Phenol red

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mustangsally
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Phenol red

Post by mustangsally » Fri Jul 29, 2005 2:50 am

I understand that phenol red is used in experiments in photosynthesis/respiration and that the color change indicates whether CO2 is present or not. The question I have is the importance of the use of phenol red in these experiments. Can you help me to better understand this?

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Post by chemistry_freako » Fri Jul 29, 2005 4:34 am

Phenol red is an indicator also used in cell and tissue culture - it does have alot of indications. Carbon dioxide, being acidic when aqueous, decreases the pH of the medium it dissolves in, and this turns the indicator to orange, and if the pH drops further to around 6, the colour becomes yellow. Alkaline pH is indicated by pink or purple colour of the indicator.
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mustangsally
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Post by mustangsally » Fri Jul 29, 2005 4:51 am

Whoa. :oops: I don't really understand what you said, and am not sure that is what I am looking for. I need to know what the importance of phenol red is to photosyntesis/respiration experiments. I am confused, is it to show that respiration is occuring when it is red and photosynthesis is occuring when it is yellow? Or do I have that backwards? Please understand that I am older :( and going back to school, is hard for this old brain to comprehend some of this, and need more basic explanation in order to understand.

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Post by Dr.Stein » Fri Jul 29, 2005 7:23 am

The function of phenol red is to observe the product (qualitative method) and to measure its quantities. In your experiment, it functions to indicate the presence of carbondioxyde. In this experiment, if carbondioxyde presents, you will see a color changing. Which color, I will not tell, you should see from your experiment ;)

Hint:
As chemistry_freako said, there will be a color changing of this indicator, from colorless to such color depends on the product that will affect the acidity (pH) of the substrate:
pH < 6 ---> orange
pH around 6 ---> yellow
pH neutral ---> colorless
pH > 7 ---> pink, purple
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Post by mustangsally » Fri Jul 29, 2005 8:03 pm

Thank you for your anwer, and forgive this g'ma, I think I failed to mention that I am reviewing for my lab final, and have gotten confused in reviewing my notes on this. We did the experiment several weeks ago, and now I have confused myself as to which caused the color change, photosynthesis or respiration. Please help me. I have been so proud of me thus far, have been out of school for over 25 yrs, and managed to maintain an A in this class so far. It makes me so mad with myself when I don't understand something and research to the point that I confuse myself.

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Post by Dr.Stein » Sat Jul 30, 2005 7:01 am

I hope you wrote the result of the experiment in your notebook because I can't help you to answer. I already gave you a hint, and I think that's enough for you to discuss your experiment then generate a conclusion for the topic.

That is why we often to hold experiments in Biology, we need to observe something new, that we do not know yet the results. This challenges our curiosity and brings us to the nice how-and-why discussion. And as a teacher I will not tell the answer to my students until they submit their reports. Then we will discuss it in class together. Please forgive me :( :)
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Post by Poison » Sun Jul 31, 2005 3:21 pm

Sorry if someone has already said this(maybe I haven't seen) but a small hint can be the thing you are looking for. CO2 makes the envionment acidic. ( think about coke, it has CO2 and it is acidic.) Think about photosynthesis and respiration... Gases formed etc. etc... ;)
Hope this helped...
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Post by sdekivit » Sun Jul 31, 2005 3:58 pm

CO2 in the air is in equilibrium with carbonic acid:

CO2(g) + H2O <--> H2CO3

Carbonic acid is in equilibrium with bicarbonate:

H2CO3 <--> HCO3(-) + H(+).

That's why a solution becomes acid when there is CO2. To get the pH constant, for example in a tissue culture to keep the pH at a constant value.

This equilibrium is also a very important buffer in the blood.

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Post by Poison » Sun Jul 31, 2005 4:04 pm

I don't remember anybody asked the equilibrium but thanks for informing us.. :)
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Post by sdekivit » Sun Jul 31, 2005 4:33 pm

Poison wrote:I don't remember anybody asked the equilibrium but thanks for informing us.. :)


it's just to show that the solution becomes acidic.

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Post by Dr.Stein » Sun Jul 31, 2005 4:34 pm

Oh, my bad lady :twisted: :lol: But that explanation is very good to discuss the experiment. Thank you, sdekivit :wink:
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Post by mustangsally » Mon Aug 01, 2005 2:14 pm

Thanks for your help, I think I understand now.

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