pH drop checker liquid

There's no need for large containers. I mixed up a batch of 4dkh water this evening, and my largest volume was one liter. My scale is only accurate to 1 gram, so I weighed 60 grams of baking soda and dissolved it in one liter of water (which I also measured using the scale). I diluted that 1:20 for a 100 dkh stock solution, and diluted that 1:25 for 4 dkh. Which is to say I measured 10 ml with a giveaway pharmacy syringe and added it to 190 ml, again measured on the scale, and then 10 ml in 240 ml.

None of these volumes is bigger than a liter (I used a Nalgene), and the least precise measurement is the 60 g of baking soda which is +/- 1 g. The syringe probably has slightly better precision. If done carefully, I think this should give precision of something like... one part in 30. This particular serial dilution uses ~1.5 liters of diH2O, and it's $0.60 per gallon anyway.

I personally think that's pretty easy, but there was a time when I got paid to do stuff like this. Ask me sometime about the med student on a rotation in my research lab who didn't understand the concept of a 1/X dilution even after four weeks. :icon_roll

The only difference I can see is that starting with Tap water instead of distilled means that you are starting with possible contaminants and slightly diluting them out. On the other hand starting with Distilled and adding baking soda you eliminate the contaminate issue. Do you not have baking soda? Or is the issue measuring it without a scale? If you don't have access to a scale I took some measurements that maybe of use to you. I repeatedly took a 1 tablespoon packed level measuring spoon of household baking soda and weighed it on an analytical balance. I came up with a weight of 15.8 grams with a measurement repeatability of approximately 8%. Your limitation for accuracy would now be how accurately you can dilute it. I'm going to assume that you can measure water volume to atleast 5% using a 10mL graduated test tube from one of your test kits. Adding 1ml of concentrated solution to 9mL of of distilled water will divide your concentration by 10 give or take 5%. You can then take 1ml of that solution and combine it with 9mL of distilled water to divide the concentration by 10 again give or take 5%, for a total dilution of 100:1 from the origional concentrated solution.

Now using the pesimistic approach of combining measurement uncertainties you assume they are all skewed in the same direction and add them together 8% + 5% +5%, +5% = ±23% accuracy in your final solution. However if you take the more likely case that the measurements are not all skewed in the same direction you would combine the uncertainties using the root sum square method and get a combine measurement uncertainty of about 12%.

12 to 23% accuracy in your KH reference solution is plenty good enough for hobby use for measuring CO2 levels as your measurement of 30ppm CO2 will be accurate to within 7ppm assuming the pessimistic 23% KH error, and accurate to within ±3.8ppm using the more likely 12% KH error. Your ability to read your PH to within 0.2 using color matching is a much larger contributor to your CO2 measurement error at give or take 17ppm from an error of 0.2pH.

I used Chuck Gadds calculator and values of 4.0° KH, and 6.6pH as a starting point and varied the numbers by the amounts listed above to come up with the ppm CO2 errors.

I guess the point of this long winded reply was to suggest that it is easy to eliminate the additional contamination errors even without a scale as most every house has measuring spoons, and you were planning on using distilled water anyway to dilute your tapwater so why not just dilute the concentrate, and that for the DIY drop checker the accuracy of the KH reference solution is not all that critical.

Thanks for all the replies, makes sense.

Blacksunshine, you are correct, there is potential for inaccurate readings. Like mentioned before, phosphates might throw it off, etc.

Walter, exactly my point. The drop checker isn't going to tell you if you have 27 or 31 ppm of CO2. It's mainly "good" or "too much" or "oops I am out". So even if there is a little skew factor, I can probably mix it in such a way that it still indicates what I want. Example, I don't want necessarily 30 ppm, I am okay with 20 ppm. Percentage-wise, big difference, but still "good" for me.

vidiots, thanks for the explanation. Yes, I am able and probably qualified to measure some baking soda into distilled water, shake, and measure the outcome. No problem. My point is that it would be much, much easier for me to take a syringe, and squirt 4 parts tap water and 6 parts distilled water into the drop checker. My tap kH has been consistently 10 for the last couple of years, so I can pretty much count on that. Yes, there will be contaminants, and some margin of error, just like with baking soda, but I don't care -- worst thing that can happen is that the solution turns bright burgundy, and then I know something is definitely contaminated. If I can get it to be green at my normal CO2 levels, it will still turn blue if the bottle is empty, and yellow if the needle valve acts up. :smile: