What caused the low pH in my Tris-HCl buffer?

In summary, Mike has an easy way to do it, however, it would be better if you just used the Henderson-Hasselbach equation.
  • #1
rockind78
78
0
I don't know if I can effectively convey what I am doing here, but I am going to try because I need the help. I am trying to determine my ratio of acid to base and subsequently make a .5M Tris-HCl buffer utilizing Tris base, and concentrated HCl.

By my calculations I get a 19.05 parts acid/1 part base ratio.
Using this, I divide the total amount of buffer I am looking to make (200mL) by the total number of parts in solution (20.05 or 19.05+1). Given this, I get ~190mL acid to 10 mL base.

Here is where I was a little unsure of myself. To calculate the amount of base I need, I multiplied the MW of Tris-base (121.4 grams/mole) by the molarity of the solution by the number of mL specified above (10mL).

To calculate a 190mL of a .5M solution of HCL, I used (C1)(V1)=(C2)(V2), where C1=.5M, V1=.19 Liters, C2=11.6M (molarity of concentrated HCl). From this, I calculated that I needed to add 8.2mL of conc. HCl to 181.8mL dd H20.

The final step was to add my acid solution to my base solution.

NOW FOR THEMILLION DOLLAR QUESTION!

I tested some of this "buffer" on pH paper and got a pH of just above 1.0. Why?
The stated pKa value seems to fluxuate between 8.08 and 8.3. I was using 8.08 in application to HH.

Any input would be very much appreciated.

Thank you.
 
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  • #2
The problem is quite straightforward. You are asked to prepare a 200 mL solution of 0.5 M Tris buffer at pH 8.08. You only calculated to prepare 10 mL of 0.5 M Tris. You need to add 12.11 grams of Tris base (according to my calculations) to have, at the end, 200 mL of 0.5 M Tris buffer at pH 8.08.

My suggestion? Weigh out 12.11 grams of Tris base. Add to 100 mL of water. Carefully pH with HCl (or NaOH if you get too fancy free with the HCl). Bring up to 200 mL with water. And there you go.

FYI, be careful with temperature. Tris is temperature dependent when it comes to pH.

My calculation just so you can take a look: (0.2 L buffer) * (0.5 mol/L) * (121.1 grams/mol) = 12.11 grams Tris base
 
  • #3
Mike has an easy way to do it, however, it would be better if you just used the Henderson-Hasselbach equation. You could easily pull all your info from there.
 

Related to What caused the low pH in my Tris-HCl buffer?

What is a "Buffer Preparation Problem"?

A "Buffer Preparation Problem" refers to a common issue faced by scientists when preparing solutions of buffers, which are mixtures of acids and bases used to maintain a stable pH in experiments. This problem occurs when the buffer solution does not have the desired pH or is unstable, leading to inaccurate or inconsistent experimental results.

Why is buffer preparation important in scientific experiments?

Buffer solutions play a crucial role in maintaining a stable pH in many chemical and biological reactions. In experiments, a slight change in pH can greatly affect the outcome of the reaction, leading to inaccurate results. By carefully preparing and using buffers, scientists can ensure that their experiments are conducted under consistent and controlled conditions.

What are some common causes of buffer preparation problems?

There are several factors that can contribute to buffer preparation problems, including inaccurate measurements of the buffer components, using expired or contaminated buffer stock solutions, and failing to adjust for the temperature of the solution. Other factors such as incorrect pH calculations and improper storage of the buffer solution can also lead to problems.

How can buffer preparation problems be avoided?

To avoid buffer preparation problems, it is important to carefully follow the instructions for preparing the buffer solution, including accurately measuring the components, using fresh and uncontaminated stock solutions, and adjusting for the temperature. It is also recommended to regularly calibrate pH meters and store buffer solutions properly to ensure their stability.

What can be done if a buffer preparation problem occurs?

If a buffer preparation problem occurs, the first step is to identify the cause and correct it. This may involve adjusting the pH of the solution, using a different buffer component, or remaking the buffer solution altogether. In some cases, it may be necessary to troubleshoot and repeat the experiment with a properly prepared buffer solution.

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