adjusting pH - (Jan/20/2006 )
Hi
To prepare Gel buffers to make SDS-PAGE gels
a solution containing Tris and SDS I adjust the pH to 8.8 or 6.8 by ading 37% HCl.
I do it under the fume-hood because HCl vapours are of course very toxic. First I add few mls but when pH is closer to what I want, I add drop by drop because I don´t want to risk to lower the pH too much.
A friend of mine instead of taking the pH meter into the fume-hood, adds a certain amount of HCl into the solution under the fume-hood mixes and after runs into the pH meter to check. As expected the pH became too low she adds few mls of 10M Sodium hydroxide to rise again the pH.
The day after I saw that the buffer she made was full of small white precipitates floating.
I think the explanation for that is that the HCl and NaOH together will react and precipitate NaCl salt. Is this true?
As a rule you shouldn´t add acid and basic strong solutions to adjust the pH. You should add the required one, according to your needs (rising or lowering the pH), right?
Can someone explain me better the chemical reaction that occured in this case?
I told her that the way she was doing was not right but she was amazed to hear that because according to her for the past 7 years she had always made like that.
Thanks
My guess is it was SDS floating around in there, and that warming the solution up a bit would've caused it to redissolve...
But, you're right -- HCl + NaOH makes salt water. I suppose if the solution were nearly saturated, the additional salt would cause something to fall out of solution, but that rarely happens.
Still, there's now some measurable quantity of NaCl in a solution that's supposed to be Tris and SDS, so passing the desired pH and going back with the conjugate is not generally best practices.
I do it the way you've described -- get close with 10 N HCl, then switch to 1 N or 0.1 N to tweak it to the target pH.
Why not just mix Tris base and Tris HCl together to get the correct (or close to it) pH?
I have a little chart in the lab that tells me how many grams of each I need to make up 1l of a 1M solution of tris for several pH's and different temperatures.
I have a little chart in the lab that tells me how many grams of each I need to make up 1l of a 1M solution of tris for several pH's and different temperatures.
it was based on such kind of chart that my friend, passed the right pH.
But I don´t know how reliable was the source of the infomation she was using.
Can you please paste here the chart you are talking about and where did you take that information? Was it from sambrook? Actually there they give some amounts of HCl required to prepare tris buffer.
So do you recommend to make tris buffer adjust the pH and after add the SDS?
And of course re measure the pH to make sure the Final pH is not much affected.
That is probably easier.
Thanks.
But, you're right -- HCl + NaOH makes salt water. I suppose if the solution were nearly saturated, the additional salt would cause something to fall out of solution, but that rarely happens.
Still, there's now some measurable quantity of NaCl in a solution that's supposed to be Tris and SDS, so passing the desired pH and going back with the conjugate is not generally best practices.
I do it the way you've described -- get close with 10 N HCl, then switch to 1 N or 0.1 N to tweak it to the target pH.
Yes probably the things floating were SDS and probably by heating the solution it would redisolve but could I trust in such solution?
Wouldn´t the composition, more salt, affect my western at some stage?