Do check your KH test kit solution before use

[argonmt]

I just bought a KH test kit from the LFS recently and the package seems very new. I followed the instructions and used it to test the hardness of my tank water.

Till last week, when i was preparing the 4dKH solution for my permanent CO2 indicator http://www.fishforums.net/content/fo...ent-Co2-Test-/ I've noticed the reference solution of the KH kit was off by 50%.

I am very sure the 4dKH solution i prepared is accurate.

[Shadow]

is the test kit expired? I also read somewhere suggested to buy any test kit from shop with aircon/cool shop, not sure how true it is.

[argonmt]

is the test kit expired? I also read somewhere suggested to buy any test kit from shop with aircon/cool shop, not sure how true it is.

I checked .. and its not. Some more the package looks very new leh.
For a 4dKH solution, it detected as 6dKH solution leh ..

[Shadow]

care to share what brand is that?

[argonmt]

care to share what brand is that?

Er.. i think not very good to disclose the brand here .. may be only for my particular bottle. BUt i can PM you ..

[jacian]

Sorry to ask, how are you so sure that the solution you prepared is accurate?

[argonmt]

Sorry to ask, how are you so sure that the solution you prepared is accurate?

I am working in a chemical laboratory. As all the materials was weighted up to 4 decimal place. I was using pure sodium bicarbonate (may be this is the reason?? ) instead of baking soda. But I've checked from the internat that the baking soda is the same as sodium bicarbonate.

The volume for the solution was weighted. And i and using the auto pipetter during the dilution process.

I've repeated a few more sample using different batches of sodium bicarbonate to prevent any potential error during the preparation. However, KH test kit consistently measured at 6dKH.

[bossteck]

I still don't get it.
So you weigh the sodium bicarbonate, and add it to distilled water. How do you know how much sodium bicarbonate to add to what volume of distilled water would give you a KH 4 solution? I looked through the link you posted in post number 1, but don't see any recommended proportions in the discussion that will give you a KH 4 solution for sure.

Would you be getting a different brand KH test kit? If so, could you measure the same solution using both test kits and let us know the results? Thanks!

[PeterGwee]

Yes, most test kits are not very accurate. The lab method would work best if you know the exact weight and dry the baking soda powder further via an oven to remove the moisture. If it is too troublesome, simply get the standard in a very high concentration from retailers like HACH and etc and then dilute to the needed concentration.

Regards,
Peter Gwee

[jacian]

As Peter had pointed out, unless you know the exact proportions and be sure that the bicarbonate is pure, i.e. moisture free. Else, I guess the smaller amount you use, the greater is the error margin. Also, test kits are known to be not very accurate.

[argonmt]

I still don't get it.
So you weigh the sodium bicarbonate, and add it to distilled water. How do you know how much sodium bicarbonate to add to what volume of distilled water would give you a KH 4 solution? I looked through the link you posted in post number 1, but don't see any recommended proportions in the discussion that will give you a KH 4 solution for sure.

Would you be getting a different brand KH test kit? If so, could you measure the same solution using both test kits and let us know the results? Thanks!

Sorry for didn't include the exact post on this. If you look further.. at the post #31 Tom Barr (http://www.fishforums.net/index.php?...post&p=1406256), he has explained how to prepare 4dKH solution.

Basically dried the baking soda @ 300K to remove both moisture and CO2 then weighed out 4.9923 g and dissolve in 5L of DI water. This will give you a 40dKH solution. Afterward do a ten times dilution of this standard to get the 4dKH solution you want.

I used the online reef calculator (http://www.cnykoi.com/calculators/calckh.asp) to find the exact amount of baking soda need to make the standard solution.

[argonmt]

I still don't get it.
So you weigh the sodium bicarbonate, and add it to distilled water. How do you know how much sodium bicarbonate to add to what volume of distilled water would give you a KH 4 solution? I looked through the link you posted in post number 1, but don't see any recommended proportions in the discussion that will give you a KH 4 solution for sure.

Would you be getting a different brand KH test kit? If so, could you measure the same solution using both test kits and let us know the results? Thanks!

I will update if I'm able to get a different brand KH solution.

[jacian]

So did you follow the instruction to bake the sodium bicarbonate? In the later part of the thread, it was mentioned that sodium bicarbonate may convert to sodium carbonate when heated by 200C. I understand that sodium carbonate will provide double the carbonate hardness as compared to sodium bicarbonate.

[argonmt]

So did you follow the instruction to bake the sodium bicarbonate? In the later part of the thread, it was mentioned that sodium bicarbonate may convert to sodium carbonate when heated by 200C. I understand that sodium carbonate will provide double the carbonate hardness as compared to sodium bicarbonate.

Yes i did heat the sodium bicarbonate at temperature of 140oC. I'm awared that heating at higher temperature will convert sodium bicarbonate to sodium carbonate but there is no way the later one will give rise to double up the carbonate hardness. Here is my old day chemistry knowledge kick in..

Molecular weight of sodium bicarbonate (NaHCO3) is 84.01 g/mol
Molecular weight of sodium carbonate (Na2CO3) is 105.99 g/mol

Both of them can yield 1 molar to CO3 (60.01 g/mole), Thus
4.99 g of NaHCO3 produce 3.56 g of CO3.
4.99 g of Na2CO3 produce 2.82 g of CO3.

By dissolving the above powder into 5L of solution you will get 712 ppm and 564 ppm carbonate hardness for NaHCO3 and Na2CO3 respectively.

As 1dKH = 17.9 ppm of carbonate, therefore you will get
39.77 dKH from 4.99 g of NaHCO3 in 5L of DI water
31.50 dKH from 4.99 g of Na2CO3 in 5L of DI water

What I'm trying to prove here is that the more NaHCO3 get decomposed from heating, the lower the hardness you will get.

Well the safest way is to heat at 100oC for long time, but I'm too lazy

[jacian]

Yes, I did the same calculation and thought that Na2CO3 will give a lower hardness. However, based on the definition of carbonate hardness found in wiki, it states that every bicarbonate ion only counts for half as much carbonate hardness as a carbonate ion does. I do not quite understand this part as well.
http://en.wikipedia.org/wiki/Carbonate_hardness

Well, in the end of the day, does it really matter to be that accurate?