Citrate Phosphate Buffer Calculator

Calculate how much 0.1 M citric acid and 0.2 M disodium phosphate to mix for a citrate-phosphate (McIlvaine) buffer of any volume at pH 2.6 to 7.6.

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Calculator Functions

The calculator gives the volumes of 0.1 M citric acid and 0.2 M disodium phosphate (Na2HPO4) to mix for a citrate-phosphate buffer, also called McIlvaine buffer, at a chosen pH and final volume [1].

Function	Use it when	You enter
Buffer recipe for a target pH	You need a citrate-phosphate buffer between pH 2.6 and 7.6	Target pH and final buffer volume
Grams of each chemical	You want to know how much solid the buffer or the stock solutions contain	Which phosphate salt you have: anhydrous or dihydrate

The result also shows how many grams of citric acid monohydrate and phosphate salt make one liter of each stock solution. For a buffer made only from phosphate salts, use the Phosphate Buffer Calculator.

Citrate Phosphate Buffer Formulas

The two stock solutions always add up to the final volume, so the buffer is made by mixing them with no extra water. For 500 mL at pH 4.0 the table gives a = 61.45, so A = 500 × 61.45 ÷ 100 = 307.25 mL of citric acid and B = 192.75 mL of Na2HPO4.

Citrate-phosphate buffer table

pH	0.1 M citric acid (mL)	0.2 M Na2HPO4 (mL)
2.6	89.10	10.90
2.8	84.15	15.85
3.0	79.45	20.55
3.2	75.30	24.70
3.4	71.50	28.50
3.6	67.80	32.20
3.8	64.50	35.50
4.0	61.45	38.55
4.2	58.60	41.40
4.4	55.90	44.10
4.6	53.25	46.75
4.8	50.70	49.30
5.0	48.50	51.50
5.2	46.40	53.60
5.4	44.25	55.75
5.6	42.00	58.00
5.8	39.55	60.45
6.0	36.85	63.15
6.2	33.90	66.10
6.4	30.75	69.25
6.6	27.25	72.75
6.8	22.75	77.25
7.0	17.65	82.35
7.2	13.05	86.95
7.4	9.15	90.85
7.6	6.35	93.65

Each row makes 100 mL of buffer [1].

A pH between two rows

Here a₁ and a₂ are the citric acid volumes in the rows just below and just above the target pH, and pH₁ is the lower row's pH. For pH 5.3, a = 46.40 + (5.3 − 5.2) ÷ 0.2 × (44.25 − 46.40) = 45.33 mL per 100 mL. This straight-line step is an estimate between two measured points.

Stock solutions and grams

S is the strength of the stock in grams per liter: 21.01 g/L of citric acid monohydrate for the 0.1 M citric acid, and 28.40 g/L of anhydrous Na2HPO4 or 35.61 g/L of the dihydrate for the 0.2 M phosphate [1]. The 307.25 mL of citric acid above holds 307.25 ÷ 1,000 × 21.01 = 6.455 g of citric acid monohydrate.

Limits of This Estimate

• The table runs from pH 2.6 to 7.6 [1]. The calculator does not go outside it.
• A pH between two rows is interpolated in a straight line. Check the mixed buffer with a pH meter and adjust it with a little of either stock.
• The volumes assume stocks of exactly 0.1 M citric acid and 0.2 M Na2HPO4. Other hydrates of either chemical have different molar masses, so the grams per liter given here do not apply to them.
• The concentration of the finished buffer changes with pH, because the two stocks have different strengths and are mixed in different proportions.

Sources