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pH

pH

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). 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 107 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

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pH

pH

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 (18681939) 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 014 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 = log [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 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 equallyand because the bond angles of the water molecule do not cancel out this imbalancethe 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 × 107 moles per liter (0.0000001 M). The water molecule that lost the hydrogen protonbut that kept the hydrogen electronbecomes a negatively charged hydroxide ion (OH).

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

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pH

pH

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 (18681939) 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 (18591927). 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, in water.

The pH of a solution is determined by the concentration of hydrogen ions presentthat 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.

pH indicators

One way of finding the pH of a solution is with a pH meter, a mechanical device that gives very precise readings. One can easily place the probe of a pH meter into a solution and read the pH of the solution on the meter dial.

Table 1.
Some Common Solutions and Their pH

Substance Approximate pH
Battery acid (sulfuric acid) 0
Lemon juice 2
Vinegar 2.5
Coffee 5
Distilled water 7
Borax 9
Household ammonia solution 11
Lye (sodium hydroxide) 14

Table 2.
Some Common Indicators, the pH Range in Which They Change Color, and Their Color Changes

Indicator pH Range Color Changes
Methyl violet pH 0.1 to 3.2 Yellow to violet
Bromophenol blue pH 2.8 to 4.6 Yellow to blueviolet
Congo red pH 3.0 to 5.0 Blue to red
Bromocresol purple pH 5.2 to 6.8 Yellow to purple
Cresol red pH 7.2 to 8.8 Yellow to purple
Phenolphthalein pH 8.4 to 10.0 Colorless to pink
Brilliant orange pH 10.5 to 12.0 Yellow to red
Titan yellow pH 12.0 to 13.0 Yellow to red

A much older method for estimating the pH of a solution is the use of an indicator. A pH indicator is a material that changes color in solutions of different pH. One of the most common of all indicators is litmus. Litmus is a chemical obtained from lichens. In the presence of a base, litmus is blue; in the presence of an acid, it is red.

Many indicators are extracted from plants. For example, you can make a reasonably good indicator just by boiling red cabbage and extracting the colored material produced. That material, like litmus, changes color in the presence of acids and bases. A number of indicators are synthetic products made just for testing the pH of solutions.

[See also Acids and bases ]

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PH

PH

The pH scale is a measure of the acid and base concentration of a solution. A pH of 7 is neutral; 0-7 is the acid range and 7-14 is the base or alkalinity range.

The presence of dissolved carbonates, bicarbonates, and hydroxides in water is associated with alkalinity. The presence of dissolved organic matter in water causes it to be more acidic in nature.

Fast growing fish are associated with alkaline waters; whereas slow growing fish are associated with acidic waters. Alkaline waters enhance the amount of aquatic foods

and weed beds. On the other hand, acidic waters curtail this production of aquatic foods and weed beds.

Acid rain can cause water to become too acidic for life, causing both fish and insect kills. A spring snowmelt can concentrate the acid in the bottom layer of the snowbank. When this last layer melts, high concentrations of acids are released into the watershed. This can be disastrous to aquatic life.

Local areas of a lake can vary in pH. Serious bass fishermen frequently measure the waters pH to determine where bass may be concentrated.

In general, I avoid the acidic lakes and streams in favor of the alkaline ones. Perhaps this explains why so many Pacific Northwest waters are poor producers of non-anadromous fish. They are nearly barren of aquatic insect life. The abundant rainfall is so intense that these waters become soft and acidic, and the carbonates and alkaline elements have been depleted. The organic matter has concentrated in them. High desert waters favor alkalinity because less rainfall does not wash away the alkaline elements. Their weed bed growth and abundant insect life provide more ideal conditions for fast growing fish. Im always amazed that a desert state like Nevada has such productive water while the rain forest of the Oregon Coast has an abundance of waters that are nearly void of residential fish. If it werent for anadromous fish, many coastal streams would not have fish.

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pH

pH, range of numbers expressing the relative acidity or alkalinity of a solution. In general, pH values range from 0 to 14. The pH of a neutral solution, i.e., one which is neither acidic nor alkaline, is 7. Acidic solutions have pH values below 7; alkaline, or basic, solutions have pH values above 7. A pH value provides a measure of the hydrogen ion concentration of a solution. In pure water the concentration of hydrogen ions is equal to 0.0000001, or 10-7, moles per liter. (A mole is the amount of a substance, expressed in grams, that is equal to the molecular weight, or formula weight, of the substance.) When an acid is added to pure water, the hydrogen ion concentration increases above this level. When an alkaline substance, or base, is added to pure water, the hydrogen ion concentration decreases below this level. Once the concentration is determined, the pH value is found by taking the exponent used in expressing this concentration and reversing its sign. This is expressed as pH=-log10 [H+]. For example, if the hydrogen ion concentration of a solution is 10-4, or 0.0001, moles per liter, the pH is 4. See indicators, acid-base.

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pH

pH A value on a scale 0–14 which gives a measure of the acidity or alkalinity of a medium. A neutral medium has a pH of 7, acidic media have pH values of less than 7, and alkaline media of more than 7. The lower the pH the more acidic is the medium, the higher the pH, the more alkaline. The pH value is the logarithm of the reciprocal of the hydrogen ion concentration, expressed in moles per litre (pH = log10l/H+). Most pH values in natural systems lie in the range 4–9. Human blood has a pH of 7.4, ocean water 8.1–8.3, water in saline environments may have a pH around 9.0 or higher, and water in acidic soils may have a pH of 4.0 or less.

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pH

pH The negative logarithm of the hydrogen ion concentration in a solution, pH = log10 1/[H+]. If the hydrogen ion concentration of a solution increases, as happens with increasing acidity, the pH will decrease, and vice versa. The pH is measured on a scale of 0–14; a neutral medium (such as pure water) has a pH of 7, numbers above 7 indicate relative alkalinity, numbers below 7 indicate relative acidity. Most pH values in natural systems lie in the range 4–9. Human blood has a pH of 7.4, ocean water 8.1–8.3, water in saline environments may have a pH around 9.0 or higher, while for water in acidic soils it may be as low as 4.0 or less.

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pH

pH The negative logarithm of the hydrogen ion concentration [H+] in mols/litre. Lower pH therefore means greater acidity, and vice versa. Extracellular fluid (ECF), including the blood, is normally at a pH close to 7.4 which means [H+] = 10-7.4 mols/litre, or 40 nanomoles/litre. At body temperature, neutral pH would be approximately 6.8; body fluids are therefore on the alkaline side of neutral. Control mechanisms normally keep ECF pH within 0.04 of the norm either way. The pH inside cells is more acid, and more variable, related to metabolic activity.

Stuart Judge


See acid–base homeostasis.

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pH

pH A value on a scale 0–14 that gives a measure of the acidity or alkalinity of a medium. A neutral medium has a pH of 7; acidic media have pH values of less than 7, and alkaline media of more than 7. The lower the pH, the more acidic is the medium; the higher the pH, the more alkaline. The pH value is the reciprocal of the hydrogen ion concentration, expressed in moles per litre.

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pH

pH A value on a scale 0–14 that gives a measure of the acidity or alkalinity of a medium. A neutral medium has a pH of 7. Acidic media have pH values of less than 7, and alkaline media of more than 7. The lower the pH, the more acidic is the medium; the higher the pH, the more alkaline. The pH value is the reciprocal of the hydrogen ion concentration expressed in moles per litre.

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pH

pH Potential hydrogen, a measure of acidity or alkalinity. Defined as the negative logarithm of the hydrogen‐ion concentration. The scale runs from 0, which is very strongly acid, to 14, which is very strongly alkaline. Pure water is pH 7, which is neutral; below 7 is acid, above is alkaline. See also acid; buffer.

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pH

pH • n. Chem. a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid, and higher values more alkaline. The pH is equal to −log10 c, where c is the hydrogen ion concentration in moles per liter.

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pH

pH Numerical scale that indicates the acidity or alkalinity of a solution. The pH value measures the concentration of hydrogen ions. The scale (introduced in 1909) runs from 0 to 14. A solution is acidic if the pH is less than 7 and alkaline if greater than 7. See also acid; alkali; base

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pH

pH n. a measure of the concentration of hydrogen ions in a solution, and therefore of its acidity or alkalinity. A pH of 7 indicates a neutral solution, a pH below 7 indicates acidity, and a pH in excess of 7 indicates alkalinity.

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PH

PH (also P.H.) • abbr. ∎  Public Health. ∎  Purple Heart.

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pH

pH See pH scale.

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pH

pHmph, pH, Rh

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ph

ph Optics, symbol for phot(s)

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