In chemistry, pH is a logarithmic scale used to determine the acidity of a medium. This is an approximately negative logarithm at the base of 10 molar concentration, measured in units of moles per liter of hydrogen ions. It can also be called an indicator of the acidity of the medium. More precisely, it is the negative logarithm of base 10 activity of a hydrogen ion. At 25 ° C, solutions with a pH of less than 7 are acidic, and solutions with a pH of more than 7 are basic. The neutral pH is temperature dependent and is less than 7 with increasing temperature. Pure water is neutral, pH = 7 (at 25 ° C), is neither acid nor alkali. Contrary to popular belief, the pH may be less than 0 or greater than 14 for very strong acids and bases, respectively.
Application
PH measurements are important in agronomy, medicine, chemistry, water treatment and many other fields.
The pH scale is relevant for a set of standard solutions, the acidity of which is established by international agreement. Primary standard pH values are determined using a transported concentration cell by measuring the potential difference between a hydrogen electrode and a standard electrode such as silver chloride. The pH of aqueous solutions can be measured with a glass electrode and a pH meter or indicator.
Opening
The pH concept was first introduced by Danish chemist Søren Peter Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to its current pH level in 1924 to adapt definitions and measurements in terms of electrochemical cells. In the first papers, the notation had the letter H in the lower case p, which means pH.
origin of name
The exact meaning of the letter p is disputed, but according to the Carlsberg Foundation, pH means "hydrogen power." It has also been suggested that p means the German word potenz (“power”), others refer to the French Puisance (also means “power”, based on the fact that Carlsberg’s laboratory was French). Another assumption is that p refers to the Latin term pondus hydroii (amount of hydrogen), potentio hydroii (capacity of hydrogen) or potential hydroli (hydrogen potential). It is also assumed that Serensen used the letters p and q (usually conjugate letters in mathematics) simply to indicate the test solution (p) and the reference solution (q). At present, in chemistry, p stands for the decimal logarithm, and is also used in the term pKa, used for the dissociation constants of the acidity of the medium.
American Contribution
The bacteriologist Alice Evans, known for the influence of her work on dairy products and food safety, thanked William Mansfield Clark and his colleagues for the development of pH measurement methods in the 1910s, which subsequently had a wide impact on laboratory and industrial use. In her memoirs, she does not mention how much or how little Clark and his colleagues knew about Sorensen's work a few years before. Already at that time, scientists actively studied the acidity / alkalinity of the medium.
Acid effect
Dr. Clark's attention was directed to the effect of acid on bacterial growth. And thanks to this, he added to the idea of the then science of the hydrogen indicator of the acidity of the medium. He found that it is the intensity of the acid in terms of the concentration of hydrogen ions that affects their growth. But existing methods for measuring the acidity of the medium determined the amount, not the intensity of the acid. Dr. Clark and his colleagues then developed accurate methods for measuring the concentration of hydrogen ions. These methods have replaced the inaccurate titration method for determining acid content in biological laboratories around the world. It has also been found that they can be used in many industrial and other processes in which they are widely used.
Practical aspect
The first electronic pH measurement method was invented by Arnold Orville Beckman, a professor at the California Institute of Technology, in 1934. It was at this point that the local citrus producer Sunkist wanted the best method for quickly checking the pH of the lemons they collected in nearby gardens. The influence of the acidity of the medium was always taken into account.
For example, for a solution with an activity of hydrogen ions of 5 × 10 –6 (at this level, this is, in fact, the number of moles of hydrogen ions per liter of solution), we obtain 1 / (5 × 10 –6 ) = 2 × 105. Thus, such the solution has a pH of 5.3. It is believed that the masses of a mole of water, a mole of hydrogen ions and a mole of hydroxide ions are 18 g, 1 g and 17 g, respectively, the amount of 107 pure moles (pH 7) of water contains about 1 g of dissociated hydrogen ions (or, more precisely, 19 g of ions H 3 O + hydronium) and 17 g of hydroxide ions.
Temperature role
Please note that pH is temperature dependent. For example, at 0 ° C the pH of pure water is 7.47. At 25 ° C - 7, and at 100 ° C - 6.14.
The electrode potential is proportional to pH when the pH is determined in terms of activity. Accurate pH measurements are presented in the international standard ISO 31-8.
A galvanic cell is configured to measure an electromotive force (EMF) between a reference electrode and an electrode that is sensitive to the activity of hydrogen ions when they are both immersed in the same aqueous solution. The reference electrode may be an object of silver chloride or calomel electrode. A hydrogen-ion selective electrode is standard for such operations.
To put this process into practice, a glass electrode is used, not a bulky hydrogen electrode. It has a built-in reference electrode. It is also calibrated against buffer solutions with known activity of hydrogen ions. IUPAC proposed the use of a set of buffer solutions with known H + activity. Two or more buffer solutions are used to take into account the fact that the slope may be slightly different than ideal. To implement this calibration approach, the electrode is first immersed in a standard solution and the pH meter readings are set equal to the standard buffer value.
What's next?
The reading from the second standard buffer solution is then adjusted using the tilt control to be equal to the pH level for this solution. When more than two buffer solutions are used, the electrode is calibrated by fitting the observed pH values to a straight line relative to the standard buffer values. Commercial standard buffers usually come with information about the value at 25 ° C and the correction factor that should be applied for other temperatures.
Characteristic Definition
The pH scale is logarithmic and, therefore, pH is a dimensionless quantity, often used also for measuring the acidity of the cell’s internal environment. That was the original definition of Sorensen, which was replaced in 1909.
However, the concentration of hydrogen ions can be directly measured if the electrode is calibrated in terms of the concentration of hydrogen ions. One way to do this, which has been widely used, is to titrate a solution of a known concentration of strong acid with a solution of a known concentration of strong alkali in the presence of a relatively high concentration of background electrolyte. Since acid and alkali concentrations are known, it is easy to calculate the concentration of hydrogen ions so that the potential can be correlated with the measured value.
Indicators can be used to measure pH using the fact that their color is changing. Visual comparison of the color of the test solution with a standard color scale allows you to measure pH to the nearest integer. More accurate measurements are possible if the color is measured spectrophotometrically using a colorimeter or spectrophotometer. The universal indicator consists of a mixture of indicators, so that a constant color change occurs from about pH 2 to pH 10. The universal indicator paper is made of absorbent paper that has been impregnated with a universal indicator. Another method of measuring pH is the use of an electronic pH meter.
Measurement levels
Measuring a pH below about 2.5 (about 0.003 mol of acid) and above about 10.5 (about 0.0003 mol of alkali) requires special procedures, because when using a glass electrode the Nernst law is violated at these values. Various factors contribute to this. It cannot be assumed that the potentials of the liquid transition are pH independent. In addition, extreme pH means that the solution is concentrated, therefore, changes in ionic strength affect the potentials of the electrodes. At high pH, the glass electrode may be susceptible to alkaline error, since the electrode becomes sensitive to the concentration of cations such as Na + and K + in solution. Specially designed electrodes are available that partially overcome these problems.
Runoff from mines or mine waste can lead to very low pH values.
Pure water is neutral. It is not an acidic medium. When the acid dissolves in water, the pH will be below 7 (25 ° C). When the alkali dissolves in water, the pH will be greater than 7. A solution of a strong acid, such as hydrochloric acid, at a concentration of 1 mol has a pH of zero. A solution of strong alkali, such as sodium hydroxide, at a concentration of 1 mol has a pH of 14. Thus, the measured pH values will generally lie in the range from 0 to 14, although negative pH values and values above 14 are quite possible.
Much depends on the acidity of the solution. Since pH is a logarithmic scale, a difference of one pH unit is equivalent to a ten-fold difference in the concentration of hydrogen ions. PH neutrality does not quite reach 7 (at 25 ° C), although in most cases this is a good approximation. Neutrality is defined as a condition in which [H +] = [OH-]. Since the self-ionization of water keeps the product of these concentrations [H +] × [OH-] = Kw, it can be seen that with neutrality [H +] = [OH−] = √Kw or pH = pKw / 2.
PKw is approximately 14, but depends on ionic strength and temperature, therefore, the pH of the medium is also ph, which should be at a neutral level. Pure water and a solution of NaCl in pure water are neutral, since the dissociation of water produces the same amount of both ions. However, the pH of a neutral NaCl solution will be slightly different from the pH of neutral pure water, since the activity of hydrogen and hydroxide ions depends on the ionic strength, so Kw varies depending on the ionic strength.
Plants
Dependent plant pigments that can be used as pH indicators are found in many plants, including hibiscus, red cabbage (anthocyanin), and red wine. Citrus juice is acidic because it contains citric acid. Other carboxylic acids are found in many living systems. For example, lactic acid is produced by muscle activity. The state of protonation of phosphate derivatives such as ATP depends on the pH of the medium. The functioning of the hemoglobin oxygen transfer enzyme is affected by pH in a process known as the root effect.
Sea water
In seawater, the pH is usually limited to a range of 7.5 to 8.4. It plays an important role in the carbon cycle in the ocean, and there is evidence of constant ocean acidification caused by carbon dioxide emissions. However, pH measurement is complicated by the chemical properties of seawater, and in chemical oceanography there are several different pH scales.
Special solutions
As part of the on-line determination of the medium acidity scale (pH), IUPAC defines a series of buffer solutions in the pH range (often referred to as NBS or NIST). These solutions have a relatively low ionic strength (≈0.1) compared to seawater (≈0.7) and, as a result, are not recommended for use in characterizing the pH of seawater, since differences in ionic strength cause changes in the electrode potential. To solve this problem, an alternative series of buffers based on artificial sea water has been developed.
This new series addresses the problem of differences in ionic strength between samples and buffers, and the new pH scale for medium acidity is called the general scale, often referred to as pHT. The overall scale was determined using a medium containing sulfate ions. These ions experience protonation, H + + SO2-4 ⇌ HSO-4, so the overall scale includes the influence of both protons (free hydrogen ions) and hydrogen sulfide ions:
[H +] T = [H +] F + [HSO-4].
An alternative free scale, often referred to as pHF, omits this consideration and focuses solely on [H +] F, making it in principle a simpler representation of the concentration of hydrogen ions. Only [H +] T can be determined; therefore, [H +] F should be estimated using [SO2-4] and the stability constant HSO-4, K * S:
[H +] F = [H +] T - [HSO-4] = [H +] T (1 + [SO2-4] / K * S) -1.
However, it is difficult to evaluate K * S in seawater, limiting the usefulness of a simpler free scale.
Another scale, known as the sea water scale, often called pHSWS, takes into account the further proton bond between hydrogen ions and fluoride ions, H + + F- ⇌ HF. The result is the following expression for [H +] SWS:
[H +] SWS = [H +] F + [HSO-4] + [HF]
However, the advantage of considering this additional complexity depends on the fluorine content in the medium. For example, in seawater sulfate ions are found in much higher concentrations (> 400 times) than fluorine concentrations. As a result, for most practical purposes, the difference between the general scale and the scale of sea water is very small.
The following three equations summarize three pH scales:
pHF = - log [H +] FpHT = - log ([H +] F + [HSO-4]) = - log [H +] TpHSWS = - log ([H +] F + [HSO-4] + [HF]) = - log [H +]
From a practical point of view, the three pH scales of the acidic medium (or sea water) differ in their values to 0.12 pH units, and the differences are much larger than is usually required for the accuracy of pH measurements, in particular with respect to the carbonate system of the ocean.