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Topic: Indirect Iodometry Calculation  (Read 6302 times)

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Offline timmatkf

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Indirect Iodometry Calculation
« on: February 12, 2016, 02:17:55 PM »
Hello All,

Struggling with how to come up with this answer.  I'm running a test to determine stannous chloride content standard (35 mg in 100 mL) but my calculations are throwing me off somewhere.  Any help is appreciated.  This is a back titration where the stannous reacts with potassium iodate and then the excess potassium iodate is titrated using sodium thiosulphate.

Here are the particulars:

1 mL Potassium Iodate Solution (0.01N)
1 mL KI Solution
1 mL 1N HCL
1 mL Stannous Chloride Standard Injection

From the study I am trying to replicate, 1 mL of KIO# = 0.594 mg stannous chloride.

Attached is my spreadsheet with my current thinking (without values, which doesn't seem to work) but may help.  It may not be clear as I went back and forth with this multiple times.

KIO3 Injection (L)
KIO3 moles injected
Stannous Chloride Injection Volume (mL)
Stannous Chloride Injection Volume (L)

Sodium Thiosulfate Titre Volume (mL)
Sodium Thiosulfate Titre Volume (L)
Normality of sodium thiosulfate (0.01mol/L)
Sodium thiosulfate in titre that reacted with liberated iodine (moles)
Excess Iodine titrated by Sodium Thiosulfate (moles)
# moles iodine = # moles stannous (moles)
Stannous Chloride Concentration


Iodine Converted to  Iodide (Moles)
Molarity of Iodide (Moles/L)
Iodine Molecular Weight
Iodine Density (g/mL)
Iodine Titrated by Sodium Thiosulphate (mL)

Stannous Chloride Injected (Theoretical) (mg)
KIO3 reacted with stannous (mL)
Stannous Equivalent (0.594) Amount (mg) Recovery
Stannous Chloride mg/mL
Concentration of Stannous in Sample (ug/mL)


Thanks in advance for any help.

Offline Borek

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Re: Indirect Iodometry Calculation
« Reply #1 on: February 12, 2016, 06:52:30 PM »
I can't see anything attached, besides, I would not open most standard spreadsheet formats on my computer.

Start with reaction equations - can you list them here?
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Offline timmatkf

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Re: Indirect Iodometry Calculation
« Reply #2 on: February 16, 2016, 09:19:45 AM »
Here goes (thanks and hopefully the format comes out okay),

SN2+ + I2 (known excess)   :rarrow: SN4+ + 2I- + I2

Titration

I2 (excess unreacted) + 2S2O32-  :rarrow: S4O62- + 2I-

1 mL 0.01 N KIO3 (0.001944 moles total injected)

1 mL 10% KI injected

1 mL 0.1N HCL injected

1 mL 300ug/mL stannous chloride injected (0.000008 moles)

4.3416 mL sodium thiosulfate titrated (0.01N) = 4.54131E-05 moles sodium thiosulfate


Excess iodine titrated by sodium thiosulfate = 0.5 * moles of sodium thiosulfate = 2.27066E-05

Iodine molecular weight = 126., Density = 4.93 g/mL

Molarity of Iodine = 1.36239E-05 (3/5 * 2.27066E-05)

Iodine titrated by sodium thiosulfate (mL) = 1.36239E-05 * 126.9 / 4.93 = 0.000350685


KIO3 reacted with stannous (mL) = 1 - 0.000350685 = 0.999649

Stannous equivalent (0.594) = 0.999649 * 0.594 = 0.593791693mg

Stannous Chloride Concentration of Injection = 0.593791693mg / 1 mL = 0.593791693 mg/mL = 594 ug/mL


Forgive the significant figures as this is a rough pass trying to figure out the math.  I recovered almost 200% so that is obviously not correct.  Any red flags out there?

Offline mjc123

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Re: Indirect Iodometry Calculation
« Reply #3 on: February 16, 2016, 12:08:24 PM »
Quote
Any red flags out there?
Yes, lots. (Starting with SN and CL.) I don't want to be rude, but do you have any idea what you're doing at all?

What does iodate do? You haven't written an equation for it.

Quote
1 mL 0.01 N KIO3 (0.001944 moles total injected)
Where do you get this from?

Quote
1 mL 300ug/mL stannous chloride injected (0.000008 moles)
Where do you get this from?

Quote
4.3416 mL sodium thiosulfate titrated (0.01N) = 4.54131E-05 moles sodium thiosulfate
Where does this come from?

Quote
Molarity of Iodine = 1.36239E-05 (3/5 * 2.27066E-05)
Where does this come from?

Quote
Iodine titrated by sodium thiosulfate (mL) = 1.36239E-05 * 126.9 / 4.93 = 0.000350685
Are you working out mL of solid iodine? And then subtracting it from mL of KIO3? Why? And then where do you get the number 0.594 from? None of this makes any sense.

Let's start again. How many moles of iodate do you start with?
How many moles of iodine does this generate?
How many moles of thiosulfate were used in the titration?
How many moles of iodine does this correspond to?
So how many moles of iodine were consumed by reaction with Sn2+?
So how many moles of Sn2+ did you have?
So what was the concentration of your Sn2+ solution?

Offline aga

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Re: Indirect Iodometry Calculation
« Reply #4 on: February 16, 2016, 04:08:19 PM »
...
From the study I am trying to replicate ...

Attached is my spreadsheet with my current thinking ...
Can you give a link to the Study ?

I also see no spreadsheet.

For back-titrations it is simplest to do all the reaction formulae/stoichimetry from start to finish, then work backwards.

A spreadsheet is a good way to simplify the process.
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Offline timmatkf

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Re: Indirect Iodometry Calculation
« Reply #5 on: February 17, 2016, 09:57:04 AM »
Thanks for the replies.  It has been a while since I was in a school type setting so I know I'm a bit rusty.  I fixed up the spreadsheet and attached the study so hopefully you can follow it but unfortunately I can't attach the spreadsheet as it is not allowed on the this website. 

I am still way off and some of the mistakes may be elementary but I did put a good faith effort to try and figure it out myself.  I ended up probably spinning myself further away from the true correct method.

Thanks again for the assistance.

Offline mjc123

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Re: Indirect Iodometry Calculation
« Reply #6 on: February 17, 2016, 11:47:06 AM »
OK, I think I see where you're going now - partly at least. Your mistake was at the beginning. To find the number of moles of iodate, you must divide the mass of potassium iodate added by the molar mass of potassium iodate - not "iodate". This affects everything else.

I attach pdfs of my calculations, the numbers and the formulas so you can see how I got them. The two results are very different, but the agreement between your two titre volumes is bad. You shouldn't get such a big difference doing the same titration twice if you do it properly. (Or was your Sn2+ getting oxidised by air, as in the paper?)

I still don't understand why you seem to be calculating the volume of solid KIO3 that reacted. What is the point of this? And what is Conversion Factor Stannous? Was your solid SnCl2 a hydrate?

Offline timmatkf

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Re: Indirect Iodometry Calculation
« Reply #7 on: February 17, 2016, 01:34:29 PM »
Thanks a lot for the spreadsheets.  I was using the conversion factor because the paper had used it but your right, it is actually the anhydrous form so I was in error.  The two titrations were actually separate standards and one was a bit older than the other (although they were under a bed of nitrogen).  I was using some new software with our autotitrators so perhaps that was the problem.  There was a default setting for multiple EP which may have something to do with it.  The second standard was made fresh and I specified one EP.

Thanks again for all of the help.

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