# pH

views updated May 29 2018

# pH

The pH of a solution is a measure of the hydronium ion (H3O+) concentration in that solution. The hydronium ion in a solution results from the self-ionization of water. In other words, because hydronium ions are formed from H+ ions and water molecules, pH is also expressed as the concentration of hydrogen ions (or, positively charged hydrogen atoms) of a solution. The pH is measured on a scale from 0 to 14. Therefore, the pH is a measure of the acidity or alkalinity of a solution based upon the dissociation of water. The variability of pH can have a

pH values of common household products
SubstanceApproximate pHH2O +
battery acid01M
(sulfuric acid)
lemon juice21 × 102M
vinegar2.53 × 103M
coffee51 × 105M
distilled water71 × 107M
borax91 ×9M
household ammonia111 ×11M
solution
1M NaOH141 ×14M
(sodium hydroxide, lye)

Approximate pH of Various Substances. (Thomson Gale.)

dramatic effect on biological or physical chemical reactions (e.g., geochemical weathering processes).

The abbreviation pH is derived from the French term pauvoir hydrogéne, which means hydrogen power. The pH scale was developed by Danish chemist Søren Peter Lauritz Sørensen (1868-1939) in 1909. It is generally presented as ranging from 0-14, although there are no theoretical limits on the range of the scale (e.g., there are substances with negative pHs and with pHs greater than 14) the range is generally cited as being from pH=0 to pH=14.

A solution with a pH of less than pH = 7 is acidic and a solution with a pH of greater than pH = 7isbasic (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+. The square brackets indicate the concentration of, in moles per liter. [H30+ represents the hydro-nium ionesentially a water molecule with a proton attached. Thus, [H3O+ indicates the concentration of hydronium ions in moles per liter. Hydronium ions are important participants in chemical reactions that take place in aqueous (water, H20) solutions.

Water is a weak electrolyte. 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+). The water molecule that lost the hydrogen proton but that kept the hydrogen electronbecomes a negatively charged hydroxide ion (OH).

In dilute solutions, the product of the hydronium ion concentration and the hydroxide ion equals the ion product (Kw) or dissociation constant (Kw = 1.0×1014 at 25°C). Calculations of pH using the ion product yield a number between 0 and 14the standard pH scale.

In a sample of pure water, the concentration of hydronium ions is equal to 1× 107 moles per liter (0.0000001 M). 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 experimentally with an electronic pH meter (highly accurate pH meters can measure to 0.001 pH units) or by using acid base indicators, chemicals that change color in solutions of different pH. A crude but common test for pH involves the use of hydrion paper strips (litmus paper) that undergo changes similar to those found in indicator solutions. For example, red litmus paper turns blue in a basic solution.

Sometimes it is necessary to maintain a solution at a constant pH. This is especially true in bodily fluids such as blood, which needs to be kept neutral (between a pH of 7.35 and 7.45). If the pH of blood is allowed to vary outside of this range, serious illness or death may occur. Buffers are substances that control the pH of a solution. A buffer is usually a mixture of acids and bases. This mix of acids and bases allows the buffer to release or absorb hydronium ions, which keeps the pH of a solution constant. The most common buffers are mixtures of weak acids and their conjugate bases. A buffer cannot keep the pH of a solution under absolute control under all conditions. There is a limit to the ability of a buffer to maintain a constant pH, called the buffer capacity. In general, the greater the concentration of buffer in a solution, the greater the buffer capacity.

# pH

views updated May 18 2018

# 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 blue–violet 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.

# pH

views updated May 29 2018

# 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.

# pH

views updated Jun 08 2018

# 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.

# pH

views updated May 14 2018

# pH

pH is a measure of the acidity or alkalinity of a solution based upon the dissociation of water . The variability of pH can have a dramatic effect on biological or physical chemical reactions (e.g., geochemical weathering processes).

 Substance Approximate pH H3O+ battery acid (sulfuric acid) 0 1M lemon juice 2 1 x 10-2M vinegar 2.5 3 x 10-3M coffee 5 1 x 10-5M distilled water 7 1 x 10-7M borax 9 1 x 10-9M household ammonia solution 11 1 x 10-11M 1M NaOH (sodium hydroxide, lye) 14 1 x 10-14M

The pH scale was developed by Danish chemist Søren Peter Lauritz Sørensen (1868–1939) in 1909 and is generally presented as ranging from 0–14, although there are no theoretical limits on the range of the scale (e.g., there are substances with negative pH's and with pH's greater than 14) the range is generally cited as being from pH =0 to pH =14.

A solution with a pH of less than pH =7 is acidic and a solution with a pH of greater than pH =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+]. The square brackets indicate the concentration of, in moles per liter. [H 0+3 ] represents the hydronium ion—esentially a water molecule with a proton attached. Thus, [H Op+3 ] indicates the concentration of hydronium ions in moles per liter. Hydronium ions are important participants in chemical reactions that take place in aqueous (water, H20) solutions.

Water is a weak electrolyte . 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+ ). The water molecule that lost the hydrogen proton—but that kept the hydrogen electron—becomes a negatively charged hydroxide ion (OH-).

In dilute solutions, the product of the hydronium ion concentration and the hydroxide ion equals the ion product (Kw) or dissociation constant (Kw= 1.0×10-14 at 25°C). Calculations of pH using the ion product yield a number between 0 and 14—the standard pH scale.

In a sample of pure water, the concentration of hydronium ions is equal to 1×10-7 moles per liter (0.0000001 M). 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 experimentally with an electronic pH meter (highly accurate pH meters can measure to 0.001 pH units) or by using acid base indicators, chemicals that change color in solutions of different pH. A crude but common test for pH involves the use of Hydrion paper strips (litmus paper) that undergo changes similar to those found in indicator solutions. For example, red litmus paper turns blue in a basic solution.

# pH

views updated May 23 2018

# pH

pH is a number used to measure the degree of acidity of a solution. It is used on a pH scale that ranges from 0 to 14, with the difference between each number being a factor of 10. In the life sciences, as well as in chemistry, many chemical reactions depend on the pH of a solution. pH is also used to analyze body secretions, to test soil suitability, and for industrial purposes.

pH refers to the amount of acid in a substance. The letters are said to have come from the French for "hydrogen power," meaning how many

hydrogen atoms are concentrated in a solution. The lowercase p means its "power," or its logarithmic value. This means that each time a number is raised to another power (from a 2 to a 3), it increases by a factor of 10. Another explanation for pH is that it stands for "potential of hydrogen." Either way, it is known that the pH symbol was first used by the Danish chemist, Soren Sorenson (1868–1939), in 1909. He used the pH symbol on what he called a Sorenson scale.

Today, however, it is called a pH scale, and it is a 0 to 14 scale that tells us exactly how acidic a substance is. This scale uses as a reference point the number 7 which is the midpoint between the scale's two extremes of 0 and 14. A pH of 7 is considered to be neutral—or neither acid nor its opposite, base. Acids and bases are two types or classes of biological compounds. They affect every living cell as well as the habitats of organisms.

The pH of a solution can be measured with an electronic pH meter or by various paper or liquid indicators. These change color depending on the pH of the mixture. A pH meter will give a digital readout, or number, indicating the pH of a solution. A treated paper indicator turns darker pink for more acid and darker blue for more base. The paper color is checked against a standard chart that indicates the pH number. The scale itself tells the exact degree of acid in a solution. Starting with the lowest number, the strongest acid, a pH of 0, would be concentrated nitric acid. Following that, in approximate values, stomach acid has a pH of 1, lemon juice 2, vinegar 3, fresh tomatoes 4, black coffee 5, and peas 6. Distilled water is neutral and has a pH of 7.

After this, the base part of the scale begins. Baking soda has a pH of 8, borax 9, ammonia 10, lime 12, oven cleaner 13, and lye 14. Since these values are logarithmic, the difference between each one is a factor of 10. Thus a solution of pH 5 is 10 times more acidic than a solution of pH 6. In the living world, almost all biological processes take place in a pH environment between 6 and 8. There are, however, a few exceptions such as digestive acids that are extremely powerful (with a pH of 1). Many organisms have built-in regulators that act as buffers and either soak up or join with small amounts of excess acid or base.

# PH

views updated May 21 2018

# 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.

# pH

views updated May 18 2018

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.

# pH

views updated May 08 2018

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.

# pH

views updated May 21 2018

# pH

A measure of the acidity or alkalinity of a solution based on its hydrogen ion (H+) concentration. The pH of a solution is the negative logarithm (base 10) of its H+ concentration. Since the scale is logarithmic, there is a tenfold difference in hydrogen ion concentration for each pH unit. The pH scale ranges from 0 to 14 with 7 indicating neutrality ((H+)= (OH-)). Values above 7 indicate progressively greater alkalinity, while values below 7 indicate progressively increasing acidity.