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Thread: Why does my pH indicator turn Orange?

  1. #1 Why does my pH indicator turn Orange? 
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    We're using "Indicator Full Range Ph 1-14" from Scichem. We're using a 0.5% solution which gives us a nice light green colour. However after a few days it turns orange. The effect is prolonged if we use distilled water, but still occurs.

    Any suggestions?


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  3. #2  
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    or perhaps some of your indicator has dissolved away into your solution over time. I could imagine something similar happening to redcabbage indicator if you are using paper perhaps the paper was originally orange and the indicator is very diluted. Otherwise perhaps your solution liquid this time oxidised over time or some of the ions in your indicator sedimented to the bottom. Perhaps your reaction is dependant on temperature and/or pressure and leaving it in the fridge or somewhere warm has caused a reverse reaction. Some solutions are affected by light U.V. etc. Some can be affected if a magnetic, electric or radioactive source is nearby. Some solutions those with very small half lives could undergo radioactive decay. Or perhaps you have large clumps of solids in your solution and the reaction really takes hours. Some reaction can take place in the presence of sound waves or when some rotational force etc is exerted on it. There is also the possibility your apparatus was not completely sterile and so became contaminated or alternatively your apparatus could have started to react with your solution. Of couse you also have the action of bacteria, catalysts etc


    Last edited by fiveworlds; March 15th, 2013 at 07:20 PM.
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  4. #3  
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    Quote Originally Posted by PhDemon View Post
    Is it a 0.5% solution in water? Is it in an open beaker/bottle? If so it could be CO2 from the air dissolving in the solution, this will create a weak acid (carbonic acid) that could explain the change in indicator colour.
    Yup, it's in water. However it's completely sealed and the test ones I made up for observation weren't opened at all.

    Could there be enough CO2 sitting in the bottle to effect this change do you think?

    Quote Originally Posted by fiveworlds View Post
    or perhaps some of your indicator has dissolved away into your solution over time. I could imagine something similar happening to redcabbage indicator if you are using paper perhaps the paper was originally orange and the indicator is very diluted. Otherwise perhaps your solution liquid this time oxidised over time or some of the ions in your indicator sedimented to the bottom. Perhaps your reaction is dependant on temperature and/or pressure and leaving it in the fridge or somewhere warm has caused a reverse reaction. Some solutions are affected by light U.V. etc. Some can be affected if a magnetic, electric or radioactive source is nearby. Some solutions those with very small half lives could undergo radioactive decay. Or perhaps you have large clumps of solids in your solution and the reaction really takes hours. Some reaction can take place in the presence of sound waves or when some rotational force etc is exerted on it. There is also the possibility your apparatus was not completely sterile and so became contaminated or alternatively your apparatus could have started to react with your solution. Of couse you also have the action of bacteria, catalysts etc
    Not convinced about the time factor as it was left less than a week. The previous Technician left the bottles in the same place and didn't encounter this problem. It's made up from a liquid, so unlikely there would be any solids loitering.

    I thought of the contamination issue as well, but I repeated it with VERY clean bottles and observed the same result.

    Thanks for your suggestions though, I'm glad I've been thinking along the right lines.
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  5. #4  
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    Quote Originally Posted by PhDemon View Post
    I've just done a quick back of the envelope calculation: if the flask is 1L containing half a litre of your solution and the half a litre of air and is sealed so no more CO2 can dissolve other than what is in the flask and also assuming the pH of the solution is intially 7, the CO2 in the air will lower the pH from 7 to 6.999 so I don't think it will be this...
    Thanks, PhDemon. That's actually pretty useful to know.

    So, I wonder what it is...
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