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I need help making a borate buffer of 0.4N, pH 10.2

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cpt49:
Good day to all users here,

Now, judging from the topic itself, it sound like I asked a simple question, probably making myself sound lazy, but believe me, this is not the case! I'm a Master student and usually in the lab, I make my own calculations and dilutions for many different experiment, but this particular borate buffer has seem to make me question a lot... I need to make the chemical for my HPLC usage, not for assignment, so I need to make sure I do it right.  :-[

Okay, I will tell what I have done so far for this question. Now, if I'm not wrong, the first step to making solution based on normality is to know the equivalent weight of the chemical. Usually if I'm making solution like HCl, NaOH or H2SO4, the equivalent weight is easy to tell from their compound names, one hydrogen ions from HCl, 1 hydroxide ions from NaOH, and two hydrogen ions from H2SO4. But in this case here, I have the chemical in the lab, sodium tetraborate decahydrate, Na2[B4O5(OH)4]·8H2O
I made a similar calculations before, based on boric acid, you can see it here: http://tiny.cc/b0eydz

Now this puzzle me, if only from the formula itself, does Borax dissolved in water and released 12 OH- ions (Four from OH, and 8 from H2O)? Now, of course that doesn't sound right, looking at the chemical structure of borax (see link: http://tiny.cc/yxeydz , figure 1), without considering the water molecule, it seems that Borax would only released 4 hydroxide ions. So the equivalent weight should be = 381.38/4 = 95.345, where 381.38 is the molecular weight of Borax. Does this sound right for you?

I should have go with 4, but further reading (same link as above for structure of borax) showed that Borax actually dissolved in water to release tetraboarte anion and eight water molecules, no hydroxide ions is written in the equation here, then, tetraboarte anion further react with water to form boric acid and one hydroxide ions. For a student like me who can only perform basic calculations in the lab, this really added so much more further confusion to me. Furthermore, I also came across this website that make the whole borate buffer chemistry more complicated for me, if you like, you can read it here: http://tiny.cc/3ufydz
If anyone can guide me on the equivalent weight of Borax with its unique chemistry behavior, I'm sure I can proceed with the calculations for making the solutions.

Some additional notes for clarification:
- This product is used a buffer for HPLC amino acids, it is intend to buffer my sample to alkali level for better sensitivity and improved peak recovery.
- I actually have both boric acid and sodium tetraborate decahydrate in the lab, and I'm aware either or a combination of these two can make borate buffer, but since I'm going for high pH, so I decided to go with borax, since I'm not sure if too much salt formation from the addition of NaOH to boric acid buffer would affect my HPLC run.
- There are many examples of readily calculated answers for borate buffer online, but almost none reflect the actual requirement of my solution, and many of them provided weight of chemical without calculations, this make me worry to use their work without me understanding them yet.

Any helps is appreciated here. I'm exhausted of option to get expert advice, and I came across this wonderful website on reddit, so please help me.

AWK:
There are several problems with your buffer. First of all, it is difficult to calculate reliably. Regardless of whether you use Na[B(OH)4] and boric acid or borax, the concentration of the buffer will never be accurate, as the residues of multi-boric acids will also be in equilibrium. In addition, the required pH is at the upper limit of this borax-NaOH buffer solution. Biochemists have never played such detailed calculations. They either buy a ready buffer or make a solution with the required concentration of ingredients in a slightly smaller volume, correct the pH by adding small amounts of HCl or NaOH solution and supplement with a small amount of water to a certain volume. In your case, 0.1 mol of borax and 0.1 mol of NaOH in almost 1 liter of the solution will probably be a good approximation. In the calculations you can treat it as 0.3 M Na[B(OH)4] + 0.1 M H[B(OH)4].

cpt49:


--- Quote from: AWK on October 04, 2019, 04:15:44 PM ---
In your case, 0.1 mol of borax and 0.1 mol of NaOH in almost 1 liter of the solution will probably be a good approximation. In the calculations you can treat it as 0.3 M Na[B(OH)4] + 0.1 M H[B(OH)4].

--- End quote ---

Hi, thanks for the reply. I see you used a different borate name in your explanation, I assumed you are still implying the sodium tetraborate decahydrate I'm using right? Also, in your sentence, I believe you mean in my calculations it should be 0.3 M of Borax + 0.1 M NaOH, right? So that will bring me to approximately 0.4 N and pH 10.2?

AWK:
Borax (commonly known as sodium tetraborate decahydrate) shows structure Na2[B4O5(OH)4]·8H2O
Hydrolysis of borax:
Na2[B4O5(OH)4]·8H2O = 2H[B(OH)4] + 2Na[B(OH)4] + H2O
Hence in water you have equimolar mixture of  sodium borate and boric acid (this is approximation used for calculations).
Borax itself forms the buffer solution. The addition of NaOH to borax solution neutralizes part of the boric acid changing pH of this solution from 9.2 up. Borax works as the buffer chemical up to pH 10.1-10.2

Check the latest discussion on borax
https://www.chemicalforums.com/index.php?topic=101063.msg354498#msg354498

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