Pressure at Titration Curve: How It Works

Updated 10 November 2026 · Standard formula, shown below

This page explains what “pressure” means in the context of a titration curve and how to read it. In most chemistry contexts, pressure is not a direct variable on a standard acid-base titration curve; instead, the curve plots pH (or another property) against titrant volume. However, pressure becomes relevant in specific titration types, such as gas-evolving reactions or manometric titrations, where the pressure change is measured to determine the endpoint. This article clarifies the role of pressure, how it appears in different titration setups, and how to interpret those curves.

Background: What a Titration Curve Normally Shows

A classic titration curve graphs the pH of a solution as a function of the volume of titrant added. The curve has a characteristic sigmoidal (S) shape for a strong acid-strong base titration, with a steep vertical section at the equivalence point. The midpoint of that steep section is the equivalence point, where the moles of acid equal the moles of base. For weak acids or bases, the curve is less steep and shows a buffer region. The pH at the half-equivalence point equals the pKa of the weak acid (or pKb of the weak base).

Pressure does not appear on this standard curve because the reaction occurs in a liquid solution at constant atmospheric pressure. The pH is measured with a glass electrode, not a pressure sensor. So when someone asks about “pressure at titration curve,” they are usually either (a) thinking of a different type of titration, or (b) confusing titration with another analytical method.

When Pressure Is Actually Measured: Manometric Titration

In a manometric titration, the pressure change in a closed vessel is monitored as the titrant is added. This technique is used when the reaction produces or consumes a gas. For example, measuring the amount of carbonate in a sample by adding acid and tracking the carbon dioxide gas produced. As acid is added, CO2 is released, increasing the pressure inside the sealed flask. The pressure versus titrant volume curve shows a break at the equivalence point, similar to a pH curve. The endpoint is found where the slope changes abruptly.

Another example is the determination of oxygen in a sample using a gas-producing reaction. The pressure increase is proportional to the amount of gas generated, and the titration curve is a plot of pressure (in kPa or mmHg) against titrant volume. This method is common in biochemical oxygen demand (BOD) testing, where pressure changes in a sealed bottle reflect oxygen consumption by microorganisms.

How to Interpret a Pressure-Based Titration Curve

In a manometric titration, the curve typically has two linear regions with different slopes. The first region corresponds to the reaction before the equivalence point, where each drop of titrant produces a certain amount of gas. The slope is proportional to the stoichiometric yield of gas per mole of titrant. After the equivalence point, the titrant is in excess and no more gas is produced, so the slope flattens (if no other gas is involved) or changes to a different value (if a secondary reaction occurs). The intersection of the two linear extrapolations is the endpoint.

To read the curve accurately, you must know the ideal gas law: PV = nRT. If the volume and temperature are constant, pressure is directly proportional to the number of moles of gas. So a pressure change of, say, 10 kPa corresponds to a specific number of moles of gas generated, which you can convert to the concentration of the analyte using the reaction stoichiometry.

Pressure in Gas-Evolving Reactions: The Setup

For a gas-evolving titration, the apparatus includes a sealed reaction vessel, a pressure sensor (or a manometer), and a burette that can deliver titrant without opening the system. Often, the titrant is injected through a septum, and the solution is stirred to ensure equilibrium. The pressure sensor records the total pressure, which includes the partial pressure of the gas of interest plus the vapor pressure of the solvent (usually water). To get the partial pressure of the gas, you subtract the vapor pressure of water at the experimental temperature. For example, at 25°C, water vapor pressure is about 3.17 kPa (23.8 mmHg). This correction is essential for accurate results.

Temperature must be held constant because pressure is temperature-dependent. A water bath or a thermostatted jacket is typically used. Also, the gas must not dissolve significantly in the liquid; if it does, Henry’s law applies and the relationship between pressure and moles becomes more complex.

Common Mistakes and Misconceptions

Confusing pressure with pH: On a standard acid-base titration curve, pressure is not plotted. If you see a curve labeled “pressure,” it is a manometric titration, not a pH titration.
Ignoring water vapor: The measured pressure includes water vapor. Always subtract the vapor pressure at the given temperature to get the gas partial pressure.
Assuming constant temperature: If the temperature drifts, the pressure changes even without any reaction. Use a thermostat.
Forgetting the volume change: Adding titrant increases the liquid volume, which can compress the gas headspace. In a rigid vessel, the gas volume decreases slightly, increasing pressure. This effect is usually small but can be corrected if needed.
Misreading the endpoint: The endpoint is the intersection of the two linear segments, not the point of maximum curvature. Use linear extrapolation for accuracy.

Comparison: Manometric vs. pH Titration

pH titration is simpler, cheaper, and more common. It works for any acid-base reaction in aqueous solution. Manometric titration is specialized for reactions that produce or consume a gas. It is used when a pH electrode is not suitable, such as in non-aqueous solvents, or when the analyte itself is a gas (e.g., dissolved CO2). Manometric methods are also used in respirometry (measuring oxygen consumption) and in the determination of carbonate/bicarbonate mixtures. The main advantages of manometric titration are that it does not require a pH electrode (which can be fouled by certain samples) and it directly measures a physical property (pressure) that is proportional to the amount of gas.

The main disadvantage is the need for a sealed apparatus and careful temperature control. Also, the sensitivity is limited by the accuracy of the pressure sensor. For high-precision work, pH titration is usually preferred when applicable.

Limits of the Pressure Method

The ideal gas law assumes ideal behavior, which is valid at low pressures and high temperatures. At high pressures or low temperatures, real gases deviate. For most titration applications, pressures are near atmospheric, so the ideal gas law is adequate. Another limit is that the gas must not react with the solvent or other components. For example, ammonia gas dissolves readily in water, so a manometric titration for ammonia would be inaccurate unless the solution is basic enough to keep ammonia in the gas phase. Also, the reaction must be fast and go to completion; otherwise, the pressure may not stabilize quickly, making endpoint detection difficult.

Frequently Asked Questions

What does pressure mean on a titration curve?

On a standard acid-base titration curve, pressure is not plotted. The curve shows pH vs. titrant volume. However, in a manometric titration, pressure is plotted on the y-axis. That pressure is the total pressure in a sealed vessel, which changes as gas is produced or consumed during the reaction. The curve is used to find the equivalence point by locating the break in the slope.

How do you convert pressure to concentration in a titration?

Use the ideal gas law: n = PV/RT, where n is moles of gas, P is the partial pressure of the gas (total pressure minus water vapor pressure), V is the headspace volume, R is the gas constant (8.314 J/(mol·K) or 0.0821 L·atm/(mol·K)), and T is the absolute temperature in Kelvin. Then use the stoichiometry of the reaction to relate moles of gas to moles of analyte. For example, if 1 mole of CO2 is produced per mole of carbonate, then moles of carbonate equal moles of CO2.

Why is my pressure titration curve not linear?

Non-linearity can be due to several factors: temperature drift, gas dissolving in the liquid (Henry’s law), a slow reaction, or a secondary reaction producing another gas. Also, if the titrant volume is large, the headspace volume decreases, causing a non-linear pressure increase. Check temperature control, stir rate, and the reaction kinetics. If the gas is soluble, you may need to correct for dissolved gas or use a different method.

Can I use a pressure sensor for a regular acid-base titration?

Yes, but it is not standard. A pressure sensor would only detect pressure changes if the reaction produces or consumes a gas. For a typical acid-base reaction like HCl + NaOH, no gas is involved, so the pressure would remain constant. You would not get a useful curve. Use a pH electrode for acid-base titrations.

What is the difference between endpoint and equivalence point?

The equivalence point is the theoretical point where the moles of titrant added are stoichiometrically equal to the moles of analyte. The endpoint is the point where the indicator changes color or where the curve shows a measurable change (e.g., the intersection of two linear segments in a manometric titration). In an ideal titration, they are the same, but in practice, the endpoint may be slightly different due to indicator error or detection method.

How do I account for water vapor in a pressure titration?

Subtract the vapor pressure of water at the experimental temperature from the total pressure. For example, at 20°C, the vapor pressure of water is about 2.34 kPa (17.5 mmHg). If your total pressure is 101.3 kPa, the partial pressure of the gas is 101.3 – 2.34 = 98.96 kPa. Use this corrected pressure in the ideal gas law.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.