Quality of Limewater based on Ph

[j0tca]

Hi guys,

I tested my limewater reactor by measuring the Ph over the course of 8 Litres of product. My reactor only has a volume of about 3.2 litres so I was concerned that it was not truly adding saturated limewater to the tank over the course of a day's topoff.

Essentially my results were:

Limewater Ph Volume dispensed
12.29 0L
12.22 1L
12.09 2L
12.01 3L
12.00 4L
11.96 5L
11.85 6L
11.78 7L
11.71 8L


My question is: Is Ph a good indication of Limewater quality?
Since a saturated solution at Ph 12.54 has 808 ppm Ca, shouldn't a solution of limewater at a Ph of 11.54 have only a [Ca] of 80.8 ppm? However since I'm only getting a Ph of 12.29 for my saturated solution, does this mean my limewater is only 390 ppm?

This confuses me because 808 ppm of Ca means a Molarity of 0.00202. The Ph of this solution would be 12.60 I thought. For a Ph of 12.54, the Ca would have to be present at about 700 ppm. My Ph would be 390 ppm. None of this seems to add up. I'm thining that I have some fundamental problem in my understanding of acid/base reacions here. How is it possible after adding so much Lime that I would only have a [Ca] of 390? I'm not trying to split hairs, just being a confused student again. This is based on a molecular weight of 40.08 for Ca. The calculation of [H] I did was Kwater/[OH-] or 10^-14/.0202.

Thanks for any help
Will

[j0tca]

Well, I'm still feeling dumb.

I can't figure out how to attach a Ca concentration in ppm to a Ph value for a limewater solution. Assuming no carbon dioxide exchange, this should be easy.

The Kb for Ca(OH) is 269, does anyone know what the molarity of a saturated Ca(OH) solution is? I think it's .14% by weight but that gives me a molarity that wouldn't match a Ph of 12.54.

How about the cnversion between ppm and Molarity? I read that .001M was the same as a the molecular weight of the solute, i.e. a .001M solution of Ca(OH) would have a concentration of 74 ppm.

[Randy Holmes-Farley]

The pH of saturated limewater at 25 deg C is 12.45, but it is rather temperature dependent. I'd also not use the absolute pH reading, since the calibration may not be very good at such a high pH.

A drop of 0.3 pH units is a drop in potency of about 50%. A drop of 0.1 pH unit is about 21% (0.2 units is 37%). The best way to do it carefully is to make a for sure saturated solution (say, 1 teaspoon lime in a cup of water), and measure the pH. Then see how much lower the effluent is, rather than any absolute pH value which will be very prone to error.

The approximate potencies for your solutions, if the pH is totally accurate and it is at 25 deg C is about:


12.29 69%
12.22 59%
12.09 44%
12.01 36%
12.00 35%
11.96 32%
11.85 25%
11.78 21%
11.71 18%

Since a saturated solution at Ph 12.54 has 808 ppm Ca, shouldn't a solution of limewater at a Ph of 11.54 have only a [Ca] of 80.8 ppm? However since I'm only getting a Ph of 12.29 for my saturated solution, does this mean my limewater is only 390 ppm?

Yes. Approximately, anyway. The small complication is that there is some soluble CaOH+ in solution at pH above 12, but it is reasonable for our purposes to ignore that contribution. That is probably why the pH is of saturated limewater is not 12.6, but a tad lower.

[j0tca]

Thanks again Randy,

Is the Ph of a saturated limewater solution 12.45 or 12.54? At 25C, is this a reliable enough fact to use to calibrate my Ph probe at high Phs? Or are their inconsitancies due to purity of Lime etc. that fluctuate the saturated Ph of the solution?

I still a bit confused. I have found in several places online that the conversion between ppm and Molarity is the molecular weight of the species being equal to the ppm at 10-3 M. This would give me a [Ca] of 564 at Ph 12.45. When we measure the Calcium in ppm in our tanks are we measuring for just [Ca] or is [Ca(OH)2], [Ca(OH)-], etc. also contributing?

How did you arrive at a [Ca] of 808 for a saturated solution?

Thanks for all the help Randy. I hope I'm not being a bother and if you can think of a book or site that offers a good explanation, I'de be happy to go read there instead of constantly asking you questions.

Will

[Randy Holmes-Farley]

The pH at saturation is 12.454 I think there have been some typos around, including my own, where it came out 12.54.

When we measure the Calcium in ppm in our tanks are we measuring for just [Ca] or is [Ca(OH)2], [Ca(OH)-], etc. also contributing?

Any form can contribute, and in the case of calcium in limewater, it is Ca++ and CaOH+ that are significant above pH 12. At saturation, the CaOH is on the order of 25% of the total.

I'll have to come back to the other questions later. :)

[Randy Holmes-Farley]

I don't recall where I got the exact info, but in some scientific reference I saw that the saturation point of calcium hydroxide at 25 deg C was 0.0204 M (Craig Bingman quoted 0.0203 M in his article). That works out to about 818 ppm.

0.0204 moles/L x 40.078 g/mole = 818 mg/L which is about 818 ppm

[j0tca]

The saturation I got was .0202M, pretty close to everyone else here. I didn't realise that ppm was mg/L, I thought is was a particule comparison, Msolvent/Msolute. Actually, my .0202M gives 809 mg/L, are our test kits actually measuring mg/L though?

How about the reliablilty of Ph at saturation, do you think it close enough with Kent Kalkwasser/RODI mix to use to calibrate a ph probe? A totally saturated solution gives me a ph of 12.33 (1 Tablespoon Lime/1 Cup water), this is a far way off 12.454.

The results of my Kalk reactor are what was expected for the titration. Since my probe was calibrated yielding a saturation ph of 12.33 I can assume my initial readings of 12.29 are close enough to saturation. My effluent reduced to 12.03 after 2.13 Liters where it plateaued for a while. It reaches it's half potency here, this is what I'll take as the useful amount of effluent it can produce per mixing. I think even at 50% I am adding enough calcium and hydroxide to be usefull to the tnak. Although I am adding 400 ppm [Ca] water, it is replacing evaporated water, thus I'm still adding calcium without diluting it.

Again Thanks,

Will

[Randy Holmes-Farley]

are our test kits actually measuring mg/L though?

I don't now whether they truly report ppm or mg/L, but I don't think any are so accurate that there is any difference.


How about the reliablilty of Ph at saturation, do you think it close enough with Kent Kalkwasser/RODI mix to use to calibrate a ph probe? A totally saturated solution gives me a ph of 12.33 (1 Tablespoon Lime/1 Cup water), this is a far way off 12.454.

If you give it enough time to dissolve and don't allow too much CO2 into it, yes. It is a recognized pH standard. But temperature is VERY important:

"Limewater that is saturated with calcium hydroxide has a pH of 12.54 at 25ºC. It is actually recognized as a secondary pH standard. The pH is substantially higher at lower temperature (12.627 at 20ºC and 13.00 at 10ºC), and lower at higher temperature (12.289 at 30ºC; 11.984 at 40ºC). "

from

What Your Grandmother Never Told You About Lime
http://reefkeeping.com/issues/2005-01/rhf/index.htm


Since my probe was calibrated yielding a saturation ph of 12.33 I can assume my initial readings of 12.29 are close enough to saturation.

Yes, I would, if the temperature is the same. :)
I think even at 50% I am adding enough calcium and hydroxide to be usefull to the tnak.

That could be. I use about half saturated limewater (at least many months of the year).