bottle. Mark this bottle, "500 dKH alkalinity standard."

 

Rinse the 500 ml graduated cylinder again with RO/DI. Remove 10 ml of the 500 dKH alkalinity standard solution (using the smaller graduated cylinder or syringe) and place it in the cylinder. Fill the cylinder to the 500 ml mark using RO/DI water. Pour this solution into the second empty water bottle. Mark this bottle, "10 dKH alkalinity standard."

 

10 dKH = 178 ppt CaCO3 = 3.571 meq/liter

[zoozilla]

Thank you for sharing Tom.

[DaveS]

This is great. Anyone want to make a few bottles of solution for others in the club? I'd be willing to pay a few bucks for it to help offset the cost of the equipment and time!

[Origami]

I can make some extra. I just need to get some empty water bottles for the final packaging.

[DaveS]

That's very kind of you Tom!

 

Maybe some plastic soda bottles? Empty hotel shampoo bottles? Obviously all should be adequately cleaned first.

 

Let me see what I can dig around and find.

[Origami]

Plastic water bottles are probably the cleanest. Little 4 or 8 ounce versions are probably the most convenient to store.

[Chad]

Awesome, Tom. Did you pull that recipe from somewhere or did you run numbers and figure it out? Either way, I am impressed: I do neutrons, not electrons.

[Origami]

I worked it out once a while ago and posted in a thread over at RC. It's easy enough to figure it out, though, recalling some high school chemistry and taking some practical steps that allow you to use practical equipment to measure things out. Here's the math for those interested:

 

Working backwards, my target was to make a 10 dKH standard. I want to convert this number to milliequivalents per liter, so I divide by 2.8.

 

(10 dKH) / (2.8 dKH/(meq/l)) = 3.5714 meq/l

 

When a molecule of sodium bicarbonate (NaHCO3) dissociates in water, it produces two ions (Na+ and HCO3-) with a single charge, thus 1 Equivalent of baking soda is the same as 1 mole of baking soda. (And, one milliequivalent is equal to one millimole, or 0.001 mole.) In this case, it's the HCO3- (bicarbonate) alkalinity that we're looking for.

 

The molar mas of sodium bicarbonate is 84.01 grams. Thus, 1 millimole is 0.08401 grams by weight.

 

Thus, to make a solution of 10 dkH requires (3.5714 * 0.8401 = ) 0.30 grams of baking soda in enough water to make 1 liter.

 

Measuring out 0.30 grams is a bit too precise and subject to large error if done in one step, so I took two steps instead. (For example, using 0.31 grams results in a 3% error in the standard.)

 

The first step was to make a solution at 15 grams per liter (178.55 meq/l or 500 dKH). Then, I took 10 ml of this solution (containing 0.15 grams of baking soda*) and diluted it into 1/2 liter, giving me the target concentration of 0.30 grams per liter.

 

0.30 g/l = 3.571 meq/l = 10 dKH = 178 ppm CaCO3 equivalent

 

After mixing it up, I checked it using my Hanna Checker which read 181 CaCO3. This reading was within the error range of the Checker, thus confirming the calculation and mixing procedure.

 

*(The math here is as follows: 15 grams/liter * 10 ml * .001 liter/ml = 0.15 grams)