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Identification of HEPES and MES on the basis of pH and pK

You and a lab parnter prepare buffers to use in an experiment. Your partner prepares 200 ml 50 mM MES pH 6.0 while you prepare 200 ml of 50 mM HEPES pH 6.0. (MES and HEPES are the abbreviated names for two biological buffers often used in research labs).

HEPES has MW 238.3 and pK = 7.48
MES has MW 195 and pK = 6.1

1. How many grams of HEPES did you need to weigh out to prepare this buffer?

2. Unfortunately, both you and your partner forgot to label the beakers containing the buffers. Now that the beakers are on the lab bench next to each other, you can't tell which buffer is in which beaker. The third member of your group remembers what you learned about buffers in Lab 2. She labels one beaker "A" and the other "B". She takes 5 ml of buffer from "A", adds 1.0 ml dilute HCL to the 5 ml, mixes, and checks the pH. It is 5.9. She repeats the process with buffer from "B" but the pH of this samples is now 4.0. Which beaker (A or B) has the HEPES buffer? Explain your answer.

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Here comes your answers.
You and a lab parnter prepare buffers to use in an experiment. Your partner prepares 200 ml 50 mM MES pH 6.0 while you prepare 200 ml of 50 mM HEPES pH 6.0. (MES and HEPES are the abbreviated names for two biological buffers often used in research labs).

HEPES has MW 238.3 and pK = 7.48
MES has MW 195 and pK = 6.1

1. How many grams of HEPES did you need to weigh out to prepare this buffer?

We need to add 2.383 g of HEPES to ...

Solution Summary

In case of HEPES, pK> pH, hence the reaction moves to protonation. It is a stronger acid will cause the formation of HA, the protonated form. It is not a good buffer and it cannot resist pH changes, while in case of MES, pH = pK (approx 6).

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