Standard Buffer Solutions -Phosphate Buffer (Monobasic)

 

In a previous post entitled "Buffer Solutions - How to prepare buffer solutions" the basic steps for designing buffers were presented. 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 standard buffer solutions:

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 a phthalate standard buffer solution. In this post it will be shown how to prepare a phosphate buffer (pH 5.8 to 8.0 including a 7 buffer).

 

Phosphate Buffer 

Prepare the following solutions:

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:  

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

 

Potassium Phosphate, Monobasic 0.2 M:  

Dissolve 27.22 g of monobasic potassium phosphate (KH₂PO₄) in water
Dilute with water to 1000 ml

Place 50 ml of the monobasic 0.2 M potassium phosphate solution in a 200 ml volumetric flask
Add the volume of the 0.2 M NaOH solution shown in the table below
Add water to volume 200 ml

 

Table 1: Phosphate Buffer - Standard Buffer Solution

pH	0.2 M NaOH (ml)	0.2 M monobasic Potassium Phosphate
5.8	3.6	50
6.0	5.6	50
6.2	8.1	50

6.4	11.6	50
6.6	16.4	50
6.8	22.4	50
7.0	29.1	50

7.2	34.7	50
7.4	39.1	50

7.6	42.4	50
7.8	44.5	50

8.0	46.1	50

 

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Relevant Posts

Buffer Solutions - How to prepare buffer solutions

Preparing Standard Buffer Solutions -Phthalate Acid and Neutralized Buffers

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, phosphate buffer, buffers, 7 buffer,

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