DIY alkalinity standard

WallyB

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FWIW, my recipe above called for sodium carbonate (washing soda) as the starting material, not sodium bicarbonate (baking soda). The results look OK, but folks doing that should bake it extra long and hot.
Ahhh.

I didn't catch the sodium carbonate (washing soda) vs sodium carbonate (washing soda)

Just goes to show what I lousy chemist I am. Glad my mix up wasn't a dangerous thing.

I will re-try with Washing Soda when I find some.

Thanks.
 
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WallyB

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FWIW, Hanna does not do an ordinary alkalinity titration, but uses a slightly different method. I expect it is a one point "titration" like described here:


Essentially, they add an amount of acid similar to that needed in an ordinary alkalinity titration, and then use the color to determine the pH, and back calculate what the alkalinity would have been to give that pH with that amount of added acid.

Without seeing some data on how the dye they use responds (in terms of absorbance and also pKa), I cannot be sure this method is reliable for the Hanna, and would be wary of over interpreting the Hanna accuracy based on it.

You can get there with one additional step if you have another kit

1. Carefully check your normal kit for accuracy.
2. Use it to measure tank water or new salt water
3. use the Hanna to measure that same tank water or new salt water and compare.

That's exactly what I've been trying to do, to kind of Calibrate the offset for my Hanna (From the Range of 7.0 dkH, to 8.5 dkh which is the the limit that I wish my DT to stay in).

and (1. Carefully check your normal kit for accuracy.) is why I'm persuing a accurate Alk Reference Solution.
 

barista7105

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For folks who may want to evaluate the accuracy of their alkalinity test kits, here are two suggested ways to do it. One is DIY and one is a mostly commercial standard with one DIY step.

Method 1
Standard solutions could be made with sodium bicarbonate, but it can be hard to be sure it hasn't begin to absorb moisture and/or decompose into sodium carbonate.

Thus my suggestion is to start with sodium carbonate, and bake it yourself to be sure it is dry. If not dry, the weight of the water will make the standard less potent than you expect.

To fully dry sodium carbonate before use we will bake it in a home oven.

Preheat the oven to about 400 deg F.

Spread about a quarter cup of sodium carbonate on a pyrex casserole dish or a roasting pan. You want CO2 and H2O gas to readily escape from it, hence the spreading out.

Place it in the oven and leave it there for 60 minutes.

Remove and cool the dish. Don't place it right near a pot of boiling water, you do not water it reabsorbing moisture.

Weight out 10.0 grams of the dry solid powder (scale the whole recipe down if your scale cannot weigh 10 grams). This 10 grams contains 0.0943 moles (94.3 mmoles) of sodium carbonate. Each carbonate contributes 2 units of alkalinity, so this 10 grams contains 2 x 94.3 mmoles = 189 milliequivalents of alkalinity.

Dissolve the 10 grams of powder in 1.0 liter (1000 g) of RO/DI or distilled water. The more accurate you are with this measurement of water, the more accurate the standard will be.

This solution has an alkalinity of 189 meq/L, which equals 528 dKH. Call this stock solution #1.

Remove 15.1 grams (or 15.1 mL) of Stock Solution #1 and combine the 15.1 g with Ro/DI or distilled water to a final total mass of 1000 g or a volume of 1000 mL.

This final solution will contain 189 meq/L x 0.0151 L = 2.86 meq in one liter = 8.0 dKH.

This final solution can be used to test a titration test kit for total alkalinity.

Method 2.
For those who do not want a pure DIY, but want a highly accurate standard, you can start with a premade solution of 0.01 N sodium hydroxide, such as this one from amazon for $16.45:

LabChem LC242001 Sodium Hydroxide Solution, 0.01N (0.01M), 500 mL Volume

As it arrives, this solution contains 10 meq/L of alkalinity, or 28 dKH. You could use that without dilution with a test kit, but will typically need a lot of titrant to reach the endpoint, wasting the titrant.

If you take 250 mL of this fluid and 750 mL of RO/DI water (or any volumes that are a 1:3 ratio), you will have a fluid that is 7 dKH and is ready for titration kit testing.

If anyone notes a math error in either recipe, please let me know :)

Happy Reefing
If I use sodium bicarbonate can I bake it to ensure minimum moisture inside it?
 

Miller535

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For folks who may want to evaluate the accuracy of their alkalinity test kits, here are two suggested ways to do it. One is DIY and one is a mostly commercial standard with one DIY step.

Method 1
Standard solutions could be made with sodium bicarbonate, but it can be hard to be sure it hasn't begin to absorb moisture and/or decompose into sodium carbonate.

Thus my suggestion is to start with sodium carbonate, and bake it yourself to be sure it is dry. If not dry, the weight of the water will make the standard less potent than you expect.

To fully dry sodium carbonate before use we will bake it in a home oven.

Preheat the oven to about 400 deg F.

Spread about a quarter cup of sodium carbonate on a pyrex casserole dish or a roasting pan. You want CO2 and H2O gas to readily escape from it, hence the spreading out.

Place it in the oven and leave it there for 60 minutes.

Remove and cool the dish. Don't place it right near a pot of boiling water, you do not water it reabsorbing moisture.

Weight out 10.0 grams of the dry solid powder (scale the whole recipe down if your scale cannot weigh 10 grams). This 10 grams contains 0.0943 moles (94.3 mmoles) of sodium carbonate. Each carbonate contributes 2 units of alkalinity, so this 10 grams contains 2 x 94.3 mmoles = 189 milliequivalents of alkalinity.

Dissolve the 10 grams of powder in 1.0 liter (1000 g) of RO/DI or distilled water. The more accurate you are with this measurement of water, the more accurate the standard will be.

This solution has an alkalinity of 189 meq/L, which equals 528 dKH. Call this stock solution #1.

Remove 15.1 grams (or 15.1 mL) of Stock Solution #1 and combine the 15.1 g with Ro/DI or distilled water to a final total mass of 1000 g or a volume of 1000 mL.

This final solution will contain 189 meq/L x 0.0151 L = 2.86 meq in one liter = 8.0 dKH.

This final solution can be used to test a titration test kit for total alkalinity.

Method 2.
For those who do not want a pure DIY, but want a highly accurate standard, you can start with a premade solution of 0.01 N sodium hydroxide, such as this one from amazon for $16.45:

LabChem LC242001 Sodium Hydroxide Solution, 0.01N (0.01M), 500 mL Volume

As it arrives, this solution contains 10 meq/L of alkalinity, or 28 dKH. You could use that without dilution with a test kit, but will typically need a lot of titrant to reach the endpoint, wasting the titrant.

If you take 250 mL of this fluid and 750 mL of RO/DI water (or any volumes that are a 1:3 ratio), you will have a fluid that is 7 dKH and is ready for titration kit testing.

If anyone notes a math error in either recipe, please let me know :)

Happy Reefing

Randy, for method 2, could I instead make a 1:1 ratio? My hanna checker uses 10ml of sample solution for testing. Could I just take 5ml of the sodium hydroxide and 5ml of RODI water and mix them? And expect a dkh of 14?
 

Rybren

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... is why I'm persuing a accurate Alk Reference Solution.

Fauna Marin makes a Multi-Reference solution - in my bottle, alk was listed at 7.5 dKH. When I checked it using a Salifert kit, I came up with 7.5 dKH.

Apparently, each batch can vary from other batches, but the bottle lists the verified values on the label.
 

WallyB

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Fauna Marin makes a Multi-Reference solution - in my bottle, alk was listed at 7.5 dKH. When I checked it using a Salifert kit, I came up with 7.5 dKH.

Apparently, each batch can vary from other batches, but the bottle lists the verified values on the label.
Thanks. I ordered one and then once I have a valid Test will make more.
 
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Randy Holmes-Farley

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Randy, for method 2, could I instead make a 1:1 ratio? My hanna checker uses 10ml of sample solution for testing. Could I just take 5ml of the sodium hydroxide and 5ml of RODI water and mix them? And expect a dkh of 14?

Yes, with the caveat that I stated above that I am not certain how well the hydroxide solution works with the Hanna.
 
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Randy Holmes-Farley

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Randy, do you see any problem using an inexpensive 0.01N sodium hydroxide like this one.....or is there too great a risk that it may not be as "good" as the one you suggested?


I don't know, but my concern would be how carefully they target that concentration in a product like that.
 

Dkeller_nc

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A few procedural notes that might assist those that wish to do this.

The first is that (as Randy notes) the more accurate your volumetric and mass measurements are, the more accurate your test results will be. The first thing to address is the mass measurement. There's a bazillion options on Amazon and other places for a scale that will read to at least 0.1 gram. Note that the decimal places that the scale reads to is the "precision" of the scale, not the accuracy. Accuracy can only be established with a bonafide mass standard. It used to be that a set of standardization weights for a scale were harshly expensive (and still are if you want traceability to National Institutes of Standards (NIST)). However, you can get a set of standards for a scale for less than $20 nowadays that aren't certified.

The second part to address is volumetric measurements. I'd strongly recommend ordering at least one (two is better) one liter volumetric flask. Again, these used to be really expensive, but nowadays they can be had for less than $20. The Karter Scientific ones off of Amazon are perfectly fine, and cost $16 each. You can use the volumetric flasks on their own by putting the material to be dissolved in them, adding enough water to come to the 1 liter mark on the flask, stoppering it, and inverting it repeatedly until the material dissolves. You will need to check the volume measurement again after all of the material dissolves, as it will be less than one liter - simply add sufficient water to come to the mark with an eyedropper or plastic pipette, re-stopper the flask, and invert several times to mix the solution.

While the above works, it's also a pain in the rear, especially with materials that dissolve slowly. For convenience, you may wish to acquire a magnetic stir plate and a magnetic stir bar. Again, those used to be really expensive, but are now very cheap.

Finally, one of the issues with converting sodium bicarbonate to sodium carbonate, (or drying out a bulk sodium carbonate material) is that as you allow it to cool off on the counter after removing it from the oven, you will absorb some amount of moisture from the air that will convert a small amount of the freshly-made sodium carbonate to sodium bicarbonate. If you wish to avoid this, it's quite easy with the help of a 1 quart mason jar. Simply (using gloves!) pour or spoon the hot sodium carbonate into the glass mason jar, seal the top, and allow it to cool to room temperature on the counter.
 
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Miller535

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Yes, with the caveat that I stated above that I am not certain how well the hydroxide solution works with the Hanna.

I just did it and got 13.5dkh. Which is probably pretty accurate being that its outside of the range usually measured in aquaria. Comparatively I also did a test with fauna marins multi reference that is supposed to be 6.6dkh and I got 6.7.
 
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Randy Holmes-Farley

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I just did it and got 13.5dkh. Which is probably pretty accurate being that its outside of the range usually measured in aquaria. Comparatively I also did a test with fauna marins multi reference that is supposed to be 6.6dkh and I got 6.7.

Sounds good!
 
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Randy Holmes-Farley

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A few procedural notes that might assist those that wish to do this.

The first is that (as Randy notes) the more accurate your volumetric and mass measurements are, the more accurate your test results will be. The first thing to address is the mass measurement. There's a bazillion options on Amazon and other places for a scale that will read to at least 0.1 gram. Note that the decimal places that the scale reads to is the "precision" of the scale, not the accuracy. Accuracy can only be established with a bonafide mass standard. It used to be that a set of standardization weights for a scale were harshly expensive (and still are if you want traceability to National Institutes of Standards (NIST)). However, you can get a set of standards for a scale for less than $20 nowadays that aren't certified.

The second part to address is volumetric measurements. I'd strongly recommend ordering at least one (two is better) one liter volumetric flask. Again, these used to be really expensive, but nowadays they can be had for less than $20. The Karter Scientific ones off of Amazon are perfectly fine, and cost $16 each. You can use the volumetric flasks on their own by putting the material to be dissolved in them, adding enough water to come to the 1 liter mark on the flask, stoppering it, and inverting it repeatedly until the material dissolves. You will need to check the volume measurement again after all of the material dissolves, as it will be less than one liter - simply add sufficient water to come to the mark with an eyedropper or plastic pipette, re-stopper the flask, and invert several times to mix the solution.

While the above works, it's also a pain in the rear, especially with materials that dissolve slowly. For convenience, you may wish to acquire a magnetic stir plate and a magnetic stir bar. Again, those used to be really expensive, but are now very cheap.

Finally, one of the issues with converting sodium bicarbonate to sodium carbonate, (or drying out a bulk sodium carbonate material) is that as you allow it to cool off on the counter after removing it from the oven, you will absorb some amount of moisture from the air that will convert a small amount of the freshly-made sodium carbonate to sodium bicarbonate. If you wish to avoid this, it's quite easy with the help of a 1 quart mason jar. Simply (using gloves!) pour or spoon the hot sodium carbonate into the glass mason jar, seal the top, and allow it to cool to room temperature on the counter.

Great tips!
 
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Dkeller_nc

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Thanks, Randy.

By the way - this is tangentially related to the subject of this thread, but Randy published a DIY procedure for doing precise alkalinity measurements with a pH meter and a standardized acid solution. Folks like Randy and I that have done extensive work in a chemistry lab would do this with a few pieces of equipment that make the job far easier.

The first has already been mentioned - a magnetic stir plate and a stir bar. And typically, we would either weigh the aquarium water sample, and together with its specific gravity would calculate its volume, or we would measure the volume with a graduated cylinder or 100mL volumetric flask. That's because just like the alkalinity standard that's the subject of this thread, the accuracy of your determination is in part dependent on the accuracy of your volumetric measurements of the aquarium water sample and the titrant that you use during the test (the 0.1N HCl solution).

That last point was the motivation for me to add this reply to the thread. I'd think most reefers would be familiar with magnetic stir plates, scales and even volumetric flasks. They might not be familiar with the standard tool for measurement of the titrant in titration procedures, which is a burette and burette stand. They look like this, and can be had for relatively little money compared to what they cost in the 1980's and 1990's before India got into the game of making scientific glassware. The stand that you use to hold the burette in use looks like this.

By the way - there's a large market for used scientific equipment on the 'net. You should easily find a used burette clamp and ringstand on fleabay or any of the used scientific equipment websites, which may save you some $$ in case you want the equipment I've mentioned in this post.
 
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Dennis Cartier

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Sigh. I am trying to validate a dkh reading I am getting and was happy to find this post about making a standard. I was really pleased when I realized that I had an unopened bottle of sodium hydroxide on hand. I followed the directions and mixed up 250ml of the standard along with 750ml RODI. Right about then is when I took a closer look at the sodium hydroxide and realized I had 0.1N, not 0.01N.

My thinking was that I should have ended up with a 70 dkh solution after the faux pas, so I tried to dilute it down to 7 dkh by mixing 50 ml of the 70 dkh solution to 450 ml of RODI.

I then tested the supposed 7 dkh solution with a Salifert test and got between 2.5 ~ 2.8 dkh, less than half of what I was expecting. Where did I go wrong (other than using 0.1N)?

The bottle of sodium hydroxide is a bit past it's expiry (Sep 17, 2020), but it was unopened, so hopefully still good.

I should also mention, all volumes were measured using volumetric flasks. So reasonably accurate (hopefully).

Dennis
 

Dkeller_nc

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Well, the first thing to note is that there's a difference between sodium hydroxide solutions sold as a titration standard and sodium hydroxide solutions sold as a general laboratory reagent. Titration standards are generally far more accurately made; typical specifications for a solution like that from Fisher Scientific are generally accurate to within 1% of the stated concentration.

That said, I would definitely not expect that a solution that is labeled as 0.1N sodium hydroxide would be one half the listed concentration. You mentioned measuring the volumes with volumetric flasks. That's certainly possible for the 250mL of the 0.1N NaOH, as 250mL volumetric flasks are quite common. However, a 750mL volumetric flask is decidedly uncommon, as is a 450mL volumetric (for the last dilution that you did).

There is, of course, a way to do the dilutions that you did with a 1L and 500mL volumetric, which would be to measure 250mL of the 0.1N NaOH solution into a 1L volumetric, then add RODI to the marked line on the flask. Similarly, you can measure 50mL of the 25% 0.1N NaOH solution into a 500mL volumetric flask and dilute it up to the mark. Using this procedure to make the dilutions will be very accurate.

Without standing over you and watching what you did, everything that follows is pure speculation, but might help you or someone else reading this to troubleshoot on your own:

1) Sodium hydroxide solutions have a density considerably greater than water, though in the case of a 0.1N NaOH solution, the difference is not great. I only note this for others that might be starting with a 1N or even a 10N solution - you can't weigh, for example, 50g of a 10N NaOH solution and assume that it's 50mL. As a matter of principle, you want to measure the actual volume of the concentrated solution, though in this case it's unlikely that the error of assuming that the specific gravity of a sodium hydroxide solution is 1.0 will have all that much effect - certainly not 50%.

2) There's a special method for determining whether you've sufficient volume in a graduated cylinder or a volumetric flask - you have to determine the position of the bottom of the meniscus relative to the graduations on the flask or cylinder. If someone reading this isn't familiar with this concept, google it - there will be lots of graphical illustrations on how to do this.

3) If you're truly using volumetric flasks to do your dilutions, realize that manually mixing a volumetric flask by inversion needs to be done thoroughly - invert it (with the stopper in place, obviously) at least 10 times. Otherwise, your solution is not likely to be uniform in the flask, and further serial dilution of it by pouring out of the flask is going to have inaccurate concentration.

4) Make absolutely sure that your glassware that you use to do this procedure is clean and that it's rinsed thoroughly with RODI. Otherwise, residual materials in the flask may have undesirable effects on the accuracy of your final 1:4 dilution of 0.01N NaOH solution by either neutralizing some of the sodium hydroxide or adding to the solution's alkalinity.

Dennis - I posted the above as just general principles of preparing laboratory solutions. In your particular case, unless you made a mistake during the dilution process (there's no shame in that - we all do it, even those of us with extensive laboratory experience), I'd guess the most likely reason for the result that you got was a very sub-standard starting sodium hydroxide solution, or a mistake in executing the salifert test.
 
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Dennis Cartier

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@Dkeller_nc I used the LabChem Sodium Hydroxide that Randy had mentioned, though I used the wrong concentration (LC242701). I also diluted the way you suggested. I started with a 250ml flask for the reagent, added the contents of that to a 1L flask and filled the rest with RODI. For the attempted 'save' I used a 50ml flask and 500ml flask. So it sounds like my methodology was sound, though not my performance.

My flasks are only grade B, but I figure that should still be close enough for aquarium use. All volumes were finished off with single drops from a syringe to hit the meniscus on the etched line on the flask neck.

For anyone using volumetric flasks for the first time and needs reading glasses. I find it helps to illuminate the etched line with a small flash light from the side, while you dribble in the final amount.

Thanks for confirming the correct process. I must have messed up along the way. I am going to test the 70 dkh to see if I can ascertain at what point things went awry.

Dennis
 

Dennis Cartier

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I decided to just attempt to make another batch of solution rather than worrying about how I had messed up. This time forewarned with the fact that I am using a 0.1N standard, I mixed 25ml of 0.1N with 1L of RODI. On my first test of the new standard, I got some really low reading (around what I had before). I re-tested, but with better light and I stopped as soon as the test changed to purple. When I compared the chart, 7.0 dkh. Yay! What I realized is that I had been going too far past the colour change. I was shooting for pink, but the change from blue to purple seems to be the correct point.

I went back and re-tested my tank water, being more careful to nail the colour shift, and got a reading that matched my automated tester, which is what I have been trying to validate.

Now I can crank up my CaRx effulent being assured of the numbers I am targetting. I was able to use the 7.0 dkh standard to not only validate the test, but to validate the tester (me). So a success all around.

Dennis
 

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