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pH Scale

pH Scale

Hydronium and hydroxide ions are present both in pure water and in all aqueous solutions, and their concentrations are inversely proportional as determined by the ion product of water (Kw). The concentrations of these ions in a solution are often critical determinants of the solution’s properties and the chemical behaviors of its other solutes. Two different solutions can differ in their hydronium or hydroxide ion concentrations by a million, billion, or even trillion times. A common means of...

pH

The potential of hydrogen (pH) is a measure of the acidity or basicity of a water-based solution determined by the concentration of hydronium ions (H3O+). In one liter of pure water at neutral pH, there are 1×10−7 moles of hydronium ions. However, the extensive range of hydronium ion concentrations present in water-based solutions makes measuring pH in moles cumbersome. Therefore, a pH scale was developed to convert moles of hydronium ions into the negative logarithm of the hydronium ion...

pH

The potential of hydrogen (pH) is a measure of the acidity or basicity of a water-based solution determined by the concentration of hydronium ions (H3O+). In one liter of pure water at neutral pH, there are 1×10−7 moles of hydronium ions. However, the extensive range of hydronium ion concentrations present in water-based solutions makes measuring pH in moles cumbersome. Therefore, a pH scale was developed to convert moles of hydronium ions into the negative logarithm of the hydronium ion...

Determining the pH of Salt Solutions

Determining the pH of Salt Solutions

The pH of a salt solution is determined by its component anions and cations. Salts that contain pH-neutral anions and the hydronium ion-producing cations form a solution with a pH less than 7. For example, in ammonium nitrate (NH4NO3) solution, NO3− ions do not react with water whereas NH4+ ions produce the hydronium ions resulting in the acidic solution. In contrast, salts that contain pH-neutral cations and the hydroxide ion-producing anions form a solution with a pH greater than 7. For...

Acid–Base Equilibria: Activity-Based Definition of pH

Acid–Base Equilibria: Activity-Based Definition of pH

For an ideal solution, the pH is defined as the negative logarithm of the hydrogen ion concentration. For a non-ideal solution, an accurate measurement of the pH must consider the negative logarithm of the hydrogen ion activity rather than concentration. In such a solution, the pH can be more accurately defined as the negative logarithm of a product of the hydrogen ion concentration and its activity coefficient.
In solutions of very low ionic strength—for example, pure water—the activity...

Composition of Polyprotic Acid Solutions as a Function of pH

Composition of Polyprotic Acid Solutions as a Function of pH

Polyprotic acids of the type H2M constitute two ionizable protons. As a result, on titration with a base, they exhibit two equivalence points in the titration curve. During titration, the species H2M, HM−, and M2− will be present in the solution at different points. The fractions of H2M, HM−, and M2− present at the various instances of the titration are denoted by α0, α1, and α2, respectively.
A graph with the alpha values is plotted against the volume of base added during titration. Here, a...

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Related Experiment Video

Updated: Jun 20, 2026

Determination of the Gas-phase Acidities of Oligopeptides

Determination of the Gas-phase Acidities of Oligopeptides

Published on: June 24, 2013

The Pacific Ocean's acidification laboratory.

Christopher Pala

Environmental Science & Technology

|September 22, 2009

Summary

Ocean acidification can be studied in real-time at Maug Island

Area of Science:

Marine Biology
Oceanography
Geochemistry

Background:

Ocean acidification is a growing global concern.
Natural analogues provide valuable insights into future conditions.

Purpose of the Study:

To investigate the effects of in situ ocean acidification.
To understand the ecological and chemical changes associated with elevated CO2.

Main Methods:

Utilizing the unique hydrothermal vent system at Maug Island.
Conducting in situ measurements of seawater chemistry and biological responses.

Main Results:

Documenting the chemical changes in the vent fluid.
Observing the impact on local marine life.

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Last Updated: Jun 20, 2026

Determination of the Gas-phase Acidities of Oligopeptides

Determination of the Gas-phase Acidities of Oligopeptides

Published on: June 24, 2013

Implantation of a New Micro Acoustic Tag in Juvenile Pacific Lamprey and American Eel

Implantation of a New Micro Acoustic Tag in Juvenile Pacific Lamprey and American Eel

Published on: March 16, 2019

Establishment of an Extracellular Acidic pH Culture System

Establishment of an Extracellular Acidic pH Culture System

Published on: November 19, 2017

Conclusions:

Maug Island's vents serve as a natural laboratory for ocean acidification.
Findings offer a preview of potential future ocean conditions.