The term is used in chemistry, biology, and agronomy. PH translates the values of the concentration of the hydrogen ion into numbers between 0 and 14. <ul><li>Pure water corresponds to a pH of 7, which is neutral (neither acidic nor alkaline). </li><li>A pH less than 7 is considered acidic.</li><li>A pH greater than 7 is considered basic or alkaline.</li></ul>

PH measures the acidity or basicity of liquid solutions

Danish biochemist S.P.L Sorensen used the measurement to represent the hydrogen ion concentration, expressed in equivalents per litre of an aqueous solution: pH = −log[H+] The pH can be read with a less accurate litmus paper or with indicator dyes. However, it is usually measured with a pH meter that consists of a voltmeter attached to a pH-responsive electrode (usually mercury-mercurous chloride electrode) and a reference electrode (usually glass that are immersed in a solution. The voltmeter measures the potential difference between the glass and reference electrodes

Ways to take a pH measurement

In agriculture, the pH of the soil shows what crops will grow easily in the soil and what adjustments are needed to adapt it for growing other crops. Acidic soils are often considered infertile as the solubility of aluminium and manganese also increase with acidity. However, conifers and members of the family Ericaceae, such as blueberries, prefer acidic soils. Acidic soil can be neutralized by treating it with lime.

PH is most important in agriculture

<div dir="ltr">
				Alternative Titles:
				hydrogen ion concentration, potential of hydrogen</div>PH, quantitative measure of the acidity or basicity of aqueous or other <a href="https://www.britannica.com/science/liquid-state-of-matter" dir="ltr">liquid</a> solutions. The term, widely used in <a href="https://www.britannica.com/science/chemistry" dir="ltr">chemistry</a>, <a href="https://www.britannica.com/science/biology" dir="ltr">biology</a>, and <a href="https://www.britannica.com/science/agronomy" dir="ltr">agronomy</a>, translates the values of the concentration of the <a href="https://www.britannica.com/science/hydrogen-ion" dir="ltr">hydrogen ion</a>—which ordinarily ranges between about 1 and 10−14 gram-equivalents per litre—into numbers between 0 and 14. In pure <a href="https://www.britannica.com/science/water" dir="ltr">water</a>, which is neutral (neither acidic nor alkaline), the concentration of the hydrogen ion is 10−7 gram-equivalents per litre, which corresponds to a pH of 7. A <a href="https://www.britannica.com/science/solution-chemistry" dir="ltr">solution</a> with a pH less than 7 is considered <a href="https://www.britannica.com/science/acid" dir="ltr">acidic</a>; a solution with a pH greater than 7 is considered <a href="https://www.britannica.com/science/base-chemical-compound" dir="ltr">basic</a>, or alkaline.<figure><img src="https://cdn.britannica.com/s:690x388,c:crop/85/138885-050-B1E3988A/Indicator-paper-liquid-pH-solution.jpg" alt="alkaline testing" width="359" height="202"><figcaption class="card-body" dir="ltr"><a class="md-assembly-title font-weight-bold mb-5 d-inline-block font-16 font-sans-serif media-overlay-link" href="https://cdn.britannica.com/85/138885-050-B1E3988A/Indicator-paper-liquid-pH-solution.jpg" data-href="/media/1/454823/141985" dir="ltr">alkaline testing</a><div class="md-assembly-caption text-muted font-14 font-serif" dir="ltr">Indicator paper is used to determine the pH of a liquid. The paper will turn blue when a solution is alkaline.</div><cite class="credit d-block mt-5" dir="ltr">© Sabine Kappel/Shutterstock.com</cite></figcaption></figure><img src="https://cdn.britannica.com/s:225x225/20/114420-050-78309493/environment-Earth-hydrosphere-atmosphere-biosphere-lithosphere.jpg" alt="Earth's environmental spheres" width="162" height="120">The measurement was originally used by the Danish biochemist <a href="https://www.britannica.com/biography/S-P-L-Sorensen" dir="ltr">S.P.L. Sørensen</a> to represent the hydrogen ion concentration, expressed in equivalents per litre, of an aqueous solution: pH = −log[H+] (in expressions of this kind, enclosure of a <a href="https://www.britannica.com/science/chemical-symbol" dir="ltr">chemical symbol</a> within square brackets denotes that the concentration of the symbolized species is the quantity being considered).<figure><img src="https://cdn.britannica.com/s:800x450,c:crop/46/216546-138-F43C96E1/Fundamentals-of-acids-and-bases-pH-scale-to-measure.jpg" alt="pH scale" width="359" height="202"><figcaption class="card-body" dir="ltr"><div class="md-assembly-title font-weight-bold mb-5 d-inline-block font-16 font-sans-serif" dir="ltr">pH scale</div><div class="md-assembly-caption text-muted font-14 font-serif" dir="ltr">The fundamentals of acids and bases and how the pH scale is used to measure them.</div><cite class="credit d-block mt-5" dir="ltr">© American Chemical Society (<a class="external" rel="noopener" href="http://corporate.britannica.com/american-chemical-society/" dir="ltr">A Britannica Publishing Partner</a>)</cite><a class="font-14 mt-10 d-inline-block" href="https://www.britannica.com/science/pH/images-videos" dir="ltr">See all videos for this article</a></figcaption></figure>Because of uncertainty about the physical significance of the hydrogen ion concentration, the definition of the pH is an operational one; i.e., it is based on a method of measurement. The U.S. <a href="https://www.britannica.com/topic/National-Institute-of-Standards-and-Technology" dir="ltr">National Institute of Standards and Technology</a> has defined pH values in terms of the <a href="https://www.britannica.com/science/electromotive-force" dir="ltr">electromotive force</a> existing between certain standard electrodes in specified solutions.The pH is usually measured with a <a href="https://www.britannica.com/technology/pH-meter" dir="ltr">pH meter</a>, which translates into pH readings the difference in electromotive force (electrical potential or voltage) between suitable <a href="https://www.britannica.com/science/electrode" dir="ltr">electrodes</a> placed in the solution to be tested. Fundamentally, a pH meter consists of a <a href="https://www.britannica.com/science/voltmeter" dir="ltr">voltmeter</a> attached to a pH-responsive electrode and a reference (unvarying) electrode. The pH-responsive electrode is usually glass, and the reference is usually a mercury-mercurous chloride (<a href="https://www.britannica.com/science/calomel" dir="ltr">calomel</a>) electrode, although a silver-silver chloride electrode is sometimes used. When the two electrodes are immersed in a solution, they act as a <a href="https://www.britannica.com/technology/battery-electronics" dir="ltr">battery</a>. The glass electrode develops an <a href="https://www.britannica.com/science/electric-potential" dir="ltr">electric potential</a> (charge) that is directly related to the hydrogen-ion activity in the solution, and the voltmeter measures the potential difference between the glass and reference electrodes. The meter may have either a digital or an <a href="https://www.merriam-webster.com/dictionary/analog" data-term="analog" dir="ltr">analog</a> (scale and deflected needle) readout. Digital readouts have the advantage of exactness, while analog readouts give better indications of rates of change. Battery-powered portable pH meters are widely used for <a href="https://www.britannica.com/science/field-physics" dir="ltr">field</a> tests of the pH of soils. Tests of pH may also be performed, less accurately, with <a href="https://www.britannica.com/science/litmus" dir="ltr">litmus</a> paper or by mixing indicator dyes in liquid suspensions and matching the resulting colours against a colour chart <a href="https://www.merriam-webster.com/dictionary/calibrated" data-term="calibrated" dir="ltr">calibrated</a> in pH.<figure><img src="https://cdn.britannica.com/s:690x388,c:crop/95/172595-050-810CCA8F/pH-meter-acidity-liquids-basicity.jpg" alt="pH meter" width="359" height="359"><figcaption class="card-body" dir="ltr"><a class="md-assembly-title font-weight-bold mb-5 d-inline-block font-16 font-sans-serif media-overlay-link" href="https://cdn.britannica.com/95/172595-050-810CCA8F/pH-meter-acidity-liquids-basicity.jpg" data-href="/media/1/454823/216891" dir="ltr">pH meter</a><div class="md-assembly-caption text-muted font-14 font-serif" dir="ltr">A pH meter is used to measure the acidity or basicity of liquids.</div><cite class="credit d-block mt-5" dir="ltr">© photongpix/Fotolia</cite></figcaption></figure><div>
Get a Britannica Premium subscription and gain access to exclusive content. 
</div>In agriculture, the pH is probably the most important single property of the moisture associated with a <a href="https://www.britannica.com/science/soil" dir="ltr">soil</a>, since that indication reveals what crops will grow readily in the soil and what adjustments must be made to adapt it for growing any other crops. Acidic soils are often considered infertile, and so they are for most conventional agricultural crops, although <a href="https://www.britannica.com/plant/conifer" dir="ltr">conifers</a> and many members of the family <a href="https://www.britannica.com/plant/heath-family" dir="ltr">Ericaceae</a>, such as <a href="https://www.britannica.com/plant/blueberry-plant" dir="ltr">blueberries</a>, will not thrive in alkaline soil. Acidic soil can be “sweetened,” or neutralized, by treating it with lime. As soil acidity increases so does the <a href="https://www.britannica.com/science/solubility-chemistry" dir="ltr">solubility</a> of <a href="https://www.britannica.com/science/aluminum" dir="ltr">aluminum</a> and <a href="https://www.britannica.com/science/manganese" dir="ltr">manganese</a> in the soil, and many plants (including agricultural crops) will tolerate only slight quantities of those metals. Acid content of soil is heightened by the decomposition of organic material by microbial action, by fertilizer salts that hydrolyze or nitrify, by oxidation of sulfur <a href="https://www.merriam-webster.com/dictionary/compounds" data-term="compounds" dir="ltr">compounds</a> when <a href="https://www.britannica.com/science/salt" dir="ltr">salt</a> marshes are drained for use as farmland, and by other causes.

pH, quantitative measure of the acidity or basicity of aqueous or other liquid solutions. The term, widely used in chemistry, biology, and agronomy, translates the values of the concentration of the hydrogen ion into numbers between 0 and 14. Learn more about pH.

scienceandnature

Science & Nature

PH , quantitative measure of the acidity or basicity of aqueous or other <a dir="ltr" href="https://www.britannica.com/science/liquid-state-of-matter">liquid </a>solutions. The term, widely used in <a dir="ltr" href="https://www.britannica.com/science/chemistry">chemistry </a>, <a dir="ltr" href="https://www.britannica.com/science/biology">biology </a>, and <a dir="ltr" href="https://www.britannica.com/science/agronomy">agronomy </a>, translates the values of the concentration of the <a dir="ltr" href="https://www.britannica.com/science/hydrogen-ion">hydrogen ion </a>—which ordinarily ranges between about 1 and 10−14 gram-equivalents per litre—into numbers between 0 and 14. In pure <a dir="ltr" href="https://www.britannica.com/science/water">water </a>, which is neutral (neither acidic nor alkaline), the concentration of the hydrogen ion is 10−7 gram-equivalents per litre, which corresponds to a pH of 7. A <a dir="ltr" href="https://www.britannica.com/science/solution-chemistry">solution </a>with a pH less than 7 is considered <a dir="ltr" href="https://www.britannica.com/science/acid">acidic </a>; a solution with a pH greater than 7 is considered <a dir="ltr" href="https://www.britannica.com/science/base-chemical-compound">basic </a>, or alkaline.

The measurement was originally used by the Danish biochemist <a dir="ltr" href="https://www.britannica.com/biography/S-P-L-Sorensen">S.P.L. Sørensen </a>to represent the hydrogen ion concentration, expressed in equivalents per litre, of an aqueous solution: pH = −log[H+ ] (in expressions of this kind, enclosure of a <a dir="ltr" href="https://www.britannica.com/science/chemical-symbol">chemical symbol </a>within square brackets denotes that the concentration of the symbolized species is the quantity being considered).

Because of uncertainty about the physical significance of the hydrogen ion concentration, the definition of the pH is an operational one; i.e., it is based on a method of measurement. The U.S. <a dir="ltr" href="https://www.britannica.com/topic/National-Institute-of-Standards-and-Technology">National Institute of Standards and Technology </a>has defined pH values in terms of the <a dir="ltr" href="https://www.britannica.com/science/electromotive-force">electromotive force </a>existing between certain standard electrodes in specified solutions.

The pH is usually measured with a <a dir="ltr" href="https://www.britannica.com/technology/pH-meter">pH meter </a>, which translates into pH readings the difference in electromotive force (electrical potential or voltage) between suitable <a dir="ltr" href="https://www.britannica.com/science/electrode">electrodes </a>placed in the solution to be tested. Fundamentally, a pH meter consists of a <a dir="ltr" href="https://www.britannica.com/science/voltmeter">voltmeter </a>attached to a pH-responsive electrode and a reference (unvarying) electrode. The pH-responsive electrode is usually glass, and the reference is usually a mercury-mercurous chloride (<a dir="ltr" href="https://www.britannica.com/science/calomel">calomel </a>) electrode, although a silver-silver chloride electrode is sometimes used. When the two electrodes are immersed in a solution, they act as a <a dir="ltr" href="https://www.britannica.com/technology/battery-electronics">battery </a>. The glass electrode develops an <a dir="ltr" href="https://www.britannica.com/science/electric-potential">electric potential </a>(charge) that is directly related to the hydrogen-ion activity in the solution, and the voltmeter measures the potential difference between the glass and reference electrodes. The meter may have either a digital or an <a data-term="analog" dir="ltr" href="https://www.merriam-webster.com/dictionary/analog">analog </a>(scale and deflected needle) readout. Digital readouts have the advantage of exactness, while analog readouts give better indications of rates of change. Battery-powered portable pH meters are widely used for <a dir="ltr" href="https://www.britannica.com/science/field-physics">field </a>tests of the pH of soils. Tests of pH may also be performed, less accurately, with <a dir="ltr" href="https://www.britannica.com/science/litmus">litmus </a>paper or by mixing indicator dyes in liquid suspensions and matching the resulting colours against a colour chart <a data-term="calibrated" dir="ltr" href="https://www.merriam-webster.com/dictionary/calibrated">calibrated </a>in pH.

In agriculture, the pH is probably the most important single property of the moisture associated with a <a dir="ltr" href="https://www.britannica.com/science/soil">soil </a>, since that indication reveals what crops will grow readily in the soil and what adjustments must be made to adapt it for growing any other crops. Acidic soils are often considered infertile, and so they are for most conventional agricultural crops, although <a dir="ltr" href="https://www.britannica.com/plant/conifer">conifers </a>and many members of the family <a dir="ltr" href="https://www.britannica.com/plant/heath-family">Ericaceae </a>, such as <a dir="ltr" href="https://www.britannica.com/plant/blueberry-plant">blueberries </a>, will not thrive in alkaline soil. Acidic soil can be “sweetened,” or neutralized, by treating it with lime. As soil acidity increases so does the <a dir="ltr" href="https://www.britannica.com/science/solubility-chemistry">solubility </a>of <a dir="ltr" href="https://www.britannica.com/science/aluminum">aluminum </a>and <a dir="ltr" href="https://www.britannica.com/science/manganese">manganese </a>in the soil, and many plants (including agricultural crops) will tolerate only slight quantities of those metals. Acid content of soil is heightened by the decomposition of organic material by microbial action, by fertilizer salts that hydrolyze or nitrify, by oxidation of sulfur <a data-term="compounds" dir="ltr" href="https://www.merriam-webster.com/dictionary/compounds">compounds </a>when <a dir="ltr" href="https://www.britannica.com/science/salt">salt </a>marshes are drained for use as farmland, and by other causes.

pH | Definition, Uses, & Facts

PH measures the acidity or basicity of liquid solutions

Ways to take a pH measurement

PH is most important in agriculture

CURATED BY

Related collections

More like this