tags: [] - coffee/brewing - coffee/brewing/water aliases: - pH scale explained - Logarithmic pH scale - Hydrogen ion scale

pH Scale¶

Tags: #coffee/brewing #coffee/brewing/water Aliases: pH scale explained, Logarithmic pH scale, Hydrogen ion scale Related: Water in Coffee MOC | pH | pH in Coffee Water | Alkalinity Status: ✅ Complete

Overview¶

The pH scale is a logarithmic measure of hydrogen ion (H⁺) activity in aqueous solution, ranging from 0 (most acidic) to 14 (most alkaline), with 7.0 representing chemical neutrality at 25°C. The scale was devised by Danish chemist Søren Peder Lauritz Sørensen in 1909 and is fundamental to water chemistry, food science, and coffee water assessment. Because the scale is logarithmic, each unit represents a tenfold change in hydrogen ion concentration — a difference of two pH units corresponds to a 100-fold difference in acidity.

The Mathematics of pH¶

pH = −log₁₀[H⁺]

Where [H⁺] is the molar concentration of hydrogen ions (equivalently, hydronium ions H₃O⁺) in mol/L:

[H⁺] (mol/L)	pH	Character
1.0	0	Strongly acidic
0.001 (10⁻³)	3	Acidic
0.0000001 (10⁻⁷)	7	Neutral
10⁻¹⁰	10	Alkaline
10⁻¹⁴	14	Strongly alkaline

At neutrality (pH 7.0 at 25°C), [H⁺] = [OH⁻] = 10⁻⁷ mol/L. Above pH 7, hydroxide ions dominate; below pH 7, hydrogen ions dominate.

Logarithmic Nature¶

The logarithmic relationship means changes in pH represent multiplicative, not additive, changes in acidity: - pH 6 vs pH 7: 10× more acidic - pH 5 vs pH 7: 100× more acidic - pH 4 vs pH 7: 1000× more acidic (typical brewed coffee acidity vs neutral water)

This is significant in coffee water chemistry: small changes in water pH do not linearly correspond to small changes in the buffering environment or flavour outcome — the underlying ion concentrations change exponentially.

Reference Points¶

Substance	pH (approx.)
Hydrochloric acid (concentrated)	0–1
Lemon juice	2.0–2.5
Vinegar	2.5–3.0
Brewed coffee	4.5–5.5
Black tea	4.5–5.5
Pure water (25°C)	7.0
Most municipal tap water	7.0–8.0
Baking soda solution	8.3
Milk of magnesia	10.5
Ammonia solution	11.5
Bleach	12.5
Sodium hydroxide	~13–14

Temperature and pH¶

pH is temperature-dependent. Pure water at 25°C has pH 7.0; at 4°C it is closer to 7.5 and at 80°C closer to 6.5 — without becoming acidic in a chemical sense, as the neutral point itself shifts with temperature. When measuring brewing water pH, temperature compensation is essential for accurate readings. Calibrated pH meters include automatic temperature compensation (ATC) for this reason.

pH and Brewed Coffee¶

Brewed coffee has pH approximately 4.5–5.5, depending on: - Roast level: Lighter roasts preserve more organic acids (citric, malic, phosphoric), yielding lower pH; darker roasts degrade acids, yielding slightly higher pH (less acidic) - Water alkalinity: High-bicarbonate water neutralises extracted acids during brewing, raising finished coffee pH toward 5.5–6.0 even with light roast coffees - Extraction yield: Under-extracted coffee (low yield) may be more acidic; over-extracted coffee can become bitter rather than more acidic

Key Facts¶

pH scale runs from 0 (most acidic) to 14 (most alkaline); 7.0 is neutral at 25°C
Logarithmic: each pH unit = 10× change in hydrogen ion concentration
Brewed coffee pH is 4.5–5.5; brewing water target is pH 7.0 (SCA)
pH is temperature-dependent — calibrated meters with ATC are required for accurate measurement
Water pH is an indicator but not a determinant of flavour impact; alkalinity (bicarbonate buffering capacity) is the critical parameter

References¶

Changelog¶

Date	Change
2026-04-28	Note created

This article is part of All-About-Coffee.com - The comprehensive coffee knowledgebase.