PH

pH is a measure of the activity of hydrogen ions (H+) in a solution and, therefore, its acidity or alkalinity. In aqueous systems, the hydrogen ion activity is dictated by the dissociation constant of water (Kw) = 1.011 × 10−14 at 25 °C) and interactions with other ions in solution. Due to this dissociation constant a neutral solution (hydrogen ion activity equals hydroxide ion activity) has a pH of approximately 7. Aqueous solutions with pH values lower than 7 are considered acidic, while pH values higher than 7 are considered alkaline.

Definition

Though a pH value has no unit, it is not an arbitrary scale; the number arises from a definition based on the activity of hydrogen ions in the solution.

Related Topics:
Activity - Hydrogen

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The formula for calculating pH is:

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:mbox{pH} = -log_{10} left

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denotes the activity of H+ ions (or more accurately written, , the equivalent hydronium ions), measured in moles per litre (also known as molarity). In dilute solutions (like river or tap water) the activity is approximately equal to the concentration of the H+ ion.

Related Topics:
Hydronium - Moles - Litre - Molarity

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Log10 denotes the base-10 logarithm, and pH therefore defines a logarithmic scale of acidity. For example, a solution with pH=8.2 will have an activity (concentration) of 10−8.2 mol/L, or about 6.31 × 10−9 mol/L; a solution with an activity of 4.5 × 10−4 mol/L will have a pH value of −log10(4.5 × 10−4), or about 3.35.

Related Topics:
Logarithm - Logarithmic scale

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In aqueous solution at standard temperature and pressure (STP), a pH of 7 indicates neutrality (i.e. pure water) because water naturally dissociates into H+ and OH− ions with equal concentrations of 1×10−7 mol/L. A lower pH value (for example pH 3) indicates increasing strength of acidity, and a higher pH value (for example pH 11) indicates increasing strength of alkalinity.

Related Topics:
Standard temperature and pressure - Water

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Neutral pH is not exactly 7; this would imply that the H+ ion concentration is exactly 1×10−7 mol/L, which is not the case. The value is close enough, however, for neutral pH to be 7.00 to three significant figures, which is near enough for most people to assume it is exactly 7. In nonaqueous solutions or non-STP conditions, the pH of neutrality may not even be close to 7. Instead it is related to the dissociation constant for the specific solvent used. (Note also that pure water, when exposed to the atmosphere, will take in carbon dioxide, some of which reacts with water to form carbonic acid and H+, thereby lowering the pH to about 5.7.)

Related Topics:
Significant figure - Dissociation constant - Solvent - Carbon dioxide - Carbonic acid

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Most substances have a pH in the range 0 to 14, although extremely acidic or basic substances may have pH < 0, or pH > 14.

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Some common pH values

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