Buffer Solution Formulas

The following formulas produce buffers with pH values close to that indicated. The

weights give 0.1 M concentrations of each species, when diluted to 1.0 L, so that the

[acid]:[conjugate base] ratio is unity. In most cases the pH will have to be adjusted using

a pH meter and the dropwise addition of either 6 M HCl or 6M NaOH to one liter of

solution.

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Acid, formula and wt.or volume of

standard reagent (mL)

Conjugate base, formula and wt. pH

Phosphoric acid 85% H3PO4

6.8 mL

Monopotassium phosphate, KH2PO4

12.8 g

2.1

Hydrochloric acid, HCl, 8.6 mL Potassium hydrogen phthalate,

KHC8H4O4, 20.4 g

2.8

Citric acid, HOC(CH2CO2H)2CO2H,

19.2 g

monopotassium citrate,

HOC(CH2CO2H)2CO2K, 23.0 g

3.1

Methyl succinic acid,

HO2CCH2CH2CO2H, 11.8

Monosodium methylsuccinate,

HO2CCH2CH2CO2Na, 14.0

4.1

Acetic acid,CH3CO2H, glacial, 5.7

mL, or

Acetic acid,CH3CO2H, dilute (36%),

15.9 mL

Sodium acetate,CH3CO2Na ,

8.2 g

4.75

Potassium hydrogen phthalate,

KHC8H4O4, 20.4 g,

sodium hydroxide, NaOH, 4.0 g 4.8

Monosodium methylsuccinate,

NaO2CCH2CH(CH3)CO2H, 15.3 g

Disodium methylsuccinate,

NaO2CCH2CH(CH3)CO2Na, 17.5 g

5.6

Monosodium citrate,

HOC(CH2CO2H)2CO2Na, 21.4 g

Disodium citrate,

NaOC(CH2CO2H)2CO2Na, 23.6 g

5.9

Disodium citrate,

HOC(CH2CO2Na)2CO2H, 23.6 g

Trisodium citrate,

HOC(CH2CO2Na)2CO2Na, 25.8 g

6.4

Monopotassium phosphate, KH2PO4,

12.8 g

Dipotassium phosphate, K2HPO4

15.8 g

7.2

DL-Cysteine,

HSCH2CH(NH2)CO2H, 12.1 g

Sodium DL-Cysteinate,

HSCH2CH(NH2)CO2Na, 14.3 g

8.1

Boric acid

H2B4O7, 31.4 g

Sodium Hydroxide, NaOH, 4.0 g 9.1

Sodium bicarbonate, NaHCO3, 8.4 g Sodium carbonate, Na2CO3, 10.6 g 10.2

Monosodium tellurate, NaHTeO4,

21.6g

Disodium tellurate, Na2TeO4, 23.8g 11.3

Monosodium ascorbate, NaC6H7O6,

19.8g

Disodium ascorbate, Na2C6H7O6, 22.0g 11.8

Dipotassium phosphate, K2HPO4

15.8 g

Tripotassium phosphate, K3PO4

19.6 g

12.7

Boric acid

H2B4O7, 31.4 g

Sodium hydroxide, NaOH, 12.0 g 12.7

Phosphate Buffer Materials

monosodium phosphate disodium phosphate water phosphoric acid to make the pH more acidic or sodium hydroxide to make the pH more

alkaline pH meter glassware hot plate with stirring bar

Prepare the Phosphate Buffer(H3PO4 (pKa) 2.148, 7.198, 12.375)

1. Decide on the concentration of the buffer. Most buffers are used at a concentration between 0.1 M and 10 M. If you make up a concentrated buffer solution, you can dilute it as needed.

2. Decide on the pH for your buffer. This pH should be within one pH unit from the pKa of the acid/conjugate base. So, you can prepare a buffer at pH 2 or pH 7, for example, but pH 9 would be pushing it.

3. Use the Henderson-Hasselbach equation to calculate how much acid and base you need. You can simplify the calculation if you make 1 liter of buffer. Select the pKa value that is closest to the pH of your buffer. For example, if you want the pH of your buffer to be 7, then use the pKa of 6.9:

pH = pKa + log ([Base]/[Acid])

ratio of [Base]/[Acid] = 1.096

The molarity of the buffer is the sum of the molarities of the acid and conjugate

base or the sum of [Acid] + [Base]. For a 1 M buffer (selected to make the

calculation easy), [Acid] + [Base] = 1

[Base] = 1 - [Acid]

substitute this into the ratio and solve:

[Base] = 0.523 moles/L

Now solve for [Acid]. [Base] = 1 - [Acid] so [Acid] = 0.477 moles/L

4. Prepare the solution by mixing 0.477 moles of monosodium phosphate and 0.523 moles of disodium phosphate in a little less than a liter of water.

5. Check the pH using a pH meter and adjust the pH as necessary using phosphoric acid or sodium hydroxide.

6. Once you have reached the desired pH, add water to bring the total volume of phosphoric acid buffer to 1 L.

7. If you prepared this buffer as a stock solution, you can dilute it to make up buffers at other concentrations, such as 0.5 M or 0.1 M.