PH

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In chemistry, pH ( ) is the logarithmic scale used to determine the acidity or alkalinity of an aqueous solution. This is approximately negative from the base logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the base logarithm of the hydrogen ion activity. A solution with a pH less than 7 is acidic and a solution with a pH greater than 7 is a base. Pure water is neutral, at pH 7 (25 Â ° C), not being acidic or alkaline. Contrary to popular belief, pH values ​​can be less than 0 or greater than 14 for very strong acids and bases.

PH measurements are important in agronomy, medicine, chemistry, water treatment, and many other applications.

The pH scale can be traced to a set of standard solutions whose pH is fixed by an international agreement. The primary pH standard value is determined using a concentration cell with transference, by measuring the potential difference between the hydrogen electrode and the standard electrode such as silver chloride electrode. The pH of the aqueous solution can be measured with a glass electrode and pH meter, or indicator.

There are three current theories used to describe acid-base reactions: Arrhenius, Bronsted-Lowry and Lewis when determining pH.

Video PH

Histori

The pH concept was first introduced by Danish chemist SÃÆ'¸ren Peder Lauritz SÃÆ'¸rensen at the Carlsberg Laboratory in 1909 and revised into a modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first paper, the notation has "H" as a subscript to the lowercase "p", such as: p _H .

The exact meaning of "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH means "hydrogen power". He has also suggested that "p" stands for German Potenz (meaning "force"), others refer to French puissance (also means "force", based on the fact that the Carlsberg Laboratory speaks French). Another suggestion is that "p" stands for the Latin term pondus hydrogenii (hydrogen quantity), potentia hydrogenii (hydrogen capacity), or potential hydrogen. It is also suggested that SÃÆ'¸rensen uses the letters "p" and "q" (letters normally matched in mathematics) just to label test solutions (p) and reference solutions (q). Currently in chemistry, p stands for "decimal cologarithm", and is also used in the term p K _a , used for acid dissociation constants.

Bacteriologist Alice C. Evans, best known for his work influences on food-generating and food safety jobs, credited William Mansfield Clark and his colleagues (among whom he was one) by developing a pH measurement method in the 1910s, with extensive influence on laboratory and use industry afterwards. In his memoirs, he did not say how many, or how little, Clark and his colleagues knew about SÃÆ'¸rensen's work several years earlier. He says:

In this study [bacterial metabolism]. Clark is directed at the acid effect on bacterial growth. He found that it was the intensity of the acid in the concentration of hydrogen ions that affected their growth. But the method of measuring the acidity that exists determines the quantity, not the intensity, of the acid. Next, along with his collaborator, Dr. Clark developed an accurate method for measuring the concentration of hydrogen ions. These methods replace inaccurate titration methods in determining the acid content used in biological laboratories around the world. Also they are found to apply in many industrial and other processes where they come to widespread use.

The first electronic method for measuring pH was created by Arnold Orville Beckman, a professor at the California Institute of Technology in 1934. This was in response to the local Sunkist citrus farmers who wanted a better method to quickly test their chosen lemon pH from nearby gardens.

Maps PH

Definitions and measurements

pH

pH didefinisikan sebagai logaritma desimal dari kebalikan dari aktivitas ion hidrogen, a _H , dalam sebuah solusi.

$            {\displaystyle                   \text{pH}                =        -        {}_{}}$ log                         10                                       (                     a                                                         H                                                                                                              )          =                     log                         10                                                  (                                        1                                 a                                                                                 H                                                                                                                                                                                  )                           {\ displaystyle {\ ce {pH}} = - \ log _ {10} (a _ {{\ ce {H }}}) = \ log _ {10} \ kiri ({\ frac {1} {a _ {{\ ce {H }}}}} \ right)}   

For example, a solution with the activity of hydrogen ions 5ÃÆ' â € "10 ^-6 = 1/(2ÃÆ' â €" 10 ⁵ ) (at that level basically the number of moles of hydrogen ions per liter of solution) has a pH log ₁₀ (2ÃÆ' â € "10 ⁵ ) = 5.3 . For a typical example based on the fact that the mass of one mole of water, one mole of hydrogen ion, and one mole of hydroxide ions are 18 g, 1 g, and 17 g, each of 10 pure moles of water pH 7), or 180 tons (18ÃÆ'â € "10 ⁷ g), containing nearly 1 g of dissociated hydrogen ion (or more precisely 19 g H ₃ O hydronium ion) and 17 g of hydroxide ions.

Note that the pH depends on the temperature. For example at 0 Â ° C the pH of pure water is 7.47. At 25 Â ° C it is 7.00, and at 100 ° C it is 6.14.

Definisi ini diadopsi karena elektroda selektif ion, yang digunakan untuk mengukur pH, merespons aktivitas. Idealnya, potensial elektroda, E , mengikuti persamaan Nernst, yang, untuk ion hidrogen dapat ditulis sebagai

${\ displaystyle E = E ^ {0} {\ frac {RT} {F}} \ ln (a _ {{\ ce {H }}}) = E ^ {0} - {\ frac {2.303RT} {F}} {\ ce {pH}}}  ÂÂ$

where E is a measurable potential, E ⁰ is a standard electrode potential, R is a gas constant, T is the temperature in kelvin, F is the Faraday constant. For H the number of electrons transferred is one. It follows that the electrode potential is proportional to the pH when pH is defined in terms of activity. Appropriate pH measurements are presented in International Standard ISO 31-8 as follows: A galvanic cell is formed to measure electromotive force (emf) between reference electrodes and electrodes that are sensitive to hydrogen ion activity when both are immersed in the same aqueous solution. The reference electrode may be a silver chloride electrode or a calomel electrode. The hydrogen-ion selective electrode is a standard hydrogen electrode.

Reference electrode | concentrated solution of KCl | | test solution | H ₂ | Pt

Pertama, sel diisi dengan larutan dari aktivitas ion hidrogen yang dikenal dan emf, E _S , diukur. Kemudian emf, E _X , dari sel yang sama yang berisi larutan pH yang tidak diketahui diukur.

${\ displaystyle {\ ce {pH (X)}} = {\ ce {pH (S)}} {\ frac {E _ {{\ ce {S}} } -E _ {{\ ce {X}}}} {z}}}  ÂÂ$

Perbedaan antara dua nilai emf terukur sebanding dengan pH. Metode kalibrasi ini menghindari kebutuhan untuk mengetahui potensi elektroda standar. Konstanta proporsionalitas, 1/ z secara ideal sama dengan ${\ displaystyle {\ frac {1} {2.303RT/F}} \}  ÂÂ$ "Lereng Nernstian".

To apply this process in practice, a glass electrode is used rather than a complicated hydrogen electrode. A combined glass electrode has a built-in reference electrode. It is calibrated to the buffer solution of the known hydrogen ion activity. IUPAC has proposed the use of a set of buffer solutions from known H activities. Two or more buffer solutions are used to accommodate the fact that the "slant" may be slightly different from the ideal. To apply this approach to calibration, the electrode is first dipped in standard solution and the readings at the pH meter are adjusted to match the standard buffer value. The readings from the second standard buffer solution are then adjusted, using a "slant" control, to match the pH for the solution. Further details, given in the IUPAC recommendations. When more than two buffer solutions are used, the electrode is calibrated by adjusting the observed pH value to a straight line with respect to the standard buffer value. Standard commercial buffer solutions usually come with information about values ​​at 25 ° C and correction factors to apply to other temperatures.

The pH scale is logarithmic and hence the pH is a dimensionless quantity.

p [H]

This is the original definition of SÃÆ'¸rensen, which replaced the pH in 1909. However, it is possible to measure the concentration of the hydrogen ions directly, if the electrode is calibrated in terms of the hydrogen ion concentration. One way of doing this, which has been used extensively, is to titrate the known strong acid concentration solution with a known base-concentration solution in the presence of relatively high background electrolyte concentrations. Since acid and base concentrations are known, it is easy to calculate the hydrogen ion concentration so that measurable potential can be correlated with concentration. Calibration is usually done using Gran plot. The calibration yields a standard electrode potential value, E ⁰ , and a slope factor, f , so the Nernst equation in the form

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂEÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â=Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂE^{Â\; Â\; Â\; Â\; ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â}Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂfÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\frac{Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\mathrm{2,303}Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂRÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂTÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â}{}Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â}$ log                [                       ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂï m  H                                                             ]               {\ displaystyle E = E ^ {0} f {\ frac {2.303RT} {F}} \ log [{\ ce {H}}]}  Â

can be used to decrease the hydrogen ion concentration from experimental measurements E . The slope factor, f , is usually slightly less than one. The slope factor of less than 0.95 indicates that the electrode is not working properly. The presence of a background electrolyte ensures that the hydrogen ion activity coefficient is effectively constant during the titration. Because it is constant, its value can be set to one by defining the default state as a solution containing the background electrolyte. Thus, the effect of using this procedure is to create an activity equal to the numerical value of the concentration.

Glass electrodes (and other ion selective electrodes) should be calibrated in media similar to those under investigation. For example, if one wants to measure the pH of seawater samples, the electrode should be calibrated in a solution that resembles sea water in its chemical composition, as described below.

The difference between p [H] and pH is quite small. It has been stated that pH = p [H] 0.04. It is common practice to use the term "pH" for both types of measurement.

pH indicator

Indicators can be used to measure pH, taking advantage of the fact that their colors change with pH. A visual comparison of the color of the test solution with a standard color chart provides a means for measuring accurate pH to the nearest integer. More precise measurements are possible if the color is measured by spectrophotometry, using a colorimeter or spectrophotometer. The universal indicator consists of a mixture of indicators so that there are continuous color changes from about pH 2 to pH 10. The universal indicator paper is made of absorbent paper impregnated with universal indicators. Another method of measuring pH is using an electronic pH meter.

pOH

pOH kadang-kadang digunakan sebagai ukuran konsentrasi ion hidroksida. OH ^- . nilai pOH berasal dari pengukuran pH. Konsentrasi ion hidroksida dalam air berhubungan dengan konsentrasi ion hidrogen oleh

${\ displaystyle [{\ ce {OH ^ -}}] = {\ frac {K _ {{\ ce {W}}}} {[{\ ce {H}} }}]}}}  ÂÂ$

di mana K _W adalah konstanta self-ionisasi air. Mengambil logaritma

${\ displaystyle {\ ce {pOH}} = {\ ce {p}} K _ {{\ ce {W}}} - {\ ce {pH}}}  ÂÂ$

So, at room temperature, pOH? 14 - pH. However this relationship does not fully apply in other circumstances, as in the measurement of soil alkalinity.

Extreme pH

The pH measurement is below about 2.5 (about 0.003 mol dm ^-3 acid) and above about 10.5 (ca. 0.0003Ã, mol dm ^-3 alkaline) requires special procedures because, when using glass electrodes, Nernst law is damaged under these conditions. Various factors contribute to this. It can not be assumed that the potential for a liquid junction is independent of pH. Also, extreme pH implies that the solution is concentrated, so that the electrode potential is affected by the variation of ionic strength. At high pH glass electrodes can be affected by "alkaline error", because the electrodes are sensitive to cation concentrations such as Na and K in solution. Specially built electrodes are available that partially solve this problem.

Runoff from mine or mine tailings can produce very low pH values.

The solution is not watery

Concentration of hydrogen ions (activity) can be measured in non-aqueous solvents. the pH value based on this measurement belongs to a different scale than the aqueous pH value, since the activity relates to different standard states. The activity of hydrogen ions, a _H , can be defined as:

${\ displaystyle a _ {{\ ce {H}}} = \ exp \ left ({\ frac {\ mu_ {{\ ce {H}} } - \ mu _ {{\ ce {H}}} ^ {\ ominus}} {RT}} \ right)} Â Â$

di mana ? _H adalah potensi kimia dari ion hidrogen, ${\ displaystyle \ mu _ {{\ ce {H }}} ^ {\ ominus}}  ÂÂ$ adalah potensi kimianya dalam keadaan standar yang dipilih, R adalah konstanta gas dan T adalah termodinamika suhu. Oleh karena itu, nilai pH pada skala yang berbeda tidak dapat dibandingkan secara langsung karena ion proton terlarut yang berbeda seperti ion lyonium, membutuhkan skala intersolvent yang melibatkan koefisien aktivitas transfer ion hidronium/lyonium.

pH is an example of acidity function. Other acidity functions can be defined. For example, the Hammet acidity function, H ₀ , has been developed with respect to superacids.

Uniform pH absolute scale

The concept of "integrated pH scale" has been developed on the basis of the absolute chemical potential of protons. This model uses Lewis's acid-base definition. This scale applies to liquids, gases and even solids. In 2010, a new "integrated absolute pH scale" has been proposed that will allow a wide range of pH range in different solutions to use common proton reference standards.

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Apps

Pure water is neutral. When the acid is dissolved in water, the pH will be less than 7 (25 Â ° C). When a base, or alkali, is dissolved in water, the pH will be greater than 7. A strong acid solution, such as hydrochloric acid, at a concentration of 1 mol dm ^-3 has a pH of 0. A strong alkaline solution, such as sodium hydroxide, at a concentration of 1 mol dm ^-3 , has a pH of 14. Thus, the measured pH value will lie largely in the range of 0 to 14, although negative pH values ​​and values ​​above 14 are quite possible. Since pH is a logarithmic scale, the difference of one pH unit is equivalent to a tenfold difference in hydrogen ion concentration.
PH neutrality is not exactly 7 (25 ° C), although this is a good approximation in most cases. Neutrality is defined as the condition in which [H ] = [OH ^- ] (or similar activity). Since the self-ionization of water holds the product from this concentration [H ] ÃÆ'â € "[OH ^- ] = K _w , it can see that at neutrality [H ] = [OH ^- ] = ? K _w , or pH = pK _w /2. pk _w approx 14 but depending on ionic strength and temperature, and also pH neutrality as well. Pure water and NaCl solution in pure water are both neutral, since dissociation of water produces the same amount of ions. However, the pH of neutral NaCl solution will be slightly different from neutral pure water because the activity of hydrogen ions and hydroxide depends on ionic strength, so K _w varies with ionic strength.

Jika air murni terpapar udara, ia menjadi sedikit asam. Ini karena air menyerap karbon dioksida dari udara, yang kemudian secara perlahan diubah menjadi ion bikarbonat dan hidrogen (pada dasarnya menciptakan asam karbonat).

${\ displaystyle {\ ce {CO2 H2O & lt; = & gt; HCO3 ^ - H }}}  ÂÂ$

pH dalam tanah

Klasifikasi rentang pH tanah

Natural Resources Conservation Service of the United States Department of Agriculture, formerly the Soil Conservation Service classified the range of soil pH as follows:

pH in nature

plant pigment-dependent pH that can be used as an indicator of pH occurs in many plants, including hibiscus, red cabbage (anthocyanin) and red wine. Orange juice juice is acidic mainly because it contains citric acid. Other carboxylic acids occur in many living systems. For example, lactic acid is produced by muscle activity. The state of protonation of phosphate derivatives, such as ATP, is dependent on pH. The hemoglobin function of the oxygen transport enzyme is affected by the pH in a process known as the Root effect.

Sea Water

The seawater pH is usually limited to a range between 7.5 and 8.4. It plays an important role in the ocean carbon cycle, and there is evidence of sustained ocean acidification caused by carbon dioxide emissions. However, pH measurements are complicated by the chemical properties of seawater, and several different pH scales exist in chemical oceanography.

As part of the operational definition of pH scale, IUPAC defines a series of buffer solutions at various pH values ​​(often denoted by NBS or NIST determination). This solution has relatively low ionic strength (? 0,1) compared with seawater (? 0,7), and, as a result, is not recommended for use in characterizing the pH of sea water, since ionic strength difference causes the change of electrode potential. To address this problem, a range of alternative buffers based on artificial marine water was developed. This new series solves the problem of ionic power differences between samples and buffers, and the new pH scale is referred to as 'total scale', often denoted as pH _T . The total scale is defined using a medium containing sulfate ions. These ions undergo protonation, H SO ^{2 -} < br> ₄ ? HSO ^-
₄ , so the total scale includes the effects of both protons (hydrogen free ions) and hydrogen sulfate ions:

[H ] _T = [H ] _F [HSO ^-
₄ ]

The alternative scale, 'free scale', often denoted 'pH _F ', omits this consideration and focuses only on [H ] _F , in principle making it a simpler representation of the hydrogen ion concentration. Only [H ] _T can be determined, therefore [H ] _F should be estimated using [SO ^{2 -}
₄ ] and the stability constants HSO ^-
₄ , K ^*
_S :

[H ] _F = [H ] _T - [HSO < sup style = "font-size: inherit; line-height: inherit; vertical-align: baseline"> -
₄ (1 [SO ^{2 -}
₄ ]/K ^* _S ) ^-1

However, it is difficult to estimate K ^* _S in seawater, limiting the utility of the otherwise free scales easier.

Another scale, known as the 'sea-water scale', is often denoted 'pH _SWS ', taking into account the further protonation relations between hydrogen ions and fluoride ions, H F ^- ? HF. Generate the following expression for [H ] _SWS :

[H ] _SWS = [H ] _F [HSO ^-
₄ ] [HF]

However, the advantage of considering this additional complexity depends on the abundance of fluoride in the medium. In seawater, for example, sulfate ions occur at much greater concentrations (& gt; 400 times) than with fluoride. As a result, for most practical purposes, the difference between the total scale and the seawater is very small.

Tiga persamaan berikut merangkum tiga skala pH:

pH _F = - log [H ] _F

pH _T = - log ([H ] _F [HSO ^-
₄ ]) = - log [H ] _T

pH _SWS = - log ([H ] _F [HSO ^-
₄ ] [HF]) = - log [H ] _SWS

In practical terms, three sea water pH scales differ in value up to 0.12 pH units, a far greater difference than the accuracy of pH measurements usually required, in particular, in relation to marine carbonate systems. Because it eliminates the consideration of sulfate and fluoride ions, the free scale is significantly different from the total scale and the seawater. Because relatively insignificant of the fluoride ion, the total scale and seawater differ only very slightly.

System life

Different cellular PH compartments, body fluids, and organs are usually strictly regulated in a process called acid-base homeostasis. The most common disorder in acid-base homeostasis is acidosis, which means excess acid in the body, generally determined by a pH that falls below 7.35. Alkalosis is the opposite condition, with too high blood pH.

The blood PH is usually slightly basic with a pH value of 7.365. This value is often referred to as physiological pH in biology and medicine. Plaque can create a local acidic environment that can cause tooth decay by demineralization. Enzymes and other proteins have an optimum pH range and may become inactivated or denatured beyond this range.

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Calculation of pH

Calculation of acid-and/or acid-based pH solutions is an example of calculating chemical speciation, that is, a mathematical procedure for calculating the concentration of all chemical species present in the solution. The complexity of the procedure depends on the nature of the solution. For strong and alkaline acids there is no need for calculation except in extreme situations. The pH of a solution containing a weak acid requires a solution of the quadratic equation. The pH of a solution containing a weak base may require the solution of a cubic equation. The general case requires a solution of a set of non-linear simultaneous equations.

Faktor yang menyulitkan adalah bahwa air itu sendiri adalah asam lemah dan basa lemah (lihat amfoterisme). Ia berdisosiasi menurut kesetimbangan

${\ displaystyle {\ ce {2H2O & lt; = & gt; {H3O (aq)} {OH ^ {-} (aq)}}}}  ÂÂ$

dengan konstanta disosiasi, K _w didefinisikan sebagai

$            {\displaystyle         K_{            w          }        =                              [            {\text{H}}^{                              }            ]                                [            \text{OH}     ÂÂ}$

Source of the article : Wikipedia