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Date: Sunday 22nd July 2018 at 03:46am

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Learning On Acid, Base,ph And Buffer

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Learning On Acid, Base,ph And Buffer Posted by Nollyrex(m): 03:09pm On 10 Oct 2017
Acids and bases

Solutions are classified as acidic or basic based on their hydrogen ion concentration relative to pure water. Acidic solutions have a higher H+^+​+​​start superscript, plus, end superscript concentration than water (greater than 1 × 10−7^{-7}​−7​​start superscript, minus, 7, end superscript M), while basic (alkaline) solutions have a lower H+^+​+​​start superscript, plus, end superscript concentration (less than 1 × 10−7^{-7}​−7​​start superscript, minus, 7, end superscript M). Typically, the hydrogen ion concentration of a solution is expressed in terms of pH. pH is calculated as the negative log of a solution’s hydrogen ion concentration:

pH =−log​10​​[H^+]

The square brackets around the H^+​ just mean that we are referring to its concentration. If you plug the hydrogen ion concentration of water (1 × 10−7^{-7}​−7​​start superscript, minus, 7, end superscript M) into this equation, you’ll get a value of 7.0, also known as neutral pH. In the human body, both blood and the cytosol (watery goo) inside of cells have pH values close to neutral.

H^+​ concentration shifts away from neutral when an acid or base is added to an aqueous (water-based) solution. For our purposes, an acid is a substance that increases the concentration of hydrogen ions (H^+​) in a solution, usually by donating one of its hydrogen atoms through dissociation. A base, in contrast, raises pH by providing hydroxide (OH^-) or another ion or molecule that scoops up hydrogen ions and removes them from solution. (This is a simplified definition of acids and bases that works well for thinking about biology. You may want to visit the chemistry section to see other acid-base definitions.)

The stronger the acid, the more readily it dissociates to generate H^+​. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions when it is placed in water, so it is considered a strong acid. The acids in tomato juice or vinegar, on the other hand, do not completely dissociate in water and are considered weak acids. Similarly, strong bases like sodium hydroxide (NaOH) completely dissociate in water, releasing hydroxide ions (or other types of basic ions) that can absorb H^+​.

The pH scale

The pH scale is used to rank solutions in terms of acidity or basicity (alkalinity). Since the scale is based on pH values, it is logarithmic, meaning that a change of 1 pH unit corresponds to a ten-fold change in H^+​ ion concentration. The pH scale is often said to range from 0 to 14, and most solutions do fall within this range, although it’s possible to get a pH below 0 or above 14. Anything below 7.0 is acidic, and anything above 7.0 is alkaline, or basic.


_Image modified from "Water: Figure 7," by OpenStax College, Biology, CC BY 4.0._Modification of work by Edward Stevens.*

The pH inside human cells (6.8) and the pH of blood (7.4) are both very close to neutral. Extreme pH values, either above or below 7.0, are usually considered unfavorable for life. However, the environment inside your stomach is highly acidic, with a pH of 1 to 2. How does the stomach get around this problem? The answer: disposable cells! Stomach cells, particularly those that come in direct contact with stomach acid and food, are constantly dying and being replaced by new ones. In fact, the lining of the human stomach is completely replaced about every seven to ten days.

Buffers

Most organisms, including humans, need to maintain pH within a fairly narrow range in order to survive. For instance, human blood needs to keep its pH right around 7.4, and avoid shifting significantly higher or lower – even if acidic or basic substances enter or leave the bloodstream.

Buffers, solutions that can resist changes in pH, are key to maintaining stable H^+​ ion concentrations in biological systems. When there are too many H^+​ ions, a buffer will absorb some of them, bringing pH back up; and when there are too few, a buffer will donate some of its own H^+​ ions to reduce the pH. Buffers typically consist of an acid-base pair, with the acid and base differing by the presence or absence of a proton (a conjugate acid-base pair).

For instance, one of the buffers that maintain the pH of human blood involves carbonic acid (H2_2​2​​start subscript, 2, end subscriptCO3_3​3​​start subscript, 3, end subscript) and its conjugate base, the bicarbonate ion (HCO3_3​3​​start subscript, 3, end subscript−^-​−​​start superscript, minus, end superscript). Carbonic acid is formed when carbon dioxide enters the bloodstream and combines with water, and it is the main form in which carbon dioxide travels in the blood between the muscles (where it’s generated) and the lungs (where it’s converted back into water and CO2_2​2​​start subscript, 2, end subscript, which is released as a waste product).


Image modified from "Water: Figure 8," by OpenStax College, Biology, CC BY 4.0.

If too many H^+​ ions build up, the equation above will be pushed to the right, and bicarbonate ions will absorb the H^+​ to form carbonic acid. Similarly, if H+^+​+​​start superscript, plus, end superscript concentrations drop too low, the equation will be pulled the left and carbonic acid will turn into bicarbonate, donating H^+​ ions to the solution. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy.


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