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At this temperature, then, neutral solutions exhibit pH = pOH = 6.31, acidic solutions exhibit pH less than 6.31 and pOH greater than 6.31, whereas basic solutions exhibit pH greater than 6.31 and pOH less than 6.31. This distinction can be important when studying certain processes that occur at nonstandard temperatures, such as enzyme reactions in warm-blooded organisms. Unless otherwise noted, references to pH values are presumed to be those at standard temperature (25 °C) ( [link] ).

Summary of Relations for Acidic, Basic and Neutral Solutions
Classification Relative Ion Concentrations pH at 25 °C
acidic [H 3 O + ]>[OH ] pH<7
neutral [H 3 O + ] = [OH ] pH = 7
basic [H 3 O + ]<[OH ] pH>7

[link] shows the relationships between [H 3 O + ], [OH ], pH, and pOH, and gives values for these properties at standard temperatures for some common substances.

A table is provided with 5 columns. The first column is labeled “left bracket H subscript 3 O superscript plus right bracket (M).” Powers of ten are listed in the column beginning at 10 superscript 1, including 10 superscript 0 or 1, 10 superscript negative 1, decreasing by single powers of 10 to 10 superscript negative 15. The second column is labeled “left bracket O H superscript negative right bracket (M).” Powers of ten are listed in the column beginning at 10 superscript negative 15, increasing by single powers of 10 to including 10 superscript 0 or 1, and 10 superscript 1. The third column is labeled “p H.” Values listed in this column are integers beginning at negative 1, increasing by ones up to 14. The fourth column is labeled “p O H.” Values in this column are integers beginning at 15, decreasing by ones up to negative 1. The fifth column is labeled “Sample Solution.” A vertical line at the left of the column has tick marks corresponding to each p H level in the table. Substances are listed next to this line segment with line segments connecting them to the line to show approximate p H and p O H values. 1 M H C l is listed at a p H of 0. Gastric juices are listed at a p H of about 1.5. Lime juice is listed at a p H of about 2, followed by 1 M C H subscript 3 C O subscript 2 H, followed by stomach acid at a p H value of nearly 3. Wine is listed around 3.5. Coffee is listed just past 5. Pure water is listed at a p H of 7. Pure blood is just beyond 7. Milk of Magnesia is listed just past a p H of 10.5. Household ammonia is listed just before a pH of 12. 1 M N a O H is listed at a p H of 0. To the right of this labeled arrow is an arrow that points up and down through the height of the column. A beige strip passes through the table and to this double headed arrow at p H 7. To the left of the double headed arrow in this beige strip is the label “neutral.” A narrow beige strip runs through the arrow. Just above and below this region, the arrow is purple. It gradually turns to a bright red as it extends upward. At the top of the arrow, near the head of the arrow is the label “acidic.” Similarly, the lower region changes color from purple to blue moving to the bottom of the column. The head at this end of the arrow is labeled “basic.”
The pH and pOH scales represent concentrations of [H 3 O + ] and OH , respectively. The pH and pOH values of some common substances at standard temperature (25 °C) are shown in this chart.

Calculation of ph from [h 3 O + ]

What is the pH of stomach acid, a solution of HCl with a hydronium ion concentration of 1.2 × 10 −3 M ?


pH = −log [ H 3 O + ]
= −log( 1.2 × 10 −3 )
= ( −2.92 ) = 2.92

(The use of logarithms is explained in Appendix B . Recall that, as we have done here, when taking the log of a value, keep as many decimal places in the result as there are significant figures in the value.)

Check your learning

Water exposed to air contains carbonic acid, H 2 CO 3 , due to the reaction between carbon dioxide and water:

CO 2 ( a q ) + H 2 O( l ) H 2 CO 3 ( a q )

Air-saturated water has a hydronium ion concentration caused by the dissolved CO 2 of 2.0 × 10 −6 M , about 20-times larger than that of pure water. Calculate the pH of the solution at 25 °C.



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Calculation of hydronium ion concentration from ph

Calculate the hydronium ion concentration of blood, the pH of which is 7.3 (slightly alkaline).


pH = −log [ H 3 O + ] = 7.3
log [ H 3 O + ] = −7.3
[ H 3 O + ] = 10 −7.3 or [ H 3 O + ] = antilog of −7.3
[ H 3 O + ] = 5 × 10 −8 M

(On a calculator take the antilog, or the “inverse” log, of −7.3, or calculate 10 −7.3 .)

Check your learning

Calculate the hydronium ion concentration of a solution with a pH of −1.07.


12 M

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Environmental science

Normal rainwater has a pH between 5 and 6 due to the presence of dissolved CO 2 which forms carbonic acid:

H 2 O ( l ) + CO 2 ( g ) H 2 CO 3 ( a q )
H 2 CO 3 ( a q ) H + ( a q ) + HCO 3 ( a q )

Acid rain is rainwater that has a pH of less than 5, due to a variety of nonmetal oxides, including CO 2 , SO 2 , SO 3 , NO, and NO 2 being dissolved in the water and reacting with it to form not only carbonic acid, but sulfuric acid and nitric acid. The formation and subsequent ionization of sulfuric acid are shown here:

H 2 O ( l ) + SO 3 ( g ) H 2 SO 4 ( a q )
H 2 SO 4 ( a q ) H + ( a q ) + HSO 4 ( a q )

Carbon dioxide is naturally present in the atmosphere because we and most other organisms produce it as a waste product of metabolism. Carbon dioxide is also formed when fires release carbon stored in vegetation or when we burn wood or fossil fuels. Sulfur trioxide in the atmosphere is naturally produced by volcanic activity, but it also stems from burning fossil fuels, which have traces of sulfur, and from the process of “roasting” ores of metal sulfides in metal-refining processes. Oxides of nitrogen are formed in internal combustion engines where the high temperatures make it possible for the nitrogen and oxygen in air to chemically combine.

Acid rain is a particular problem in industrial areas where the products of combustion and smelting are released into the air without being stripped of sulfur and nitrogen oxides. In North America and Europe until the 1980s, it was responsible for the destruction of forests and freshwater lakes, when the acidity of the rain actually killed trees, damaged soil, and made lakes uninhabitable for all but the most acid-tolerant species. Acid rain also corrodes statuary and building facades that are made of marble and limestone ( [link] ). Regulations limiting the amount of sulfur and nitrogen oxides that can be released into the atmosphere by industry and automobiles have reduced the severity of acid damage to both natural and manmade environments in North America and Europe. It is now a growing problem in industrial areas of China and India.

For further information on acid rain, visit this website hosted by the US Environmental Protection Agency.

Two photos are shown. Photograph a on the left shows the upper portion of trees against a bright blue sky. The tops of several trees at the center of the photograph have bare branches and appear to be dead. Image b shows a statue of a man that appears to from the revolutionary war era in either marble or limestone.
(a) Acid rain makes trees more susceptible to drought and insect infestation, and depletes nutrients in the soil. (b) It also is corrodes statues that are carved from marble or limestone. (credit a: modification of work by Chris M Morris; credit b: modification of work by “Eden, Janine and Jim”/Flickr)

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Source:  OpenStax, Ut austin - principles of chemistry. OpenStax CNX. Mar 31, 2016 Download for free at http://legacy.cnx.org/content/col11830/1.13
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