Logarithmic Decay of Hypochlorite/Chlorine by UV light

In summary, the decay of hypochlorite/free chlorine in pool water due to UV light might show a logarithmic decay (natural log) as opposed to an exponential one.
  • #1
jaamae
8
0
Hi,

Does anyone know why the decay of hypochlorite/free chlorine in pool water due to UV light might show a logarithmic decay (natural log) as opposed to an exponential one?

I did a chemistry experiment with scaled up concentrations of chlorine and cyanuric acid (scaled up 25x). Even in the solution with no cyanuric acid it still shows logarithmic decay.

Analysis was by titration. Excess potassium iodide and sulpuric acid was added and then it was titrated with sodium thiosulphate.

I'm wondering because ALL secondary data I have found shows a logarithmic decay.

With thanks,

Jaamae.
 
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  • #2
Please elaborate on how did you get to the conclusion it is logarithmic.
 
  • #3
I plotted the results as a graph in Excel and the logarithmic trendline option fits best.

Thanks,

Jaamae.
 
  • #4
On how many points of data?

And what have you plotted against what? If [itex]C=C_0e^{-kt}[/itex] you will get a perfect linear plot for [itex]\ln{C}=\ln{C_0}-kt[/itex], but it doesn't mean trend is logarithmic.
 
  • #5
Hi,

I plotted amount of thiosulfate added (which is directly proportional to concentration of chlorine left) (y axis) against time (x axis).

I have attached the data. Sheet 1 shows the data with a logarithmic trendline fitted to it. Sheet 2 shows the same data with exponential trendlines fitted to it.
As you can see the R^2 values for logarithmic are much better.

With thanks,

Jaamae.
 
  • #6
Woops forgot to attach :)
 

Attachments

  • chem data2.xls
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  • #7
I have a feeling errors are way too large for any meaningful result.
 
  • #8
OK, but can you think of any reason a logarithmic trendline fits better?
 
  • #9
By pure chance.
 
  • #10
But it fits noticably better on most of them. If we take out 1500 and 2500 (procedural anomalies) then its definitely fits better. I have a feeling there is a reason buried in the chemistry somewhere.
 
  • #11
Since exponential and logarithmic curves are inverses of each other.
Is it possible that you have exponential decay but your titration would be a
logarithmic curve ?
You can check this with: dm/dt = - km
with solution y(t) = y0e-kt
Solve for k : 1/[Chlorine] = ekt
And data check decay curve
 
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  • #12
Hi Morrobay,

Thanks for your reply. But could you please explain the bit about a logarithmic titration a bit more. Also what does 'm' represent in your equations?

Thanks very much,

Jaamae.
 
  • #13
jaamae said:
Hi Morrobay,

Thanks for your reply. But could you please explain the bit about a logarithmic titration a bit more. Also what does 'm' represent in your equations?

Thanks very much,

Jaamae.

m = [chlorine]

see these pages : https://www.physicsforums.com/showthread.php?t=291766 [Broken]

note: I had a question mark on whether the titration curve could be logarithmic.
Maybe you should plot this experiment from a decay curve, see reference
 
Last edited by a moderator:
  • #14
How did you expose your sample to UV radiation? Describe the reaction setup.
Did you calculate the quantum yield for Cl2 vs. concentration? The quantum yield can vary from 2 to 4.5 for concentrations of 500 ppm and 1500 ppm respectively. Quantum yield of concentrations less than 100 ppm are ~1.0
 
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  • #15
Hi,

Firstly thanks very much for your reply. The setup was a commercial UV light mounted above a shelf. The beakers (with different trials in them) were placed under it for 3 hours. Ever 30 mins a 20mL sample was removed and titrated to find the hypochlorite content.

How does one calculate quantum yield? Keep in mind the varying ppm in the excel data is the Cyanuric Acid, which was a test variable. The original chlorine solution was made to be 100ppm. Please elaborate on how to calculate the quantum yield as I am very interested. Thanks very much for your help.Jaamae.
 
  • #16
It is described in http://neotechaqua.com/wp-content/uploads/2011/07/NEOTECH-Photolysis-AqueousFreeChlorineSpecies-HOCl-and-OCl-withUV.pdf
 
  • #17
chemisttree said:
It is described in http://neotechaqua.com/wp-content/uploads/2011/07/NEOTECH-Photolysis-AqueousFreeChlorineSpecies-HOCl-and-OCl-withUV.pdf
So is it correct to conclude that this photolysis is exponential decay:
-d[A]/dt = k[A] and as Borek said in post #4 it is linearized to :
ln [A]t = ln [A]0 -kt
 
  • #18
Yes, this is what the OP expected but it is usually written as d[A]/dt = -k[A]. OP found that when concentration is plotted vs -k*ln(t) + C there is a better fit than when remaining concentration is plotted vs C1e-kt+ C2.

I'm not going to analyze and replot the data so I won't comment as to why OP gets these results. I thought that perhaps a change in the quantum efficiency during the experiment might affect the best-fit curve enough to skew the results.
 

1. What is logarithmic decay?

Logarithmic decay is a type of exponential decay where the decrease in a quantity follows a logarithmic function. This means that as the quantity decreases, the rate of decrease also decreases.

2. How does hypochlorite/chlorine decay by UV light?

Hypochlorite/chlorine decay by UV light occurs through a process called photolysis, where the UV light breaks down the chemical bonds in hypochlorite/chlorine molecules. This results in the formation of less reactive and less effective byproducts, leading to a decrease in the concentration of hypochlorite/chlorine over time.

3. What factors affect the rate of logarithmic decay of hypochlorite/chlorine by UV light?

The rate of logarithmic decay of hypochlorite/chlorine by UV light can be affected by several factors, including the intensity of the UV light, the initial concentration of hypochlorite/chlorine, the presence of other chemicals that may act as catalysts or inhibitors, and the temperature of the solution.

4. How is the rate of logarithmic decay of hypochlorite/chlorine by UV light measured?

The rate of logarithmic decay of hypochlorite/chlorine by UV light can be measured by regularly collecting samples from the solution and analyzing the concentration of hypochlorite/chlorine over time. This can be done using various methods such as titration or spectrophotometry.

5. What are some practical applications of studying the logarithmic decay of hypochlorite/chlorine by UV light?

Studying the logarithmic decay of hypochlorite/chlorine by UV light is important in understanding the effectiveness of UV disinfection processes in water treatment. It can also help in optimizing the use of UV light in wastewater treatment plants and swimming pools to ensure safe and efficient disinfection. Additionally, this research can aid in the development of more effective UV disinfection methods for various industries and applications.

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