Trichlor composition

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Thanks again for the responses. I did make an error in my original consumption of trichlor.

I used 6 x 6.16 lb of the New Water 407c packs. = 16.8 kg

Based on the information provided above, roughly, 0.91T = FC and 0.6FC = CyA.

Therefore, dosed FC was 148 ppm with 89 ppm CyA.

Incidentally, the measured mean rate of CyA addition was approximately 0.25 ppm/day (based on an earlier estimate of the rate) and over 136 days total (full consumption of the trichlor); it should have increased by 34 ppm. I initially had 34 ppm in there bringing the concentration to 68 ppm. Measured concentration was 65 ppm, so very close.

However, the values provided above don't come close enough. Again, I'm trying to estimate next year's liquid chlorine needs based on the consumption of trichlor (and the liquid I added). Calculations are largely verified by the reconciliation of the measured CyA. Just trying to achieve mass balance.
 
Hmm...not to beat a dead horse here, but I'm going to.

It appears that there is an 'error' in the pool math calculator. I would love someone to verify what I'm seeing. This all goes to THE problem I've been saying all along, saying "ppm chlorine" is extremely misleading and inaccurate. It must be written as "ppm chlorine as..." followed by the species you are comparing against (i.e. Cl2, NaOCl). Yes, I know there is an equilibrium but when it comes to calculations and a species you are calculating versus one.

The issue: It appears that based on the pool math calculator that the intention is to calculate chlorine as NaOCl, but it is using the molecular weight for Cl2.

The proof:

So, let's test this out. Make sure you are selecting METRIC.

Pool volume 1000 L
Scroll down to the Effects of adding Chemicals section (at the bottom)
Enter 10000 g of trichlor

Now, let's follow the stoichiometry I originally mentioned, but arrange it for NaOCl.

1 mol trichlor -> 1 mol cya + 3 mol NaOCl

MW trichlor = 232.41 g/mol
MW cya = 129.07 g/mol
MW NaOCl = 74.44 g/mol
MW Cl2 = 70.9 g/mol

Let's do the math by hand!


10000 g trichlor / 232.41 g/mol trichlor X 3 mol NaOCl / mol of trichlor X 74.44 g NaOCl / mol NaOCl X 1000 mg/g / 1000 L =

9608.9 ppm FC

But, this is NOT the answer pool math gives. Substitue 74.44 for 70.9 (MW of Cl2) and you get:

9151.9 ppm FC

Which is extremely close to the answer pool math provides (within reasonable error for significant figures).

Let's now check the CYA concentration, using the same stoichiometry:


10000 g trichlor / 232.41 g/mol trichlor X 1 mol cya / mol of trichlor X 129.07 g cya / mol cya X 1000 mg/g / 1000 L =

5553.5 ppm cya

Which is the same answer (within reasonable error for sig figs) that pool math provides.

Therefore, it seems as though the stoichiometry to get to NaOCl is using the wrong MW.

If you try this in the reverse using the FC portion at the top you'll see it works the same way.


So, if the intentions are to calculate FC as Cl2 then:

1 mol trichlor -> 1 mol cya + 3/2 mol Cl2

And thus in this example:
10000 g trichlor / 232.41 g/mol trichlor X 3/2 mol Cl2 / mol of trichlor X 70.9 g Cl2 / mol Cl2 X 1000 mg/g / 1000 L =


4576.0 ppm FC as Cl2

Now, all the background behind the equilibrium is great and certainly valid, but the pool math calculator is apprently calculating simple stoichiometry and may be using the incorrect MW.


Let's move onto the last calculation and that is the calculation of ppm chlorine in 12.5 wt% NaOCl. Entering in 1000 L volume and 1000 ppm target (start at 0) yields 7989 mL of 12.5% bleach. This would correspond to a concentation of 125172 ppm of "chlorine" in the concentrate. The question is whether or not this is as NaOCl or Cl2. It appears as though this is 125172 ppm chlorine as NaOCl as the calculated value in 12.5wt % for chlorine as Cl2 is 120549 ppm chlorine (as Cl2).

So, this calculation is pretty close (though it seems to use 1 g/cc as the density of NaOCl where it should be 1.11).

The question I have is: Is the FC species measured by the DPD test calculated as ppm of chlorine as NaOCl or Cl2?

So, finally, getting the answers to the pool math calculator will help as will the question above.

Thanks.
 
If you have 999,990 liters of water (mass = 999,990 Kg) and you add 10 Kg chlorine gas, you get 10 ppm reported in units of chlorine gas equivalent.

One kilogram of trichlor is equivalent to 0.91 kg chlorine gas. One mole trichlor is equivalent to 3 moles of chlorine gas.

If you have 999,989 liters of water (mass = 999,989 Kg) and you add 11 Kg trichlor, you get 10 ppm reported in units of chlorine gas equivalent.

The way the test is done cannot distinguish what species is present. We can determine the approximate concentration of various species by the chemical equilibriums.

The test is a titration that reduces the available chlorine until a color change indicates that all chlorine has been reduced to chloride.

The test results depend on the total molarity of all chlorine and the results are reported in units of chlorine gas equivalent.

C3Cl3N3O3 + 3H2O --> C3H3N3O3 + 3HOCl

3Cl2 + 3H2O --> 3H+ + 3HOCl + 3Cl-

As you can see from the equations above, 1 mole of trichlor is equivalent to 3 moles of chlorine gas, both produce 3 moles of hypochlorous acid.

Chlorine gas is 70.906 grams per mole.

Trichlor is 232.41 grams per mole.

232.41 grams of trichlor are equivalent to 212.718 grams of chlorine gas.

1 gram of trichlor is equivalent to 0.915 grams of chlorine gas.

Whenever a chlorine product gives a % available chlorine, it indicates how much chlorine gas the product is equivalent to.

For example, trichlor might say 91% available chlorine, which means that 1 pound of trichlor is equivalent to adding 0.91 pounds of chlorine gas.

The actual weight percent of chlorine in trichlor is 45.76%. The cyanurate in trichlor accounts for 54.24% of the weight of trichlor.

Adding 1 pound of trichlor adds 0.4576 pounds of chlorine, but it’s counted as double since it’s equivalent to adding 0.9152 pounds of chlorine gas.

It also adds 0.5424 pounds of cyanurate but it’s counted as 0.55535 pounds of cyanuric acid since it’s reported in units of cyanuric acid.

Everything that’s measured is reported in specific units to standardize the metrics.

Total Alkalinity and Calcium Hardness are reported in units of calcium carbonate equivalents even though that’s not necessarily what was added to the water.

Reporting units are reported in equivalents to avoid having to know the actual makeup.

Salt is reported in equivalents of sodium chloride but everything measured as salt won’t be from sodium chloride. Borates are reported in units of boron.

PoolMath is correct.
 
JamesW said:
PoolMath is correct.
Click to expand...

Ok. If it is correct, can you (or anyone) please go through the long hand calculation of exactly what PoolMath is doing to provide the solution of 9151.9 ppm FC and 5553.5 ppm cya when 1000L pool volume and 10,000 g trichlor is used as input values into the calculator.

Forget any chemistry and possible assumptions. Just the simple A x B x C + K etc... form of what the calculator is doing. Then, for any constant explain where the value is derived from.

That would be great.
Thanks.
 
Weight of solute x 1,000,000 ÷ weight of solution.

For example 12.5% bleach is 12.5 grams per 100ml (trade percent). 10000ml adds 1250 grams chlorine gas equivalent.
If the pool is 10,000 liters, the water had a mass of 10,000Kg.
1250 grams x 1000000 ÷ 10,000,000 grams = 125 ppm.
 

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From the Pool Equations Spreadsheet that Pool Math was based off of -

FC = (weight of trichlor in grams) * 1000 mg/g * 3 mols of Cl * ( Cl2_g_mole / Trichlor_g_mole ) / volume

I stripped out the Excel spreadsheet cell references and put in what those references are.

Cl2_g_mole = 70.906 gm/mol
Trichlor_g_mole = 232.4103 gm/mol

FC = [ (10000) * (1000) * (3) * (70.906/232.4103) ] / 1000 = 9152.69 ppm

Pool Math says the FC is 9153 ppm

Now you might say "A HA!! You're using 3 mols of Cl and the molecular weight for Cl2, therefore you are wrong!" But that is not right. As James posted, the 3 mols of Cl atoms added to the water by trichlor is EXACTLY equivalent to adding three mols of chlorine gas (Cl2) to the water. And, because the FAS-DPD test is designed to report results as units of chlorine gas equivalents, it's perfectly correct to use the MW of Cl2.

The equation for CYA yields results that are in line with Pool Math as well.
 
Thank you. This nearly answers my question. Two things:

1. "because the FAS-DPD test is designed to report results as units of chlorine gas equivalents, it's perfectly correct to use the MW of Cl2" - This is the 'smoking gun' I have been looking for and completely satisfies what I'm looking for and also exemplifies the importance of proper labeling. Though, I would stay away from calling Cl2 chlorine gas.

2. The equation as presented is not balanced and 3 mols of trichlor does not yield 3 mols of Cl2 unless the other 3 Cl is coming from some other source. However, if the DPD test is accounting for this, then the use of the molecular weight for Cl2 may be appropriate (referencing the above #1).


JoyfulNoise said:
Now you might say "A HA!! You're using 3 mols of Cl and the molecular weight for Cl2, therefore you are wrong!" But that is not right. As James posted, the 3 mols of Cl atoms added to the water by trichlor is EXACTLY equivalent to adding three mols of chlorine gas (Cl2) to the water. And, because the FAS-DPD test is designed to report results as units of chlorine gas equivalents, it's perfectly correct to use the MW of Cl2.

The equation for CYA yields results that are in line with Pool Math as well.
Click to expand...
 
If you want to understand how the DPD-FAS chemistry works, read this - https://www.hach.com/cms-portals/hach_com/cms/documents/pdf/LIT/L7019-ChlorineAnalysis.pdf

Not that I'm beating a dead horse but James already addressed the issue of trichlor dissociation so lets just state it again for completeness -

C3Cl3N3O3 + 3H2O --> C3H3N3O3 + 3 HOCl
"trichlor" + "Water" ---> "cyanuric acid" + "hypochlorous acid"

So, when the three chlorine atoms dissociate from the triazinetrione ring, they turn into hypochlorous acid in the water....actually, as the chlorine dissociates, it becomes a mixture of hypochlorous acid, hypochlorite anion and some stays bound to the CYA where the ratio of all of these components is controlled by pH of the solution and the concentrations of the various chemicals. But that is equilibrium chemistry details and not that important. The point is, all the chlorine released from the trichlor is either active chlorine or reserved chlorine. Free chlorine (FC) is defined as the sum of active chlorine plus reserve chlorine.

As far as it being equivalent to 3 mols of chlorine gas, James also posted this -

3Cl2 + 3H2O --> 3H+ + 3HOCl + 3Cl- (I made it obvious by adding the "3" in front).

So, if I gave you two beakers of solution and I made one beaker up by dissolving trichlor so that there was 10ppm FC in it and I made the second beaker up by bubbling chlorine gas through it until the FC reached 10ppm, the blind tester would have no idea which beaker was which based solely on a chlorine test. Both tests would be equivalent and so that is why one can say that 3 mols of Cl atoms added to solution by trichlor is the equivalent to bubbling 3 mols of chlorine gas (Cl2) into solution. Based on the FC test, they are the same thing. Now, in reality, one could test the two samples for the presence of CYA or, if done quickly enough, the presence of chloride ion (Cl-) and then make a better guess as to which one is which, but that's not the point.

Why is it that they use "ppm's of chlorine gas equivalent" ? One reason is that the FC test measures three species of chlorine - hypochlorous acid (HOCl) + hypochlorite anion (OCl-) + any chlorine that is bound to a cyanurate anion (we call this reserve chlorine). Since the test measures three distinct "things" but lumps them all together, one has to standardize the basis of the units. So the equivalent amount of chlorine gas is chosen as the basis.

As James mentioned, total alkalinity is similar and just that - all alkaline species in the water that can accept a proton. We don't care if the alkaline species is carbonate, or sulfate, or borate, or cyanurate, etc, etc, we only care about the total amount and then need a "measuring stick" to compare it to. That comparison is the ppm equivalent of calcium carbonate. So again, if I make up three beakers, one with 100ppm TA all from borates and the second one with 100ppm TA all from carbonates and the third with 100ppm TA all from phosphates and I hand you a standard TA test, then you have no way to distinguish which one is which - they all read 100ppm [equivalent CaCO3 units].
 
Thanks for the link, I just read through it briefly.

But, in terms of the equations below, the assertion that 1 mol of trichlor yields 3 mol of Cl2 is not correct. 1 mol of trichlor is equivalent to 3/2 mol Cl2. It cannot be 3 because there is no other source of Cl.

However....in terms of an analytical test, such as the DPD test, where the molar equivalent is used to express the analytical result I see how the conversion is made and thus how using the MW for Cl2 is appropriate. The article made this connection much clearer.

So, I'm perfectly fine now with the calculations in PM for the trichlor -> cya and Cl2 eq. calculation.

But, one question remains:
Why is the ppm concentration of 12,5wt% NaOCl in terms of NaOCl and not Cl2?
As wt%, this solution results in:
125172 ppm chlorine as NaOCl OR
120549 ppm chlorine as Cl2

PM uses 125172. Now, as wt% calculations goes you would take into account the density of NaOCl (1.11 g/cc). 12.5g NaOCl and 87.5g water you can approximate to 125000 ppm NaOCl. It's not exact because the volumes aren't completely additive, but close enough. But..if you convert to Cl2, you get 119000 - 120000 ppm. Again, close enough to make the point that the calculator is using NaOCl as the basis and not Cl2.

Why? Or, what am I missing?


JoyfulNoise said:
If you want to understand how the DPD-FAS chemistry works, read this - https://www.hach.com/cms-portals/hach_com/cms/documents/pdf/LIT/L7019-ChlorineAnalysis.pdf

Not that I'm beating a dead horse but James already addressed the issue of trichlor dissociation so lets just state it again for completeness -

C3Cl3N3O3 + 3H2O --> C3H3N3O3 + 3 HOCl
"trichlor" + "Water" ---> "cyanuric acid" + "hypochlorous acid"

3Cl2 + 3H2O --> 3H+ + 3HOCl + 3Cl- (I made it obvious by adding the "3" in front).
Click to expand...
 
Also, for chlorinating liquid, PoolMath uses different densities depending on the percentage on the bottle -

5.25% Bleach => 1.07 g/ml
6.00% Bleach => 1.08 g/ml
8.25% Bleach => 1.10 g/ml
10% chlorinating liquid => 1.14 g/ml
12.5% chlorinating liquid => 1.16 g/ml

Once you go from 8.25% to 10%, the industry switches terms and starts calling it "chlorinating liquid" instead of "bleach", they also change the labelling from weight percent to trade percent. 12.5% chlorinating liquid is NOT 12.5 wt%. 12.5 refers to the "Trade %" which is technically the volume % of available chlorine. See this thread for disambiguation - Bleach concentration per unit volume and other calculations
As for the calculation -

FC =(Volume added) * 1000 * NaOCl_percent * NaOCl_g_ml * (Cl2_g_mole/NaOCl_g_mole) / (pool volume)

So FC is still in units of equivalent chlorine gas. In the above the NaOCl_percent is weight % and if you put in the proper numbers, you get 12507.1ppm in equivalent Cl2 units
 
Thanks. That is very clear and specific, and also matches the dosed label claim on the bottle I have (52oz/10,000gal = 5ppm).

12.5 g Cl2/100mL X (52/128)/10000 = 5.1 ppm

PM provides 4.1 given the inputs above which means there are some assumptions.

In doing some additional research there are apparently different ways that the concentration of Cl2 is reached. I guess this is part of the frustration. There are hidden assumptions within the calculator and why it's important to be able to run calculations by hand.

In the end, the bottle label claim for dosing should be accurate (for fresh bottles of course).

Yes?




JamesW said:
12.5% sodium hypochlorite is trade percent, which means 12.5 grams chlorine gas equivalent per 100 ml solution.

In other words, trade percent assumes the density is the same as water.

I know that it's confusing. It's just industry convention.
Click to expand...
 
PoolMath gives 5.1 ppm for the above inputs.

Assuming that the bleach is the strength indicated, it should provide the ppm as specified by the dosing instructions.

Note that sodium hypochlorite solutions degrade over time. If the bleach is old or has been stored under poor conditions, it might be significantly degraded.
 
Oops...my mistake. When I switched from metric I must have selected Imperial instead of US. This is why I always work in metric!

That reference also made things perfectly clear. Their dilution tool was excellent since they also have calculated the densities.

Thanks again.

JamesW said:
PoolMath gives 5.1 ppm for the above inputs.

Assuming that the bleach is the strength indicated, it should provide the ppm as specified by the dosing instructions.

Note that sodium hypochlorite solutions degrade over time. If the bleach is old or has been stored under poor conditions, it might be significantly degraded.
Click to expand...
 

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