Definition (Encyclopedia of Global Warming)
pH is a quantitative measure of the acidity or basicity of a solution. The pH scale assigns a value between 0 and 14 based on the concentration of the hydrogen atoms in the solution. The more hydrogen present, the more acidic the solution and the lower the pH value. The fewer hydrogen ions present, the less acidic the solution and the higher the pH value. Pure water, which is neutral (neither acidic nor alkaline) has a pH of 7, which means that the concentration of the hydrogen ion is 10-7 gram-equivalent per liter. This mathematical system of stating pH was developed by a Danish chemist, Soren Peter Lauritz Sorenson, around 1909. “pH” is thought to be an abbreviation for the “potential of hydrogen” in English. The same abbreviation makes sense in several other languages, as it could stand for pondus hydrogenii or potentia hydrogenii in Latin or potentiel hydrogene in French.
(The entire section is 147 words.)
Significance for Climate Change (Encyclopedia of Global Warming)
Soil pH. Arguably the single most important property of the moisture that is provided to soil is its pH. Soil’s acidity determines which kinds of plants are able to grow in that soil. It is possible to adjust the pH of soil or of the water provided to it; acidic soil can be treated with lime to “sweeten” or neutralize it.
Acidic soil is usually considered to be infertile, as most conventional crops will not grow in it. Soil acidity also causes certain metals, such as aluminum and manganese, to become more soluble in the soil; many plants will not tolerate more than tiny quantities of these metals. Of the seventeen essential nutrients a plant needs to grow, it gets fourteen of them from the surrounding soil. The soil’s pH is an important factor in the solubility and availability of these nutrients.
Soil acidity can be increased by aspects of the global climate. For example, acid rain (rain with a pH of 5.6 or lower) acidifies the soil upon which it falls. Other contributors to soil acidity include the oxidation of sulfur compounds that occurs when salt marshes are drained to be used as farmland, as well as the addition of fertilizer salts that hydrolyze or the addition of microbes that decompose organic materials. The pH of soil can be determined using a pH meter, a device that uses electrodes in the soil to generate a weak charge that can measure the concentration of hydrogen...
(The entire section is 591 words.)
Further Reading (Encyclopedia of Global Warming)
Date, R. A., et al., eds. Plant-Soil Interactions at Low pH: Principles and Management. Boston: Kluwer Academic, 1995. Discusses the chemistry and physics of acid soils and how they affect crop production, particularly in developing countries.
Rengel, Z., ed. Handbook of Plant Growth: pH as the Master Variable. Boca Raton, Fla.: CRC Press, 2002. Explores the role of pH in all parts of a plant. Includes an overview of all plant mechanisms and how they are affected by pH.
_______. Handbook of Soil Acidity. Boca Raton, Fla.: CRC Press, 2003. Discusses how soil acidity affects crop production and effective strategies for balancing soil pH.
(The entire section is 100 words.)
pH (Encyclopedia of Science)
The most common method of indicating the acidity of a solution is by stating its pH. The term pH refers to a mathematical system developed by Danish chemist Søren Sørenson (1868939) around 1909. Sørenson originally suggested the term pH as an abbreviation for potential (or power) of hydrogen.
Acids and bases were first defined by Swedish chemist Svante Arrhenius (1859927). Arrhenius proposed that acids be defined as chemicals that produce positively charged hydrogen ions, H+, in water. By comparison, he suggested that bases are compounds that produce negatively charged hydroxide ions, OH/sup>, in water.
The pH of a solution is determined by the concentration of hydrogen ions presenthat is, by its acidity. The more hydrogen ions present (the more acidic the solution), the lower the pH. The fewer hydrogen ions present (the less acidic the solution), the higher the pH. The pH scale runs from 0 to 14. A pH value of 7 (in the middle of that range) represents a solution that is neither acidic nor basic.
Strong acids have very low pHs (battery acid has a pH of 0). Strong bases have very high pHs (sodium hydroxide, commonly known as lye, has a pH of 14). Lemon juice has a pH of 2; vinegar of 2.5; coffee of 5; distilled water of 7; borax of 9; and household ammonia of 11.
(The entire section is 490 words.)
pH (Science Experiments)
Quick! Get the litmus paper
Kitchen Chemistry: What is the pH of household chemicals?
Design Your Own Experiment
The numerical measurement of and basesA substance that when dissolved in water is capable of reacting with an acid to form salts and release hydrogen ions. in a solution is called (the abbreviation for potential hydrogen). Acids and bases are groups of chemicals. When dissolved in water, all acids release hydrogen atoms with a positive electric charge (H+). These atoms are known as hydrogen ionsAn atom or group of atoms that carry an electrical chargeither positive or negatives a result of losing or gaining one or more electrons.. The term pH means the strength of the hydrogen ions. The p is derived from the Danish word potenz meaning strength; H is the symbol for hydrogen. When dissolved in water, bases produce negatively charged hydroxide ions (OH. When mixed together in the right proportions, acids and bases each other and form a water and a salt.
In 1909, Danish scientist Soren Peter Lauritz Sorensen, whose wife Margarethe Hoyrup...
(The entire section is 2929 words.)
pH (World of Earth Science)
pH is a measure of the acidity or alkalinity of a solution. The variability of pH can have a dramatic effect on geochemical processes (e.g., weathering processes).
The pH scale was developed by Danish chemist Søren Peter Lauritz Sørensen (1868939) in 1909 and is generally presented as ranging from 0 to 14, although there are no theoretical limits on the range of the scale (there are substances with negative pH's and with pH's greater than 14, although for most substances the range of 04 suffices). A solution with a pH of less than 7 is acidic and a solution with a pH of greater than 7 is basic (alkaline). The midpoint of the scale, 7, is neutral. The lower the pH of a solution, the more acidic the solution is and the higher the pH, the more basic it is. Mathematically, the potential hydronium ion concentration (pH) is equal to the negative logarithm of the hydronium ion concentration: pH = og [H30+], where H3O+ represents the hydronium ion.
Essentially, the hydronium ion can be thought of as a water molecule with a proton attached. The square brackets indicate the concentration of, in moles per liter. Thus, [H3O+] indicates the concentration of hydronium ions in moles per liter.
The hydronium ion is an important participant in the chemical reactions that take place in aqueous (water, H20) solutions.
Through a process termed self-ionization, a small number of water molecules in pure water dissociate (separate) in a reversible reaction to form a positively charged H+ ion and a negatively charged OH/sup> ion. In aqueous solution, as one water molecule dissociates, another is nearby to pick up the loose, positively charged, hydrogen proton to form a positively charged hydronium ion (H3O+).
Water molecules have the ability to attract protons and form hydronium ions because water is a polar molecule. Oxygen is more electronegative than hydrogen. As a result, the electrons in each of water's two oxygen-hydrogen bonds to spend more time near the oxygen atom. Because the electrons are not shared equallynd because the bond angles of the water molecule do not cancel out this imbalancehe oxygen atom carries a partial negative charge that can attract positively charged protons donated by other molecules.
In a sample of pure water, the concentration of hydronium ions is equal to 1 10 moles per liter (0.0000001 M). The water molecule that lost the hydrogen protonut that kept the hydrogen electronecomes a negatively charged hydroxide ion (OH/sup>).
The equilibrium (balance) between hydronium and hydroxide ions that results from self-ionization of water can be disturbed if other substances that can donate protons are put into solution with water.
The pH of solutions may be measured electronically with a pH meter (better pH meters can measure to 0.001 pH units) or by using acid base indicators, chemicals that change color in solutions of different pH.
See also Acid rain; Geochemistry; Weathering and weathering series
pH (World of Microbiology and Immunology)
The term pH refers to the concentration of hydrogen ions (H+) in a solution. An acidic environment is enriched in hydrogen ions, whereas a basic environment is relatively depleted of hydrogen ions. The pH of biological systems is an important factor that determines which microorganism is able to survive and operate in the particular environment. While most microorganisms prefer pH's that approximate that of distilled water, some bacteria thrive in environments that are extremely acidic.
The hydrogen ion concentration can be determined empirically and expressed as the pH. The pH scale ranges from 0 to 14, with 1 being the most acidic and 14 being the most basic. The pH scale is a logarithmic scale. That is, each division is different from the adjacent divisions by a factor of ten. For example, a solution that has a pH of 5 is 10 times as acidic as a solution with a pH of 6.
The range of the 14-point pH scale is enormous. Distilled water has a pH of 7. A pH of 0 corresponds to 10 million more hydrogen ions per unit volume, and is the pH of battery acid. A pH of 14 corresponds to one ten-millionth as many hydrogen ions per unit volume, compared to distilled water, and is the pH of liquid drain cleaner.
Compounds that contribute hydrogen ions to a solution are called acids. For example, hydrochloric acid (HCl) is a strong acid. This means that the compounds dissociates easily in solution to produce the ions that comprise the compound (H+ and Cl/sup>). The hydrogen ion is also a proton. The more protons there are in a solution, the greater the acidity of the solution, and the lower the pH.
Mathematically, pH is calculated as the negative logarithm of the hydrogen ion concentration. For example, the hydrogen ion concentration of distilled water is 10 and hence pure water has a pH of 7.
The pH of microbiological growth media is important in ensuring that growth of the target microbes occurs. As well, keeping the pH near the starting pH is also important, because if the pH varies too widely the growth of the microorganism can be halted. This growth inhibition is due to a numbers of reasons, such as the change in shape of proteins due to the presence of more hydrogen ions. If the altered protein ceases to perform a vital function, the survival of the microorganism can be threatened. The pH of growth media is kept relatively constant by the inclusion of compounds that can absorb excess hydrogen or hydroxyl ions. Another means of maintaining pH is by the periodic addition of acid or base in the amount needed to bring the pH back to the desired value. This is usually done in conjunction with the monitoring of the solution, and is a feature of large-scale microbial growth processes, such as used in a brewery.
Microorganisms can tolerate a spectrum of pHs. However, an individual microbe usually has an internal pH that is close to that of distilled water. The surrounding cell membranes and external layers such as the glycocalyx contribute to buffering the cell from the different pH of the surrounding environment.
Some microorganisms are capable of modifying the pH of their environment. For example, bacteria that utilize the sugar glucose can produce lactic acid, which can lower the pH of the environment by up to two pH units. Another example is that of yeast. These microorganisms can actively pump hydrogen ions out of the cell into the environment, creating more acidic conditions. Acidic conditions can also result from the microbial utilization of a basic compound such as ammonia. Conversely, some microorganisms can raise the pH by the release of ammonia.
The ability of microbes to acidify the environment has been long exploited in the pickling process. Foods commonly pickled include cucumbers, cabbage (i.e., sauerkraut), milk (i.e., buttermilk), and some meats. As well, the production of vinegar relies upon the pH decrease caused by the bacterial production of acetic acid.
See also Biochemistry; Buffer; Extremophiles