Standard Buffer Solutions -Preparing Alkaline Borate Buffers

In a previous post entitled "Buffer Solutions - How to prepare buffer solutions" the basic steps for preparing buffers were presente d. It was shown that it is possible to prepare buffer solutions that maintain the pH close to any desired value by the proper choice of a weak acid and its conjugate base and their relatitive concentrations.

The following steps can be used to prepare different buffers:

• Determine the optimal pH (the required pH)
• Select a weak acid with a pka near the desired pH
• Calculate the ratio of salt to acid required to produce the desired pH (Henderson-Hasselbach equation): pH = pk_a - log [H_A]_O/[A^-]_O
• Determine the desired buffer capacity of the solution
• Calculate the total buffer concentration required to produce this buffer capacity ß (Van Slyke equation): ß = 2.3* C* (k_a * [H₃O⁺]) / (k_a + [H₃O⁺] )²
• Determine the pH and the buffer capacity of the final buffer solution using a reliable pH meter.

As an example it was described how to prepare phosphate buffer solutions and acetate buffers. In this post it will be shown how to prepare an alkaline borate buffer (pH 8.0 to 10.0).

 

Borate buffer (alkaline)

• Prepare the following solutions:

Boric Acid and Potassium Chloride, 0.2 M:

1. Dissolve 12.37 g of boric acid (H₃BO₃) and 14.91 g of potassium chloride (KCl) in water and dilute with water to 1000 ml.

Sodium Hydroxide , 1 M:

1. Dissolve 162 g of sodium hydroxide in 150 ml of carbon dioxide-free water, cool the solution to room temperature and filter through hardened filter paper. Transfer 54.5 ml of the clear filtrate to a tight, polyolefin container, and dilute with carbon dioxide-free water to 1000 ml.
2. Standarize the above solution as follows: Accurately weigh about 5 g of potassium biphthalate, previously crushed lightly and dried at 120 C for 2 hours, and dissolve in 75 ml of carbon dioxide-free water. Add 2 drops of phenolphthalein and titrate with the sodium hydroxide solution to the production of a permanent pink color.

M = g KHC₈H₄O₄ /( 0.20422 * ml NaOH solution) (M, Molarity of solution)

Sodium Hydroxide, 0.2 M:

1. Prepare a 0.2 M NaOH solution: Dilute appropriatelly the 1 M NaOH solution

• Place 50 ml of the boric acid and potassium chloride solution in a 200 ml volumetric flask
• Add the specified volume of the 0.2 M NaOH solution shown in the table below
• Add water to volume and mix.

Table 1: Alkaline Borate Buffer - Standard Buffer Solution

pH	0.2 M NaOH (ml)	0.2 M Boric Acid & Potassium Chloride solution (ml)
8.0	3.9	50
8.2	6.0	50
8.4	8.6	50
8.6	11.8	50
8.8	15.8	50
9.0	20.8	50
9.2	26.4	50
9.4	32.1	50
9.6	36.9	50
9.8	40.6	50
10.0	43.7	50

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Relevant Posts

Buffer Solutions - How to prepare buffer solutions

Standard Buffer Solutions -Preparing Na Acetate Buffers

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References

1. CRC Handbook of Chemistry and Physics, 52nd edition, The Chemical Rubber Co., (1971)
2. U.S. Pharmacopeia, 68, USP 36
3. David W. Oxtoby, H.P. Gillis, Alan Campion, “Principles of Modern Chemistry”, Sixth Edition, Thomson Brooks/Cole, 2008
4. Steven S. Zumdahl, “Chemical Principles” 6th Edition, Houghton Mifflin Company, 2009

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Key Terms

preparing standard buffer solutions, borate buffer, buffers, 10 buffer,

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