|07-11-2013 07:38 AM|
|R. David Jain||
An alternate recipe
Here are some instructions I wrote up for an alternate method to make 4dKH solution. I had two main goals in coming up with this method: The first was to avoid the need for accurate weighing equipment (in fact, there is no weighing at all in this method, just measuring volumes). The second was to have it work even if the baking soda is not "fresh"... so that it would be precise even using an already-opened package that may have absorbed some water. The answer to both these requirements was to start with a fully saturated solution of baking soda and dilute from there. Since the saturation concentrations of sodium bicarbonate in (distilled) water are well-defined and known, this should (and in my estimation DOES) give good results, without having to measure out small amounts of baking soda ("is my level teaspoon level enough? Is the soda too compacted? How do I measure out 2 grams and how much of that is water??"). After some science facts and preliminaries, I lay out the steps below. I didn't have the energy to make a separate "what you will need" section, so you may want to read through the instructions first and make a list (almost everything is something that most people, especially fish-keepers, will have around the house anyway). The steps are simple enough, if long-winded, and this level of precision really isn't necessary probably for most people, but it was fun to come up with, and gives me peace of mind knowing that my "4dKH" solution is accurate to a few percent, and I won't inadvertently gas my fish .
Without further ado...
Relevant solubility figures (in water) for sodium bicarbonate
(see full temperature range of solubility at: http://www.tatachemicals.com/europe/...solubility.pdf)
69 g/L (0 °C)
96 g/L (20 °C)
165 g/L (60 °C)
236 g/L (100 °C)
Solubility vs. temperature is highly linear in the full range 0-100C relevant for liquid water, so one can interpolate
from these figures as needed. For example, interpolating the solubility at 25C using the data points for 0C and 20C
gives (solubility at 25C) ~ [96 + 5*(96-69)/20]=102.75 g/L, and this is close enough for our purposes.
Our goal in this process will be to reach a concentration of 120mg/liter NaHCO3. From Wikipedia (edited for brevity):
"An aqueous solution containing 120 mg NaHCO3 (baking soda) per litre of water will contain 1.4285 mmol/L of bicarbonate.
...Since one degree KH = 17.848 mg/L CaCO3, this solution has a KH of 4.0052 degrees."
"Water" throughout refers to distilled (or de-ionized) water.
Dilution ratios are given as (amount total final mixture) : (amount starting solution). So, "Dilute 2:1" means
mix 1 part starting solution with 1 part distilled water.
"1 Cup" here does not necessarily have to mean 1 8oz cup (though it could be). You can decide on exactly what size
"cup" you would like to use depending on your needs. Larger quantities mean a more accurate dilution, but more wasted
materials (baking soda and distilled water). I chose to use a "cup" size which was actually 3/4cup=180mL because I had
a 3/4 measuring cup handy.
Be sure to wash and thoroughly dry (tap water is fine for this) your mixing vessels at EACH STAGE of the dilution.
Because we will be diluting over several orders of magnitude in concentration, even a small residue of higher
concentration material from an earlier step (say, from re-using the mixing vessels) will throw off the final
1) Create a saturated solution of sodium bicarbonate. First mix 2cups hot water with enough baking soda to saturate the solution. You will know that it's saturated when no more of the powder can dissolve and any additional amount added falls to the bottom of your mixing vessel. Using hot water (distilled water heated on the stove) ensures that the solution is truly saturated. After dissolving, we will cool the water back to room temperature, allowing any excess to precipitate out. Do not heat the water too much (warm to the touch is fine)!! At high temperatures, the bicarbonate ions begin releasing C02 and water to form carbonate ions, which will throw off the calculations.
2) Cool the water back to room temperature. For me, this was 25C, so my calculations in the later steps are based on this number. Adjust your numbers accordingly. Using an electronic temperature probe (like one used for cooking) submerged directly in the water will help you tell when the temperature has finished dropping and give you a more precise final temperature reading than a wall thermometer or home thermostat.
3) Do an initial dilution of 2:1. I would suggest bottling and saving a large portion (1cup or more) of this initial dilution to use for making more 4dKH water in the future, without having to repeat steps 1 and 2). You probably want to use a coffee filter for this initial dilution to get out any precipitated bits of baking soda. By diluting 2:1, we ensure no further precipitation of the bicarbonate out of solution, even if the temperature drops. Be sure to label the concentration of this solution on the bottle based on your temperature reading and the 2:1 dilution!!
4) Assuming the temperature of your starting solution was 25C, after this 2:1 dilution, you will have a concentration of 51.375 g/L (one half the saturation concentration read-off or interpolated from the solubility information above, based on your solution's temperature after cooling). Let's get diluting! We need to dilute by an overall factor of [51375mg/L]/[120mg/L]=428.125 (adjust the first number on the left as needed).
5) Dilute in a ratio of 2:1 8 times (NOT counting the initial 2:1 dilution). Keep track of your dilutions!! I STRONGLY suggest you have a pad of paper next to you to check off each 2:1 dilution, to make sure you do exactly 8). After this, the solution will be at 6.69 dKH (=428.125/2^8*4dKH if the starting solution was at 25C... adjust this number according to your situation).
6) The final step requires some precision in terms of measuring volumes, but if you have a dropper that can measure 1mL, an API test kit with test tubes marked with a 5mL line, and a larger measuring cup that can measure a few hundred mL's at a time in increments of 10 (available in most bigger supermarkets' cooking sections), it shouldn't be too hard. We need
to take the 6.69dKH solution that we now have (or whatever you got based on your starting temperature) and dilute it down to 4dKH. In the following formula, replace 6.69 with whatever your number is. Defining x as the number of mL's of distilled water that we need to add, and Y as the number of mL's in your user-defined "cup" (you should have at this point exactly 2 "cups" of diluted fluid), with some simple reasoning we can derive the equation:
x= 2Y*(6.69/4 - 1)
Once you add this many mL of water to the 2 "cups" of solution you already have, the resulting mixture will be at 4dKH! We're done!! Now bottle that sh*t! (Small refillable bottles for toiletries or what have you can be bought at a well-stocked grocery or drug store).
7) Testing: When I tested my results with the standard API KH tester, it changed color at 6 drops (indicating a KH of between 5 and 6 degrees). You will probably get a similar "off" result. Do not worry! These tests are notoriously imprecise (or rather, they are precise enough for the needs of typical aquarists, but they are not lab quality). I got independent confirmation of this fact when I had my local fish store test a sample of water that I had just tested for comparison. Their result was 2.1dKH, while mine was 4dKH (to within the error implied by counting a discrete number of drops). I don't know for sure what method they were using, but it was likely more accurate than mine (based on the fact that they can afford, and have interest in owning, more accurate equipment, coupled with the fact that they quoted me a number to the first decimal place, whereas your standard kit "rounds" to the nearest whole number). So, this tells me that my kit probably tests a little high, meaning that my reading of 5-6 was really closer to the 4 we should expect. In the end, the real "proof" of this method is in the procedure. So long as we followed the steps correctly (and that basic science works), there should be no reason not to get the 4dKH solution we were expecting. Still and all, a "sanity" check at the end is useful, just to reassure ourselves that we didn't screw something up TOO badly . If your number is way different from 4, then you might have mis-counted dilutions, have an expired test kit, or run into some other unknown gremlin. Short of this, just go ahead and assume that the solution is what it should be.
Happy drop-checking! I hope some of you out there find this useful .
|08-27-2010 11:18 PM|
As KevinC mentioned, you can easily increase your precision by increasing the volume of water to be tested.
The gH and kH test kits work on the basis of titration. As KevinC also mentioned, if your 4 dkH reference solution is slightly off (i.e. 4.1 dkH), then it will require that 5th drop to change the colour to a bright yellow.
As for where your error is coming from, I would suspect the sodium bicarbonate as well. Despite it being a brand new box, it is not completely air tight, and there are surely impurities left over from the manufacturing process, etc. However, since it is going to go into a drop checker, which is not the most precise tool anyway, you can probably get away with the reference solution you already have made up.
|08-27-2010 08:41 PM|
|08-27-2010 03:57 PM|
I measured it out again. Still coming up with 5 kh. I do get the blue to be pale greenish at four drops, but it does take 5 to seal the deal and be yellow. I guess the arm and hammer is suspect? It is a new box tho, and any added moisture would give me a lesser amount to the mixture, so a lesser concentrated mixture. I checked the scale, and it has been calibrated recently. I carefully measured the water with plastic disposable eye droppers. I checked the TDS in the mixture, and it came out at 74 ppm (which I suppose would be the most accurate way to measure it). Oh, just checked my conversion chart. It should be 71.6 ppm. It is good to write things out sometime isn’t it? That will be a good way to dial it in…
The thing is 5 kh is 89.5 ppm. So, I suspect the integrity of the API dropper test (drop size could vary). The room in my lab (not a chemistry lab) that I test in is very well lit with fluorescents, and I do it on white paper for color fidelity. The instructions say the solution should be bright yellow (not pale green). I guess I would have to calibrate my TDS know for sure, what’s going on.
|08-26-2010 09:48 PM|
It's been 30 years since I did Chemistry. A lot has vaporised.
|08-26-2010 08:02 PM|
I would just make it again, if you have the reagents to spare.
If you have access to an analytical scale (which I assume you mean by "laboratory scale"), then you can scale it down from 6 grams to (say) 0.6 grams and make 500 mL of 40 dkH water only.
From there, just take 100 mL of the 40 dkH water and add to 900 mL of DI water to get your 1 L of 4 dkH water.
You probably do not need a Pasteur pipette, as it will not add any more accuracy if you are using graduated cylinders only. The only time it may be useful is if you were using volumetric flasks to make up your solutions.
|08-26-2010 01:57 PM|
I measured out 6 grams of sodium bicarbonate from a brand new box of arm and hammer on a laboratory scale that measured to .0001 (I settled for .001, I was fine tuning by the spec). I mixed that with 5 liters of distilled water measured out with a 1000 ml graduated cylinder, in a new carboy rinsed out with DI water. Then I diluted it 1:9 with distilled water using a 100 ml graduated cylinder and the 1000 ml (the last solution I was very diligent measuring by the drop with a pipette).
So, I was hoping for a liter of 4 KH, but when I tested it with an API KH tester, it took 5 drops to turn it from blue to yellow. Would it make sense to remix, and try again, or dial it in to the drops by diluting it?
|08-26-2010 01:39 PM|
Dilutions do not work that way. You must take into account that by adding 40 mL of dilute water, you are changing the final volumes and concentrations. The proper way to calculate this:
(200 mL)(5 dkH) = (x dkH)(240 mL)
Solving for x = 4.167 dkH.
|08-26-2010 10:43 AM|
What am I forgetting?
|08-26-2010 02:20 AM|
|Hoppy||KH test kits can be inaccurate too. Here is a way to calibrate that test kit, or make 4 dKH water, that has worked pretty well for at least a few people: http://www.plantedtank.net/forums/fe...tml#post800612 The fortunate thing is that you don't need very good accuracy on the 4 dKH standard. Since the ppm of CO2 you determine with a drop checker is directly proportional to that KH, a 25% error only makes a 25% error in ppm of CO2. But the drop checker isn't that accurate anyway. When it tells you you have 30 ppm, you may have anywhere from 20 to 45 ppm, depending on how good you can interpret the color of the drop checker liquid.|
|08-25-2010 10:50 PM|
As long as your initial calculation was correct, then your dilutions seem OK.
Your last dilution is incorrect, however.
To 200 mL of 5 dkH, adding 40 mL of distilled water will yield 240 mL of 4.16 dkH reference solution, not a solution of 4 dkH.
|08-25-2010 11:37 AM|
I tried using 1 litre of de-ionised water, some Tesco Bicarbonate of soda and a API KH liquid test kit. Oh and I used some electronic kitchen scales accurate to 1 gram.
Water weights 1Kg per 1Litre so dead easy to calculate volume using weight.
I put a tiny amount of Bicarb into 100ml of the water and it went to 20dKH so I added double amount of water. Then I halved it and added double again to get 5dKH with 200ml then I added 40ml to dilute it by 1dKH.
Only took me a few minutes to get a 240ml of 4KH.
But I guess it's not going to be that accurate? But will it be accurate enough do you think for a drop checker?
I took a double sample size of water using my API KH test kit and used twice the amount of drops. I got 8 drops to make it turn yellow.
|08-24-2010 06:49 PM|
I do not believe a pH meter will aid you in making up a 4 dkH reference solution.
All you need is a scale that can measure to (at least) 0.1 grams.
|08-24-2010 05:39 PM|
I've got an accurate pH meter. Would it be possible to use this to make up a 4dKH solution?
If I let the distilled water degas to the level of the air (outside an enclosed room of course)I wonder what the pH level of 4dKH should be?
Making up 3L for me will take up a lot of room and I've got to buy it!
|02-15-2007 02:13 PM|
If you don't won't to make your own KH Standards or if you want a standard that's more accurate than the baking soda standard. You can go to the swap and trade forum or to Aqua Bid under "test kits" and buy one of several different KH Standards.
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