How do I make a 400ul Edwards Extraction Buffer

Question

Stocks that I have: 1M Tris-Cl, 5M NaCl, 0.5M EDTA in 0.5M NaOH, and 10% SDS

Recipe needs: 200mM Tris-Cl, 250mM NaCl, 25mM EDTA, and 0.5% SDS

I need calculations too

Answer 1

Firstly, we want to know how much volume of each stock should be added. Let's start with the Tris-Cl:

We start with a stock of 1M Tris-Cl, but we need 200mM. How do we achieve this concentration? We can simply dilute the 1M stock until it reaches 200mM. Since 200mM is 1/5th of 1000mM (remember, 1M = 1000mM), we can dilute the solution by 1/5x. What does this mean? It means that in our 400uL buffer, the stock 1M Tris-Cl must occupy 1/5th of the volume in order for the buffer to be 200mM Tris-Cl. Simply put, we need 400/5 uL of the 1M Tris-Cl = 80uL of 1M Tris-Cl.

Same process for the rest!

NaCl:

5M NaCl to 250mM NaCl requires a 1/20 dilution (because 5000mM/20 = 250mM). So in our 400uL buffer, 1/20th of the volume must be the 5M NaCl stock solution.

400uL/20 = 20uL of 5M NaCl.

EDTA:

0.5M EDTA to 25mM EDTA requires 1/20 dilution (500mM/20 = 25mM). In our 400uL buffer, 1/20th of the volume must be the 0.5M EDTA.

400uL/20 = 20uL of 0.5M EDTA.

SDS:

10% SDS to 0.5% SDS requires a 1/20 dilution (10%/20 = 0.5%). In our 400uL buffer, 1/20th of the volume must be the 10% SDS.

400uL/20 = 20uL of 10% SDS.

In order to make Edwards extraction buffer with the recipe's concentrations, we will need to add:

80uL of 1M Tris-Cl

20uL of 5M NaCl

20uL of 0.5M EDTA

20uL of 10% SDS

and then we would need to "top-off" to 400uL. Since 80 + 20 + 20 + 20 = 140, we need to add 260uL of water to reach the desired 400uL volume. If we didn't include this step, then the concentrations would not be correct. Our calculations are all based off the goal volume of 400uL!