The biuret test

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PODHEC1
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My textbook describes the biuret test as semi-quantitative. But I don't understand how it could be this since it is simply a colour change blue to purple so surely it must be qualitative?
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moolio20
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(Original post by HN786)
My textbook describes the biuret test as semi-quantitative. But I don't understand how it could be this since it is simply a colour change blue to purple so surely it must be qualitative?
You can measure the concentration of the protein present in your sample by observing the intensity of the colour change. One way to do this is measure the absorbance of the sample. Those containing more proteins will give a greater value. This is because of the gradient of the colour change. It can be faint or quite intense.

Hope this helps to explain it.
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username2613789
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Its qualitative because you can look at the colours and be happy about their shades and such.
Its quantitative cause you can measure that you needed 20.5 cm^3 of "Chemical" to observe the happy shades.
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PODHEC1
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(Original post by moolio20)
You can measure the concentration of the protein present in your sample by observing the intensity of the colour change. One way to do this is measure the absorbance of the sample. Those containing more proteins will give a greater value. This is because of the gradient of the colour change. It can be faint or quite intense.

Hope this helps to explain it.
What about the iodine test that must be qualitative?
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moolio20
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(Original post by tremen222)
Its qualitative because you can look at the colours and be happy about their shades and such.
Its quantitative cause you can measure that you needed 20.5 cm^3 of "Chemical" to observe the happy shades.
I am afraid this is not correct. The quantitative is referring to the idea that you can get an indication of the concentration of protein by observing the colour of the precipitate. It is only "semi" because you dont obtain numerical values.
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moolio20
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(Original post by HN786)
What about the iodine test that must be qualitative?
Yes you are correct. In the iodine test, you are just testing for presence of starch, not how much starch is present.
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