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The Thing

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Hi I have a weird question that is from a biology lab but has to do with chemistry. We were testing the effect of different pH on bacteria, and placed tablets soaked in different concentration of HCl on an agar plate and couple of days later we measured the circle of inhibition around the tablet.


My question is, how far can the HCl spread to? Like one circle of inhibition was about 2 cm in diameter (with 2.0M of HCl). Going away from the center, how does the concentration of the HCl drop? I vaguely remember reading somewhere that the concentration of HCl decreases logarithmically when moving away from the center of the circle. Is that right?


Thank you!

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To The Thing.


You hit the nail on the head when you queried the diffusion distance. This kind of test can be very misleading, since it depends so much on diffusability. You are correct about the concentration gradient. The inhibition distance will depend more on the concentration of HCl than anything else.


Another factor that can screw the result is the fact that not all species of bacteria are upset by low pH. A few species are quite happy in very acid conditions. This test is good visually - you can clearly see the results. But lousy scientifically.

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