What are the pKa values of citric acid?
Citric acid has three pKa values, one for each of its three carboxylic acid groups: pKa1 3.128, pKa2 4.761 and pKa3 6.396 at 25 °C and zero ionic strength. Rounded, they are the 3.13, 4.76 and 6.40 found in most formulation literature. The source for these numbers is the NIST review of buffer ionization data by Goldberg, Kishore and Lennen, Journal of Physical and Chemical Reference Data 31 (2002), Table 7.22, which gives them as the selected values for the three ionization steps of citric acid.
| Step | Reaction | pKa at 25 °C | Ionization enthalpy |
|---|---|---|---|
| 1 | citric acid to dihydrogen citrate | 3.128 | 4.07 kJ/mol |
| 2 | dihydrogen citrate to hydrogen citrate | 4.761 | 2.23 kJ/mol |
| 3 | hydrogen citrate to citrate | 6.396 | −3.38 kJ/mol |
A pKa is the pH at which an acid group is half dissociated. At pH 3.13, half of the citric acid in solution still carries all three protons and half has given up the first one.
Which form of citric acid is present at beverage pH
Because the three pKa values sit 1.6 units apart, the acid moves through four forms as pH rises. The table gives the share of each form, calculated from the NIST values.
| pH | Citric acid (H3Cit) | Dihydrogen citrate (H2Cit−) | Hydrogen citrate (HCit2−) | Citrate (Cit3−) |
|---|---|---|---|---|
| 2.5 | 81% | 19% | 0% | 0% |
| 3.0 | 57% | 42% | 1% | 0% |
| 3.2 | 45% | 53% | 1% | 0% |
| 3.5 | 29% | 68% | 4% | 0% |
| 3.8 | 16% | 76% | 8% | 0% |
| 4.0 | 10% | 76% | 13% | 0% |
| 4.5 | 3% | 63% | 34% | 0% |
| 5.0 | 0% | 36% | 62% | 2% |
| 6.0 | 0% | 4% | 69% | 28% |
Most acidic beverages run between pH 3.0 and 4.0. Across that band, undissociated citric acid and dihydrogen citrate make up 86% or more of the total, and the second and third pKa values barely come into play. Hydrogen citrate passes a third of the total at pH 4.5 and becomes the main form by pH 5.0.
Setting a citric to citrate ratio
Inside the beverage band the Henderson-Hasselbalch equation on the first pKa gives the ratio directly: pH = 3.13 + log([dihydrogen citrate] / [citric acid]). At pH 3.5 the ratio is 100.37, about 2.3 to 1 on a molar basis, which matches the 68% to 29% split in the table. A citric acid and sodium citrate pair is how a formulator holds that ratio from batch to batch, because the buffer resists the pH drift that water hardness, juice content or other acids would otherwise cause. Our citric acid product page notes that water above 200 ppm hardness as CaCO3 takes 10 to 25% more citric acid to reach the same pH, because calcium and magnesium neutralize part of the acid first.
Temperature and ionic strength
Temperature moves the citric acid pKa values very little. The ionization enthalpies in the NIST table are small, so a van 't Hoff calculation puts the shift between 25 °C and 5 °C at about 0.05 units for pKa1, 0.03 for pKa2 and 0.04 for pKa3. The acid dissociates to almost the same degree at a warm line reading and in a cold bottle.
Ionic strength moves them more. The NIST values are at zero ionic strength, and a finished beverage carries sugars, salts, minerals and other acids. Treat the calculated ratio as the starting point and set the final citric and citrate levels on a pH reading of the finished product.
Citric acid from PAT Ingredients
PAT supplies citric acid anhydrous, fine granular 30 to 80 mesh, USP/FCC food grade and non-Chinese origin, to North American beverage, supplement and food manufacturers. It is CAS 77-92-9, INS 330 and E330, and GRAS under 21 CFR 184.1033. For bulk and contract supply, see citric acid bulk supply.
Frequently asked
What are the pKa values of citric acid?
Citric acid has three pKa values, one for each carboxylic acid group: 3.128, 4.761 and 6.396 at 25 °C and zero ionic strength, usually rounded to 3.13, 4.76 and 6.40. These are the selected values in the NIST review by Goldberg, Kishore and Lennen, Journal of Physical and Chemical Reference Data, volume 31, 2002, Table 7.22.
Is citric acid a strong or a weak acid?
Citric acid is a weak triprotic acid. Its first pKa is 3.13, so at pH 3.13 half of the acid has given up its first proton and half has not. At pH 2.5, about 81% is still the undissociated acid.
Which form of citric acid is present at pH 3.5?
At pH 3.5, about 68% is dihydrogen citrate, 29% is undissociated citric acid and 4% is hydrogen citrate, calculated from the NIST pKa values. In the pH 3.0 to 4.0 range of most acidic beverages, the first dissociation does almost all of the work.
Does temperature change the pKa of citric acid?
Very little. The NIST ionization enthalpies are small, 4.07, 2.23 and minus 3.38 kJ per mole, so cooling from 25 °C to 5 °C moves each pKa by about 0.05 units or less. Ionic strength matters more: the NIST values are at zero ionic strength, and dissolved salts and minerals in a real beverage shift them, so the finished product is always measured.
What is the CAS number and INS number of citric acid?
Citric acid is CAS 77-92-9, INS 330 and E330. In the US it is GRAS under 21 CFR 184.1033.
Citric acid anhydrous, USP/FCC
Fine granular 30 to 80 mesh, non-Chinese origin. Tell us the application and we will send the specification and a sample.
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