TimboS
Sat Mar 15, 2008, 07:12 AM
Hi All,
I have been wondering about buffers of late, more specifically the chemistry behind them. I think in my circumstances I will avoid them but that does not stop me from trying to figure it all out.
Let's have a quick look at Seachem's acid and alkaline buffer, and, let's ignore the ratio on the back for the moment.
First, the acid buffer. The bottle says 2g for every 80 litres (per day) until the desired pH is achieved where each dose in accordance with this will reduce KH by "about" 0.2meq/L or 0.6 dKH. So, for 120 Litres, you use 3g or 3/16th of a teaspoon until you reach the desired pH. Further, if it took three applications (over three days) to achieve the desired pH, you have dropped KH by -0.6 * 3 = -1.8 dKH.
Now, the alkaline buffer. The bottle says 7g for every 40 litres (per day) until the desired pH is achieved where each dose in accordance with this will increase KH by "about" 2 meq/L or 5.6 dKH. So, for 120 Litres, you use 21g or 3 teaspoons until you reach the desired pH. Similarly, if it took three applications (over three days) to achieve the desired pH, you have increased KH by 5.6 dKH * 3 = 16.8 dKH.
Let us now consider the ratio on the back of the bottle. Let's go for a pH of 6.0 to simplify things since this requires a ratio (by weight of product added) of 1:1. Let us also assume it took one dose in the stated ratio to achieve pH 6.0.
We will now start with the acid buffer. 120 Litres needs 3g per day and had one days worth, so 3g was the total. We changed KH by -0.6 dKH.
Now, the alkaline buffer. Going on the WEIGHT ratio of 1:1, we needed 3g which is 3/7th of a teaspoon, or 3/7 * +5.6 dKH (for 40L) = +2.4 dKH for 40L, or, taking the case of 120L into account, 2.4/3 = +0.8 dKH.
The end result = -0.6 dKH + 0.8 dKH = +0.2 dKH. So, for 1:1, we increased KH and thus pH. Somehow we did not go down...
We will now start with the alkaline buffer. 120 Litres needs 7g (per 40L) * 3 = 21g. This will raise KH by 5.6 dKH. With a WEIGHT ratio of 1:1 we need 21g of acid buffer = 10.5 quarter teaspoons. This will decrease KH by 10.5 * 0.6 dKH for 80 Litres = -6.3 dKH, but treating 120 Litres this is -6.3 * 80/120 = -4.2 dKH.
The end result = +5.6 dKH - 4.2 dKH = +1.4 dKH. So, again, for 1:1, we increased KH and thus pH. Again, we did not go down...
So, we simply adjust the ratio of acid to alkaline in favour of acid to actually drop, and buffer, the pH.
Now comes the fun part. The acid buffer reduces pH by converting KH into CO2 and H20. So, to drop pH, we need to "eat away" any buffering capacity and then it's a free for all - the pH will drop really quickly with more product. But then, to actually buffer the result, we add alkaline buffer which increases KH and thus pH. So, it would seem as though we must first add acid, then alkaline - and EXACTLY in that order. But, if we don't add enough acid to fully eat up the KH, our effort to decrease pH will be in vain since the alkaline buffer will simply takes us back toward where we started!!
What if we wanted to achieve and buffer a pH that is well away from the pKa of the buffer product we are using, i.e. where the product is the most unstable? Would we every actually get to buffer the lowered pH? I know that Seachem's discus buffer has a pKa of 5.0, so with my water of pH 7.4, I would need to add a hell of a lot to get 5.0, but despite the inherent stability, my small KH of 2 dKh from the tap would not allow it to stay there.
Either I am well and truly lost in the world of buffer chemistry, or just missing a small piece of the puzzle. Ultimately, if an acid buffer has to chew up KH to drop pH, how can you lower AND buffer it??
Timbo (on a boring saturday arvo)..
(Oh yeah, forgot to ask, given all the calculations for difference in dKH, is there any way this can be connected to an expected difference in pH ?, e.g. pH drops by 0.1 units for every x many dKH. If yes, or approximately yes, I can see how to create new ratios for desired change).
I have been wondering about buffers of late, more specifically the chemistry behind them. I think in my circumstances I will avoid them but that does not stop me from trying to figure it all out.
Let's have a quick look at Seachem's acid and alkaline buffer, and, let's ignore the ratio on the back for the moment.
First, the acid buffer. The bottle says 2g for every 80 litres (per day) until the desired pH is achieved where each dose in accordance with this will reduce KH by "about" 0.2meq/L or 0.6 dKH. So, for 120 Litres, you use 3g or 3/16th of a teaspoon until you reach the desired pH. Further, if it took three applications (over three days) to achieve the desired pH, you have dropped KH by -0.6 * 3 = -1.8 dKH.
Now, the alkaline buffer. The bottle says 7g for every 40 litres (per day) until the desired pH is achieved where each dose in accordance with this will increase KH by "about" 2 meq/L or 5.6 dKH. So, for 120 Litres, you use 21g or 3 teaspoons until you reach the desired pH. Similarly, if it took three applications (over three days) to achieve the desired pH, you have increased KH by 5.6 dKH * 3 = 16.8 dKH.
Let us now consider the ratio on the back of the bottle. Let's go for a pH of 6.0 to simplify things since this requires a ratio (by weight of product added) of 1:1. Let us also assume it took one dose in the stated ratio to achieve pH 6.0.
We will now start with the acid buffer. 120 Litres needs 3g per day and had one days worth, so 3g was the total. We changed KH by -0.6 dKH.
Now, the alkaline buffer. Going on the WEIGHT ratio of 1:1, we needed 3g which is 3/7th of a teaspoon, or 3/7 * +5.6 dKH (for 40L) = +2.4 dKH for 40L, or, taking the case of 120L into account, 2.4/3 = +0.8 dKH.
The end result = -0.6 dKH + 0.8 dKH = +0.2 dKH. So, for 1:1, we increased KH and thus pH. Somehow we did not go down...
We will now start with the alkaline buffer. 120 Litres needs 7g (per 40L) * 3 = 21g. This will raise KH by 5.6 dKH. With a WEIGHT ratio of 1:1 we need 21g of acid buffer = 10.5 quarter teaspoons. This will decrease KH by 10.5 * 0.6 dKH for 80 Litres = -6.3 dKH, but treating 120 Litres this is -6.3 * 80/120 = -4.2 dKH.
The end result = +5.6 dKH - 4.2 dKH = +1.4 dKH. So, again, for 1:1, we increased KH and thus pH. Again, we did not go down...
So, we simply adjust the ratio of acid to alkaline in favour of acid to actually drop, and buffer, the pH.
Now comes the fun part. The acid buffer reduces pH by converting KH into CO2 and H20. So, to drop pH, we need to "eat away" any buffering capacity and then it's a free for all - the pH will drop really quickly with more product. But then, to actually buffer the result, we add alkaline buffer which increases KH and thus pH. So, it would seem as though we must first add acid, then alkaline - and EXACTLY in that order. But, if we don't add enough acid to fully eat up the KH, our effort to decrease pH will be in vain since the alkaline buffer will simply takes us back toward where we started!!
What if we wanted to achieve and buffer a pH that is well away from the pKa of the buffer product we are using, i.e. where the product is the most unstable? Would we every actually get to buffer the lowered pH? I know that Seachem's discus buffer has a pKa of 5.0, so with my water of pH 7.4, I would need to add a hell of a lot to get 5.0, but despite the inherent stability, my small KH of 2 dKh from the tap would not allow it to stay there.
Either I am well and truly lost in the world of buffer chemistry, or just missing a small piece of the puzzle. Ultimately, if an acid buffer has to chew up KH to drop pH, how can you lower AND buffer it??
Timbo (on a boring saturday arvo)..
(Oh yeah, forgot to ask, given all the calculations for difference in dKH, is there any way this can be connected to an expected difference in pH ?, e.g. pH drops by 0.1 units for every x many dKH. If yes, or approximately yes, I can see how to create new ratios for desired change).