How to make a reference KH solution for CO2 measurement

[Plantbrain]

Given the wide ranges of KH related issues and suspiciously high CO2 ppm readings in some folk's tap water/tank water, this KH reference method is a very useful thing.

The hardest thing is getting folks to actually make one and calibrate things.

So I did some digging and have a couple of useful links that make it clear.
Here is a good KH Calculator using baking soda to make your KH reference sample.

http://www.cnykoi.com/calculators/calckh.asp

Note: make larger volumes(5 liters) and high concentrations, and then dilute.

This reduces errors(less is better).

So adding 5l of DI H2O (1.321 gallons) and 4.992 grams of baking soda = 40KH.

Play around with the calculator, try 50 liters and 4 KH, the higher volumes and higher the concentration, the less error you have, well up to a point[]

50 liters of DI water and 40 KH will give you 49.923 grams.
Now you have pretty high accuracy but you have a lot of KH solution!!
5 liters is not bad and you can toss the rest down the drain or trade to friends etc in the hobby.

Then you may take a 10ml sample of the 40KH reference and add it to a 90mls of DI water.

This will give you a reference of 4 KH to within a very close tolernace depending on how careful you are weighing the baking soda, measuring the water volume and cooking the baking soda for 30-45 min at 400F before weighing to remove water and CO2 in the baking soda.

For diutions:

http://www.wellesley.edu/Biology/Concepts/Html/volumetovolume.html

This should help you get going on making the KH reference solutions and putting them to good use.

You may use the drop checker method, it's slow, cheap, simple, or you can DIY a DO membrane on the tip of a pH probe, and put the KH ref solution inside instead of the dissolved O2 KCL solution.

This is extremely accurate.
DO membrane material is fairly cheap per unit.

Regards,
Tom Barr

 

[dapellegrini]

Awesome... This is exactly what I was looking for to setup my Drop Checker when it arrives.

If you ever get an easy mod to fit on the end of my SMS122, I think I would do that too...

Thanks Tom

 

[Plantbrain]

Well, I'll see what they do, using a rubber hose that slips on the end of a pH probe and then another size larger over top of that to provide more surface area and then notching the outer hose to faciliate a grove for an O ring makes a simple water tight seal between the water and the KH solution and allows fast equilibration.

Very cheap once you have some poly membrane.
The KH solution is a snap to make.
I think a lot of folks will use the drop checker as it's easier etc, but there will be a crowd that likes the membranes.


Regards,
Tom Barr

 

[squawkbert]

I have GOT to make one of these...(well, 2 of 'em).

I think I'll try to just put some bends in a glass tube, mount w/ suction cups.

I have a microbalance & RO/DI at work & a moisture balance to cook the water out of some baking soda.

I'll get the indicator for xmas as part of a APA FW master kit... I think I'm set!

If I get good at it, I may buy dry indicator and premix test solutions of differing KH values and start my own ebay business. I'm thinking I can make these for <<$1 ea. (glass, propane will probably be the most costly parts).

 

[dapellegrini]

Nice! Make a simple end cap for my SMS122 prob and I would buy one of those from you too!

 

[Plantbrain]

BTW, a nice cheap scale can be bought on ebay for 10-20$ for measuring 4.99 grams of Baking soda, cook it first though.

Then if you can, find a cheap graduated cylinder etc on ebay also, or www.AquaticEco.com, chem supply place etc, they should be pretty cheap, or a volumetric flask etc to make dilutions and stock solutions for any range of of Nutrients such as NO3, K+, PO4, etc.

Then you can calibrate all those cheap test kits

Regards,
Tom Barr

 

[malkore]

I'm going to sticky this for a while to make sure everyone sees it. Thanks for sharing your wealth of knowledge Tom.

 

[dapellegrini]

Ok, so a few more questions now that I have my drop checker and am gearing up to make my kH solution.

- Cleaning the drop checker before use - I am assuming that a few rinses in the huge amount of kH solution that I will end up with would be sufficient?
- The water - Is "Distilled" water the same thing as "Deionized" water?
- The test regent - Is there a certain pH test regent that is best for this application? The regent that came with the kit was open in transit and had to be thrown away. I am considering using my AP pH test regent... Will this work?
- Refreshing the solution - How often, if ever. should I change the kH reference and Regent to maintain accuracy?

Once I have my kH solution baked up, mixed and measured, I plan on using it in the AP kit's pH test, then using that liquid, which will include the regent, to fill the drop checker. Does this sound right?

TIA

 

[squawkbert]

- Cleaning the drop checker before use - I am assuming that a few rinses in the huge amount of kH solution that I will end up with would be sufficient?

Yes

- The water - Is "Distilled" water the same thing as "Deionized" water?

Yes

- The test regent - Is there a certain pH test regent that is best for this application? The regent that came with the kit was open in transit and had to be thrown away. I am considering using my AP pH test regent... Will this work?

I think so (should be bromothymol, try it)

- Refreshing the solution - How often, if ever. should I change the kH reference and Regent to maintain accuracy?

when spilled, contaminated w/ tank water (got loaches?) or if it fades

- Once I have my kH solution baked up, mixed and measured, I plan on using it in the AP kit's pH test, then using that liquid, which will include the regent, to fill the drop checker. Does this sound right?

Just add the 4dKH solution you made to the drop checker, add a few drops of the pH reagent (hwo ever many it'll take to give decent color), that's it.

I'm loving the stickyness of this thread.

 

[dapellegrini]

squawkbert - Thanks!

One more: Concerning the air gap in the drop checker. Do I need to reduce the air gap or is it ok to leave as much as as possible in there? Will the amount of air between the aquarium water and the kH solution effect response times or accuracy?

 

[squawkbert]

I suspect that more air will lead to a somewhat slower time for the thing to settle down (equilibrate), but in the long run, it shouldn't matter much. The only way to get a really appreciable increase in reaction time for these would probably be to find a good gas permeable membrane so you could pretty much eliminate air from the system on the indicator side.

 

[dapellegrini]

So I broke down and did it today. Here is my photo journal, making the reference kH solution for the Drop Checker, per Tom Barr's recommendations.

********************************************************

THIS LOG WAS MADE WHEN THIS SUBJECT WAS STILL VERY NEW - AND A FEW THINGS HAVE BEEN CORRECTED SINCE. INSTEAD OF 4.99 GRAMS OF BAKING SODA YOU WILL NEED 6.00. AND DO *NOT* BAKE IT AT ALL - THIS WILL CHANGE THE CHEMICAL COMPOSITION OF THE POWDER. OTHERWISE THIS IS STILL AN ACCURATE REPRESENTATION OF HOW YOU DO THIS

********************************************************

First a quick list of everything I picked up for this project:

- Scale that would weigh out to the .01 grams - $20 on E-bay
- Set of graduated cylinders (10ml, 50ml, 100ml) - $20 on E-bay
- Drop Checker - Chinese ripoff version of ADA - $22 on E-bay from Hong Kong
- 10 liters of Distilled Water - $2 at local grocery store
- 2 or 3 gallon bucket - $2 at local grocery store
- 2 bottles of SOBE - cheapest way to get two glass storage bottles for the excess mixture

Things that I had laying around:

- Big sack of Baking Soda
- AP pH test kit
- Dropper from another test kit (Hagen brand)
- Liquid measure for 1 liter measurement
- small creme-brule pot to cook the Baking Soda in the oven
- little medicine dosing cup
- chopsticks - to stir my mixture

So here is the whole setup in a couple of pics:

So with everything finally gathered, off we go...

First things first, measure out a bunch of backing soda. Need quite a bit more than the required 6.00 (corrected from 4.99) grams, just incase mine is full of water and/or CO2...

This should do:

Into the Creme Brule bowl and on into the oven at 400-degrees for 1 hour (I know, the oven is a bit of a mess): *** UPDATE: Do NOT cook the baking soda - it will actually change the chemical composition of the powder and you will not have an accurate mix. I will leave the photo here for posterity sake...

One hour later, it is time to measure out 6.00 (corrected from 4.99) grams of this stuff. I have a few doubts about how accurate this cheap scale is for this, but I figure this looks about right:

Then to measure out the water. I was thinking I would find distilled water premeasured in liters, but no such luck. Good thing my wife had this pyrex measuring cup. Measured out 5 liters into the new bucket (also washed and thoroughly rinsed everything before use).

The bucket confirmed my measurements, but in quarts... I didn't think quarts were quite the same thing as liters, but again, i trust the pyrex measures.. I was eye level with the thing and perhaps a bit too anal about the measurements... you can see it exactly on the 5 line in the bucket:

Well my next dilemma was exactly what to use to mix this stuff. At each phase I was concerned about contaminating my mixture (again a bit anal, but i really didn't want to have to do this all twice). Well I settled on a super clean pair of chopsticks.

The kids drank the SOBEs and I washed, rewashed and rinsed the bottles, then labeled and filled.

The first bottle to get filled was of the 40dkH mixture straight out of the bucket. I figure 500ml should last me forever, or as long as I care to believe that it is still any good.

The second bottle required diluting... In comes the graduated cylinder. I ended up only using the 100ml one. Used a dropper from one of my Hagen test kits (again rinsed like 10 times first) to fill the first 10ml with the bucket water. Then filled the last 90ml with clean distilled water for my 4dkH solution. Repeated this 5 times, pouring each into the other SOBE bottle.

OK, now we are getting somewhere. Next, I decided to break out the AP test kit, filled the pH test to it's line and dropped in 3-drops of regent, per the instructions (UPDATE: Adding 6-8 drops will make the color much easier to read and will not otherwise change the effectiveness of your measurement - thus highly recommended to add more drops). Bingo, blue water showing 7.6 pH. Looks like we are on track:

Then I took the dropper, filled it with the test solution and squirted it into the drop checker. Bingo:

And in the aquarium it goes. Clocking the time in at 5:45pm:

And here it is now down to my base color of green (6.6 pH) at about 7:30pm. (PWC, thus all of the bubbles, and yes I know I need to pull that algae off of the spraybar)

I hope I did all of this right. Of course let me know if I did not :S ...

Also hope someone out there finds this a useful addition to this sticky. Would love to see something similar for the pH probe cap.

 

[godzilla]

no comments on this? Thanks for doing the work and journalizing it. I am ordering a hong kong knockoff drop checker also so im glad to see this post.

 

[dapellegrini]

no comments on this? Thanks for doing the work and journalizing it. I am ordering a hong kong knockoff drop checker also so im glad to see this post.

You're welcome.

I think everyone out there that does not have a fully automated CO2 system (with pH monitor), should have one of these drop checkers with reference kH solution as it seems that stable CO2 is the leading cause of problems in the planted tank...

 

[Zezmo]

Interested in how accurately you can use a test?
I posted most of this info as a response to a question about hobby tests. Since this sticky thread is about making a reference solution, I though I would add some info about how to determine if you are measuring your reference solution in a repeatable fashion.

There is a simple study called a Gage R&R (repeatability, and reproducibility). It will take some work, but you only go through this series once and it will tell you a lot.

In it's simple form a GR&R basically consists of taking 5 consecutive measurements on 2-9 separate occasions, and then plugging the results into a formula. (the link below has a spreadsheet that does all the math for you).

Taking Ph as an example because it is the most common test most folks use (though can do a Gage R&R for Kh or any other testable parameter). Ideally, you would buy a gallon of bottles spring water from the store and use it for the entire series of tests (or make a reference solution as described in this thread). This should give a reasonable constant sample to work with so that you are only focusing on accuracy of the test and the tester.

You would using this water do 5x Ph tests in a row and record the results. Do this at least 2 days in a row to get a total of 10 measurements. (minimum)
To really get good data, you should do this 3 days in a row, on 3 consecutive weeks. Sounds like a lot, but again you only ever need to do this once.
Plug these results into the spreadsheet. The results in yellow at the bottom of the spreadsheet will give you an idea of how closely you can rely on your test results.

http://webpages.charter.net/zezmo/Gage_R&R_for_aquarium_v1.xls

Note: A gage R&R looks at variations in the test and tester, not the actual test accuracy. Accuracy can only be verified against a reference solution (calibration). However, without repeatability, and reproducibility the actual accuracy of the test does not even come into play.

 

[7Enigma]

Anyone have a % error if the baking soda is not baked prior to making the standard?

Tom, thanks for posting this. I spent 20minutes trying to find this post before I realized its now a sticky.

And thanks dapellegrini for posting the step by step picture guide, it should help quite a lot of people unfamiliar with making buffers. I'll be testing mine out tonight as my RedSea drop checker came in the mail yesterday from Big Al's. Not near as attractive as the Hong Kong or ADA checkers, but it was only $8. And even at that price I feel ripped off....

 

[squawkbert]

Baking soda error shouldn't be more than about 2%. The object of baking it a little is to drive off the ~2% water weight that's there coming out of the box.

 

[dapellegrini]

FWIW, I just changed out the solution in my drop checker as it was starting to fade a bit, but was happy that it lasted about a month. In my new batch I added twice as much pH regent. While this makes the blue color very dark, the green and yellow colors tend to be much less pronounced. So I guess I am saying, don't be shy with the pH regent. The more you put in their the easier it will be to see the green/yellow color.

 

[squawkbert]

Dropchecker w/ gas-permeable membrane (won't leak), made from cheap stuff.

 

[dapellegrini]

squawkbert - How fast do you see the regent change color using the membrane instead of an air gap?