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Re: [APD] How to make a good KH reference stock solution for CO2 measurement
Of course you are right about how much more accurate it is to make a
standard KH solution by weighing and measuring the ingredients. But,
ppm of CO2=3*dKH*10exp(7-pH)
This means an error of 20% in measuring the dKH will cause an error
of 20% in measuring ppm of CO2. So, if I believe I have 5 dKH water,
but have a 20% error, my ppm of CO2, when the KH reference solution
is green will be from 30 to 46, assuming I can see green accurately.
That is darn good compared to any other easy method for measuring ppm
of CO2. It is only when you need to know the ppm much more
accurately that you would go to a quality pH probe plus very accurate
KH reference water. For us casual hobbyists, 30 -46 is a nice range
of accuracy. For you scientists, you need to use the pH probe with
condom filled with precision KH water.
I'm not disagreeing with your approach, just pointing out that I
don't need a micrometer to measure the thickness of a pine board.
On Dec 13, 2006, at 1:23 PM, Thomas Barr wrote:
> I'm going to detail out some issues for folks with making stock
> solutions. I've heard some odd stuff folks are doing and getting
> way off in their measurements and methods.
> This post will hopefully address some of this before more issues
> 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 so folks do not have do chemistry(I know you will
> all avoid it like the plague,<grin>).
> Here is a good KH Calculator using baking soda to make your KH
> reference sample.
> Note: make larger volumes(5 liters) and high concentrations, and
> then dilute (this allows you to make 5KH, 10 KH, 4KH, 1 KH etc
> from the same higher concentrated stock solution).
> This reduces errors(less error 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[:p]
> 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
> 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 100ml sample of the 40KH reference and add
> it to a 900mls 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:
> 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
> This is extremely accurate.
> DO membrane material is fairly cheap per unit.
> Using a hose on the tip of pH probe and making a melted groove
> or a cut for the O ring can make your own DIY version if you buy
> the DO membrane material.
> Now some folks have taken to using their KH test kits to make a
> reference solution!!!
> Do NOT use a KH test kit which is not calibrated to make a
> reference solution. You compare the reference solution to the KH
> test kit, not the other way around!!!!!
> I really really hope folks understand that last part.
> Play around with this.
> Try different volumes and different KH's and see how accurate
> the baking soda weight needs to be to achieve good accuracy.
> The water volume measure is also reduced when you increase the
> volume. So when you make a batch of KH=4, use 100mls, not 1 ml
> unless you have a very accurate pipette(1ml)or micro pipetter.
> That willr educe the rror but waste some ref solution which is
> cheap and now you have 5 liters:-) May as well use it.
> Note, for basic practical purposes(oh my, I do think about such
> stuff), scales are cheap that are in the 0.01 range, while they
> shoot up in cost when you get to the 0.001, 0.0001 range.
> So making 5 liters DI water with 4.99 grams of cooked baking
> soda ought to be fine. That will be pretty accurate and not
> require much in the way of expensive scales nor volumetric flask
> I know and realize some will round things off another digit, but
> the cost of a scale is pretty cheap, about 20$.
> Tom Barr
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