Dont Underestimate This Question - What Does DPD#1 Measure?

[AnotherPoolBloke]

I thought as a lot of people do I understood this question, until I thought about some history. And made some enquiries and asked myself a question.

The question I asked is:

If I measure 2 mg/l of chlorine in the pool using DPD#1 then we say we have 2 mg/l of chlorine in the pool.
If I measure 1 mg/l of ozone in the pool using DPD#1 then we would say we have 1 mg/l of ozone in the pool.
However if we measure bromine, and here it depends on test kit, we multiply by 2.2x (2.22 or 2.25) to get the bromine level.
Why?

In looking at this it looks to me like the logic is we are measuring the elemental chlorine and as bromine has a higher molecular weight we multiply to correct. That's sort of logical if we are measuring elemental substance being chlorine or bromine. However then we should also multiply up the Ozone reading as Ozone is heavier than chlorine too...but we don't.

However if you look at what the definition of the test is it implies the test is measuring the oxidation of the reagent this is nothing to do with the molecular weight. Am I so wrong in my thinking?

I am specifically talking about Halogen bromine or BCDMH
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[JamesW]

Chlorine and bromine are measured in ppm Cl₂ or Br₂. Ozone is measured in ppm O₃.

One mole of chlorine, bromine or ozone will oxidize the same amount of dpd reagent. So, you're really measuring molarity.

When the test is designed for chlorine, you multiply by 2.25 for bromine or 0.677 for ozone.

Note: To measure ozone, iodide is added so that the ozone can oxidize the iodide to iodine and the iodine then oxidizes the dpd amine into colored Würster dye.

The FAS reagent reverses the oxidation of the magenta colored Würster dye.

2Fe⁺² --> 2Fe⁺³.

The iron donates electrons to reduce the colored dpd back to colorless dpd. Every drop of R-0871 reduces the equivalent of 5 micrograms of chlorine. Every drop of fas reagent reduces 70.5 nanomoles of magenta colored dpd back to colorless amine. So, every drop is equivalent to 70.5 nanomoles of chlorine, bromine, iodine, ozone or chloramines.

Iodide is easily oxidized by chloramines, ozone and other oxidizers. That's why it's used for the CC test. The chloramines oxidize the iodide to iodine.

 

[AnotherPoolBloke]

Chlorine molar mass or atomic weight is 35.453
Ozone molar mass or atomic weight is 48.0
Bromine molar mass or atomic weight is 79.904


THe logic implied is:
79.904/35.453 = 2.253 etc


However if the above was true then the below would also be true:
48.0 / 35.453 = 1.35 etc


Meaning Ozone would be calculated by multiplying up by 1.35 not by 0.67 which suggests the equation maybe backwards.


The other problem is:


Molarity

In order to be clear about the concentration of solute to solvent in a solution, scientists use the idea of molarity. Molarity is a concentration unit of a solution expressed as moles of solute dissolved per liter of solution. Using molarity means that you can have an idea of the number of particles of solute in any volume of a given solution.


Lets say I have 5 mg/l of Chlorine in 300 litres.
(300 L)(5 mg/L) = 1,500 mg
1,500 /1000 = 1.5 grams
(1.5 g)(mol/35.453) = 0.0423 mol
0.0423 mol / 300 L = 0.000141 mol/L = 0.000141M


Now for Ozone our Chlorine test is returning for discussion 5 mg/l
(300 L) (5 mg/L) = 1,500 mg
1,500 / 1000 = 1.5 grams
(1.5g) (mol/48.0) = 0.03125 mol
0.03125 mol /300 L = 0.000104167 mol/L = 0.0001041M


Now for Bromine our Chlorine test is returning for discussion 5 mg/l
(300 L)( 5 mg/L) = 1,500 mg
1,500 /1000 = 1.5 grams
(1.5 g) (mol/79.904) = 0.018772527 mol
0.018772527 mol / 300 L = 0.0000625751 mol/L = 0.000625M


These suggest:
Chlorine we accept as 0
Ozone multiply by 1.355
Bromine multiply by 2.25


Easier to use atomic weights and just multiply through but anyway. This returns to the original question.


Is DPD #1 measuring elemental chlorine in which case this would be correct or is it measuring the ability of sanitiser to oxidise the reagent leading to an indication of the amount of sanitiser present.

As I read the answer provided it is the second that is true the ability of the sanitiser to oxidise reagent. In which case the multiplication is only required when I am specifically wanting to know the concentration of bromine specifically (when using BCMDH as donour) as in a bromine tablet situation both chlorine and bromine are present in the sample being tested.

 

[JamesW]

Chlorine is reported as ppm chlorine gas, not elemental chlorine. So, you have to use 70.9 as the molar mass.

For example, if you have 1,000 liters of water (1 million grams) and you add 5 grams of chlorine gas, that gives you 5 ppm chlorine. When the chlorine gas is dissolved, it doesn't stay as chlorine gas, it converts to hypochlorite and hypochlorous acid, but the chlorine is reported in standardized units of chlorine gas. They could have chosen to report in units of mass of the chlorine atoms, but they chose chlorine gas.

Bromine is reported as Br₂. So, you have to use 159.8 grams per mole.

There's really no good reason. There's virtually no chlorine gas or Br2 in the water.
It's simply a convention that was chosen.

Ozone is reported as O3, so that uses the regular molar mass.

The FAS reagent reduces 70.5 nanomoles per drop. So, 10 drops would mean 705 nanomoles of oxidized dpd reagent was present.

So, our assumption is that the 705 nanomoles was created by 705 nanomoles of chlorine, bromine or ozone.

Then you convert from moles to grams using the appropriate conversion factor.

So, you know the mass of solute and you know the mass of solution based on the sample size. 10 ml is 10 grams etc.

705 nanomoles of is 50 micrograms of chlorine, 112.7 micrograms of bromine or 33.8 micrograms of ozone.

In a 10 ml sample, that's 5 ppm chlorine, 11.27 ppm bromine or 3.38 ppm ozone.

If a kit is designed for a specific oxidizer, the drop equivalence will be specified and you would need to convert the number if you use it for a different oxidizer.