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pH Scale: What It Is and How to Read Acids and Bases | |||||||||||||||||||||||||||||||||||||||
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pH Scale: What It Is and How to Read Acids and BasesWhat to print Page numbers appear when printing with default margins. SlidesChoose a cut Flash10 slidesThe essential thread, to present in classFull17 slidesEvery chapter and the deeper detailBoth come with speaker notes. In 30 seconds quick readThe pH tells you how acidic or basic a water solution is. It equals minus the base-10 logarithm of the hydrogen ion concentration, and on the usual scale 7 is the neutral value. Each step down of one pH unit means ten times more hydrogen ions. A pH meter measures it, and litmus paper gives a rough reading. Key Points
Deep DiveIn 1909 the Danish biochemist Søren Sørensen saw that the ion concentrations in a solution could be expressed on a base-10 logarithmic scale. The pH that came out of it is a number with no unit of measure. What is pH? The definitionThe pH expresses the concentration of hydrogen ions in a water solution. According to the EPA it is the negative logarithm of the hydrogen ion concentration, measured in mol/L; the logarithm is base 10. It makes sense only for solutions in water: the USGS notes that vegetable oil and alcohol have no pH. According to the EPA, what is actually measured is the activity of hydrogen ions, not the concentration: the formula with concentration is the simplified version. The pH formulaThe formula fits on one line: pH = -log10 [H+] Square brackets stand for concentration in mol/L. You can also run it backward: from the pH, the concentration is [H+] = 10^(-pH). In water the H+ ion does not stay free. It attaches to a water molecule and forms the hydronium ion, H3O+, which is why many textbooks write [H3O+] in place of [H+]. The pH scale range and how to read itThe pH describes acidity or basicity on a scale that usually runs from 0 to 14. The value 7 is neutral, and at 25 °C it is the pH of pure water, because at that temperature the product [H+][OH-] of water equals 1.00 × 10^-14. The more H+ ions there are, the further the pH drops below 7 and the more acidic the solution is. Above 7 it is basic. Saying the scale runs from 1 to 14 is inaccurate, since it starts at 0. And 0 to 14 is the usual range only: a pH can fall outside it. Why each number counts ten times moreThe scale uses a logarithm, so numbers that sit close together describe quantities that are far apart. One pH unit up or down changes the H+ concentration by a factor of 10. A solution at pH 1 has ten times the H+ ions of one at pH 2, and the pH 2 solution has ten times more than one at pH 3. According to the USGS, water at pH 5 is ten times more acidic than water at pH 6.
Acid pH level and pH chart: typical valuesThe chart below gives the typical values printed in David Ball’s chemistry textbook. The book warns that real values vary with conditions.
In the table, most of the foods are acidic. Gastric juice, at pH 1.7, is the most acidic of the group; digestion in the stomach has a Recap of its own. According to the USGS, normal rain has a pH of about 5.6, slightly acidic because of atmospheric carbon dioxide. For the rest of the water’s journey there is the Recap on the water cycle. Acid base examples and definitionsThe Swedish chemist Svante Arrhenius proposed the two classes of compounds in the 1880s. In his view an acid, dissolved in water, raises the concentration of H+ ions, and a base produces OH- ions. The theory has a limit: ammonia (NH3) releases hydroxide ions in water although it contains none, and Arrhenius cannot explain that. In 1923 the Dane Johannes Brønsted and the Englishman Thomas Lowry, working independently, gave a wider definition of acids and bases. An acid is a compound that donates a proton, H+, in a reaction, and a base is a compound that accepts it. Every Arrhenius acid and base also qualifies under Brønsted-Lowry, though the reverse does not hold. Strong and weak acids and basesStrong acids ionize almost completely in water. Common ones include HCl, HNO3, H2SO4, HBr, HI and HClO4. All the others are weak acids and ionize only partly. Acetic acid is a weak acid and ammonia a weak base: in water they dissociate only a little, typically a few percent according to LibreTexts, depending on concentration. Strong bases include the hydroxides of the alkali metals, such as NaOH and KOH. How pH is measuredA pH meter uses two electrodes. One sits in a liquid of fixed pH and the other responds to the acidity of the sample, and the voltage difference between them is converted into a pH. Before measuring, the instrument is calibrated with a solution of known pH. At school or at home, litmus paper gives a rough estimate. A worked calculationTake a solution with [H3O+] = 1.00 × 10^-7 mol/L. The base-10 logarithm of 10^-7 is -7, and changing the sign gives pH 7.00, the value of a neutral solution. With 1.00 × 10^-8 mol/L the pH is 8.00, so that solution is basic. The first holds ten times more hydronium ions than the second.
The Heartland course writes pH with two decimal places. Slide deckSlides ready to download and make your own in PowerPoint or Google Slides, with speaker notes. Pick the Flash cut or the Full one.     ![Slide 5 of the presentation on pH Scale: pH = -log10 [H+], and it applies only to water solutions: oil has no pH](./ph-scale-slide-05-ph-log10-h-and-it-applies-only-to-water-solutions-oil-has-no.webp)           ![Slide 16 of the presentation on pH Scale: A solution has [H3O+] = 1.00 × 10^-8 mol/L. What is its pH?](./ph-scale-slide-16-a-solution-has-h3o-1-00-10-8-mol-l-what-is-its-ph.webp)  Common myths
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Frequently asked questionsHow do you calculate pH?Take the hydrogen ion concentration in mol/L, find its base-10 logarithm and change the sign. A concentration of 1.00 × 10^-7 mol/L gives pH 7.00. To go the other way, the concentration is 10 raised to minus the pH. What is the pH scale range?The scale is usually drawn from 0 to 14, with 7 in the middle as the neutral value. Ball's textbook says that range holds usually, and that a pH can fall outside it. What are some acid and base examples, and what is the pH level of an acid?Any acidic solution has a pH below 7. In Ball's table of typical values, gastric juice (1.7), lemon juice (2.2), vinegar (2.9), wine (3.5) and black coffee (5.0) are acidic, while milk of magnesia (10.5) and an ammonia solution (12.5) are basic. Pure water at 25 °C is neutral at 7.0. Real values change with conditions. What is the difference between pH and alkalinity?The pH measures the concentration of hydrogen ions. Alkalinity, for aquatic environments, is about how much acid a solution can neutralize. What pH does rain have?Normal rain has a pH of about 5.6, according to the USGS. It is slightly acidic because of the carbon dioxide in the atmosphere. Every Recap goes through an independent review before publication. |