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Re: [APD] How to make a good KH reference stock solution for CO2 measurement
I will say, I agree with all Tom says, in theory. There are a few holes
that an over achiever like myself has issues with though. First, drying
the powder in the oven is not only a good idea, it is necessary if your
already weighing samples to 3 decimal places. Time consuming and
tedious but necessary non-the-less. Remember, water is heavy and dry
powders are usually hydroscopic, meaning they can suck up their weight
in water after a while . Place some freshly dried powder on an
accurate, 3-4 decimal place scale and you can almost watch the weight
increase as it re-absorbs water. Most people can do this at home though
so no real big deal there.
Second, weighting to 3 decimal places is impossible with normal hobby
equipment. A .1 scale will cost around $30-40, a .01 around $75-100 and
a .001, who the hell wants to spend that much money. I'd rather buy
enough Aquasoil for the tank so that non of this matters. I'll get
some nice ADA blackwood while I'm at it and use the winnings from the
contest to buy whatever I want.... well, you get the idea. In truth
though, if you have a descent, .1 accurate scale, or a balance you would
probably have enough accuracy. As Tom said, larger volumes and amount
equals more accuracy, less error. If you wanted to make 100ml of this
solution, you'll need a .001 scale but if you make 5liters, you should
be fine with a .1 scale.
That last, and maybe hardest part for most folks is measuring out 5
liters of DI water, or accurately measuring out any volume of water
larger than a few milliliters. I have access to nanopure water and .001
scales through school but accurately measuring 5liters would be a royal
pain. If you already have a .001 scale, use 1 liter. Is easy to find
accurate measuring devices for this and you then simply use 0.998(4)
grams of dried NaHCO3. Don't have that accurate scale or want to make 5
liters anyway? Your best bet is to use a square container, like a small
AGA aquarium and very carefully measure the inside of the tank. Using
the internal length and width will allow you to find the height
necessary to obtain, accurately, whatever volume you wish. Of course,
you'll need to make sure the tank is level. Again, you will have a
smaller error factor with larger amounts.
For diluting, the best way I have found using stuff laying around the
house is to use a syringe and suck up the desired amount of solution and
the corresponding DI water. Of course, using graduated cylinders is the
way to go but not everyone has them.
As I said, from what little I know of such things, Tom is certainly
correct in telling use good ways to make reference solution. Yes,
making the solution from the standard kit is a crap shoot though there
is something to be said for consistancy. If someone finds they need
yellow rather than green in the drop checker for good plant health, they
may not have kH5 but at least they are always the same. As for an
accurate solution, I am just trying to share some ideas to make this
accessible/doable for more people.
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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