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Topic: No Color Change During Manual Titration  (Read 599 times)

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Offline znaes97

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No Color Change During Manual Titration
« on: October 19, 2022, 01:20:11 PM »
Hello,
I am attempting to perform a titration to determine the concentration of sodium dichromate in a nitric acid bath. The titration is done in two steps. The first step is to titrate (with 0.1N sodium thiosulfate) to a straw yellow color. Once this change occurs, you are supposed to add starch indicator and continue titrating until a clear-green endpoint is reached. My problem is that I am not seeing any color change to straw yellow during the first step. Does this mean that there is little to no sodium dichromate in the solution? Before titration, the solution is prepared with 20% potassium iodide and a few mLs of concentrated HCl. The final result should only be a few mL, but I titrated ~25mL without seeing any changes. Any help or suggestions are greatly appreciated. Thanks in advance

Offline Borek

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Re: No Color Change During Manual Titration
« Reply #1 on: October 19, 2022, 06:02:34 PM »
Can you elaborate on the procedure you are following?
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Offline znaes97

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Re: No Color Change During Manual Titration
« Reply #2 on: October 20, 2022, 11:18:06 AM »
Can you elaborate on the procedure you are following?

The full procedure is as follows:
1. Pipette a 1ml sample into a 250ml Erlenmeyer flask
2. Add 150ml of DI water
3. Add 10ml of concentrated HCl
4. Add 5ml of 20% potassium iodide solution
5. Titrate with 0.1N sodium thiosulfate to a straw color
6. Add 2ml of starch indicator and continue titrating to a clear light green color

% by weight sodium dichromate = ml of sodium thiosulfate used * 0.498
The target concentration is 2.0 - 2.5%

Offline Borek

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Re: No Color Change During Manual Titration
« Reply #3 on: October 20, 2022, 11:56:35 AM »
The first step is to titrate (with 0.1N sodium thiosulfate) to a straw yellow color.

What is the initial color of the solution?
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Offline znaes97

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Re: No Color Change During Manual Titration
« Reply #4 on: October 20, 2022, 12:28:41 PM »
The first step is to titrate (with 0.1N sodium thiosulfate) to a straw yellow color.

What is the initial color of the solution?

The initial color of the solution was clear. When adding the potassium iodide, I dimmed the lights and the solution appeared to be very faintly yellow, but when I turned the light back up, the solution appeared to change from faint yellow to clear.

Offline Borek

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Re: No Color Change During Manual Titration
« Reply #5 on: October 20, 2022, 12:34:55 PM »
If the solution is clear it probably doesn't contain any chromate/dichromate, they are both quite intense.
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