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October 20 2008

1. yuchi_sachiro
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pH is the measure of the acidity or alkalinity of a solution. It is formally a measure of the activity of dissolved hydrogen ions (H+), but for very dilute solutions, the molarity (molar concentration) of H+ may be used as a substitute with little loss of accuracy. In solution, hydrogen ions occur as a number of cations including hydronium ions (H3O+).

In pure water at 25 °C, the concentration of H+ equals the concentration of hydroxide ions (OH-). This is defined as "neutral" and corresponds to a pH level of 7.0. Solutions in which the concentration of H+ exceeds that of OH- have a pH value lower than 7.0 and are known as acids. Solutions in which OH- exceeds H+ have a pH value greater than 7.0 and are known as bases. Because pH is dependent on ionic activity, a property which cannot be measured easily or fully predicted theoretically, it is difficult to determine an accurate value for the pH of a solution. The pH reading of a solution is usually obtained by comparing unknown solutions to those of known pH, and there are several ways to do so.

The concept of pH was first introduced by Danish chemist S. P. L. Sørensen at the Carlsberg Laboratory in 1909. Sørensen suggested the notation "PH" for convenience, standing for "power of hydrogen", using the negative logarithm of the concentration of hydrogen ions in solution.

The operational definition of pH is officially defined by International Standard ISO 31-8 as follows: [6] For a solution X, first measure the electromotive force EX of the galvanic cell

reference electrode | concentrated solution of KCl || solution X | H2 | Pt

and then also measure the electromotive force ES of a galvanic cell that differs from the above one only by the replacement of the solution X of unknown pH, pH(X), by a solution S of a known standard pH, pH(S). The pH of X is then

\text{pH(X)} = \text{pH(S)} + \frac{(E_{\text{S}} - E_{\text{X}})F}{RT \ln 10}

where

F is the Faraday constant;
R is the molar gas constant;
T is the thermodynamic temperature.

Defined this way, pH is a dimensionless quantity. Values pH(S) for a range of standard solutions S, along with further details, are given in the relevant IUPAC recommendation.[7]

pH has no fundamental meaning as a unit; its official definition is a practical one. However in the restricted range of dilute aqueous solutions having an amount-of-dissolved-substance concentrations less than 0.1 mol/L, and being neither strongly alkaline nor strongly acidic (2 < pH < 12), the definition is such that

\text{pH} = -\log_{10}\left[\frac{\gamma_1 [\text{H}^+] }{ \text{1 mol L}^{-1} } \right] \pm 0.02

where [H+] denotes the amount-of-substance concentration of hydrogen ion H+ and γ1 denotes the activity coefficient of a typical univalent electrolyte in the solution.

Simplified definition

pH is a measurement of the concentration of hydrogen ions in a solution. Because of its mathematical formulation, low pH values are associated with solutions with high concentrations of hydrogen ions, while high pH values occur for solutions with low concentrations of hydrogen ions. Pure water has a pH of 7.0, and other solutions are usually described with reference to this value. Acids are defined as those solutions that have a pH less than 7 (i.e. more hydrogen ions than water); while bases are defined as those solutions that have a pH greater than 7 (i.e. less hydrogen ions than water).

pH is a logarithmic scale, that is, the integer part expresses the order of magnitude, e.g. pH 5 has an acidity ten times weaker than pH 4. The fractional part is also logarithmic; pH 4.3 is approximately two times weaker, and pH 4.5 three times.

The definitions of weak and strong acids, and weak and strong bases do not refer to pH, but instead describe whether an acid or base ionizes in solution.

pH can be measured:

* by addition of a pH indicator into the solution under study. The indicator colour varies depending on the pH of the solution. Using indicators, qualitative determinations can be made with universal indicators that have broad colour variability over a wide pH range and quantitative determinations can be made using indicators that have strong colour variability over a small pH range. Precise measurements can be made over a wide pH range using indicators that have multiple equilibriums in conjunction with spectrophotometric methods to determine the relative abundance of each pH-dependent component that make up the colour of solution ,[citation needed] or
* by using a pH meter together with pH-selective electrodes (pH glass electrode, hydrogen electrode, quinhydrone electrode, ion sensitive field effect transistor and others).
* by using pH paper, indicator paper that turns colour corresponding to a pH on a colour key. pH paper is usually strips of paper that has been soaked in an indicator solution, and is used for approximations.

As the pH scale is logarithmic, it does not start at zero. Thus the most acidic of liquids encountered can have a pH as low as −5.[citation needed] The most alkaline typically has pH of 14. Measurement of extremely low pH values has various complications. Calibration of the electrode in such cases can be done with standard solutions of concentrated sulfuric acid whose pH values can be calculated with the Pitzer model.

As an example of home application, the measurement of pH value can be used to quantify the amount of acid in a swimming pool.

An indicator is used to measure the pH of a substance. Common indicators are congo red, phenolphthalein, methyl orange, phenol red, bromothymol blue, bromocresol green and bromocresol purple. To demonstrate the principle with common household materials, red cabbage, which contains the dye anthocyanin, is used.[9]

In addition to red cabbage, some flower petals (such as hibiscus and marigold) impart a bluish stain when crushed onto white paper, and may be used as 'homemade indicators'. Addition of acidic substances will turn the paper red, after which alkaline substances will return it to blue.

The pH of different body fluids, including urine, saliva, and blood, varies with function and other factors. They are mostly tightly regulated systems to keep the acid-base homeostasis. A notable acidic substance in the body is plaque. Plaque's pH is low and will dissolve teeth if not removed. The pH of blood is known to be slightly basic, at a value of 7.4. pH is vital in maintaining the functioning of cells. For example, enzymes are heavily affected by changes in pH, and have an optimum pH at which they operate. Outside a small range they can denature and cease to catalyse vital reactions.


Knowledge is power.
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Posted 10/19/08

Cuddlebuns wrote:

pH is the measure of the acidity or alkalinity of a solution. It is formally a measure of the activity of dissolved hydrogen ions (H+), but for very dilute solutions, the molarity (molar concentration) of H+ may be used as a substitute with little loss of accuracy. In solution, hydrogen ions occur as a number of cations including hydronium ions (H3O+).

In pure water at 25 °C, the concentration of H+ equals the concentration of hydroxide ions (OH-). This is defined as "neutral" and corresponds to a pH level of 7.0. Solutions in which the concentration of H+ exceeds that of OH- have a pH value lower than 7.0 and are known as acids. Solutions in which OH- exceeds H+ have a pH value greater than 7.0 and are known as bases. Because pH is dependent on ionic activity, a property which cannot be measured easily or fully predicted theoretically, it is difficult to determine an accurate value for the pH of a solution. The pH reading of a solution is usually obtained by comparing unknown solutions to those of known pH, and there are several ways to do so.

The concept of pH was first introduced by Danish chemist S. P. L. Sørensen at the Carlsberg Laboratory in 1909. Sørensen suggested the notation "PH" for convenience, standing for "power of hydrogen", using the negative logarithm of the concentration of hydrogen ions in solution.

The operational definition of pH is officially defined by International Standard ISO 31-8 as follows: [6] For a solution X, first measure the electromotive force EX of the galvanic cell

reference electrode | concentrated solution of KCl || solution X | H2 | Pt

and then also measure the electromotive force ES of a galvanic cell that differs from the above one only by the replacement of the solution X of unknown pH, pH(X), by a solution S of a known standard pH, pH(S). The pH of X is then

\text{pH(X)} = \text{pH(S)} + \frac{(E_{\text{S}} - E_{\text{X}})F}{RT \ln 10}

where

F is the Faraday constant;
R is the molar gas constant;
T is the thermodynamic temperature.

Defined this way, pH is a dimensionless quantity. Values pH(S) for a range of standard solutions S, along with further details, are given in the relevant IUPAC recommendation.[7]

pH has no fundamental meaning as a unit; its official definition is a practical one. However in the restricted range of dilute aqueous solutions having an amount-of-dissolved-substance concentrations less than 0.1 mol/L, and being neither strongly alkaline nor strongly acidic (2 < pH < 12), the definition is such that

\text{pH} = -\log_{10}\left[\frac{\gamma_1 [\text{H}^+] }{ \text{1 mol L}^{-1} } \right] \pm 0.02

where [H+] denotes the amount-of-substance concentration of hydrogen ion H+ and γ1 denotes the activity coefficient of a typical univalent electrolyte in the solution.

Simplified definition

pH is a measurement of the concentration of hydrogen ions in a solution. Because of its mathematical formulation, low pH values are associated with solutions with high concentrations of hydrogen ions, while high pH values occur for solutions with low concentrations of hydrogen ions. Pure water has a pH of 7.0, and other solutions are usually described with reference to this value. Acids are defined as those solutions that have a pH less than 7 (i.e. more hydrogen ions than water); while bases are defined as those solutions that have a pH greater than 7 (i.e. less hydrogen ions than water).

pH is a logarithmic scale, that is, the integer part expresses the order of magnitude, e.g. pH 5 has an acidity ten times weaker than pH 4. The fractional part is also logarithmic; pH 4.3 is approximately two times weaker, and pH 4.5 three times.

The definitions of weak and strong acids, and weak and strong bases do not refer to pH, but instead describe whether an acid or base ionizes in solution.

pH can be measured:

* by addition of a pH indicator into the solution under study. The indicator colour varies depending on the pH of the solution. Using indicators, qualitative determinations can be made with universal indicators that have broad colour variability over a wide pH range and quantitative determinations can be made using indicators that have strong colour variability over a small pH range. Precise measurements can be made over a wide pH range using indicators that have multiple equilibriums in conjunction with spectrophotometric methods to determine the relative abundance of each pH-dependent component that make up the colour of solution ,[citation needed] or
* by using a pH meter together with pH-selective electrodes (pH glass electrode, hydrogen electrode, quinhydrone electrode, ion sensitive field effect transistor and others).
* by using pH paper, indicator paper that turns colour corresponding to a pH on a colour key. pH paper is usually strips of paper that has been soaked in an indicator solution, and is used for approximations.

As the pH scale is logarithmic, it does not start at zero. Thus the most acidic of liquids encountered can have a pH as low as −5.[citation needed] The most alkaline typically has pH of 14. Measurement of extremely low pH values has various complications. Calibration of the electrode in such cases can be done with standard solutions of concentrated sulfuric acid whose pH values can be calculated with the Pitzer model.

As an example of home application, the measurement of pH value can be used to quantify the amount of acid in a swimming pool.

An indicator is used to measure the pH of a substance. Common indicators are congo red, phenolphthalein, methyl orange, phenol red, bromothymol blue, bromocresol green and bromocresol purple. To demonstrate the principle with common household materials, red cabbage, which contains the dye anthocyanin, is used.[9]

In addition to red cabbage, some flower petals (such as hibiscus and marigold) impart a bluish stain when crushed onto white paper, and may be used as 'homemade indicators'. Addition of acidic substances will turn the paper red, after which alkaline substances will return it to blue.

The pH of different body fluids, including urine, saliva, and blood, varies with function and other factors. They are mostly tightly regulated systems to keep the acid-base homeostasis. A notable acidic substance in the body is plaque. Plaque's pH is low and will dissolve teeth if not removed. The pH of blood is known to be slightly basic, at a value of 7.4. pH is vital in maintaining the functioning of cells. For example, enzymes are heavily affected by changes in pH, and have an optimum pH at which they operate. Outside a small range they can denature and cease to catalyse vital reactions.


Knowledge is power.


I want you to learn me something new o.o GIR IS ALMIGHTY!

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Finally, the freedom and entertainment of the internet meets the boring monotony of attendance sheets! A match made in heaven.
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what the?
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May you so kindly explain what this is supposed to be about?
Posted 10/19/08

Nyarth wrote:

what the?

couldn't express my thoughts better than you did already
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Great, people come here to forget school and college, you know
Posted 10/19/08
Pointless thread

~locked
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