A buffer solution is a type of solution that resists changes in pH when small amounts of acid or base are added. Pure water has a neutral pH of 7.0 at 25°C, but when left exposed to air, it absorbs carbon dioxide from the atmosphere, forming carbonic acid and lowering the pH to around 5.5. This shows how sensitive pure water is to even minor environmental changes.
If just one drop of concentrated hydrochloric acid (approximately 12.4 mol/L) is added to 1 liter of pure water, the concentration of hydrogen ions ([Hâº]) increases dramatically—from 1.0×10â»â· mol/L to about 5×10â»â´ mol/L, which is an increase of nearly 5,000 times. Similarly, adding one drop of a concentrated sodium hydroxide solution (also around 12.4 mol/L) to 1 liter of pure water can cause a pH change of about 3 units. These examples clearly demonstrate how easily the pH of pure water is affected by even a small amount of strong acid or base.
In contrast, if the same amount of hydrochloric acid is added to a buffer solution such as a mixture of acetic acid (HOAc) and sodium acetate (NaOAc), or a mixture of sodium dihydrogen phosphate (NaH₂PO₄) and disodium hydrogen phosphate (Na₂HPO₄), the pH change is minimal. This is because buffer solutions contain both a weak acid and its conjugate base (or a weak base and its conjugate acid), which can neutralize added H⺠or OH⻠ions, thereby maintaining a relatively stable pH. This ability to resist pH changes is known as the buffering effect, and it is essential in many biological and chemical processes.
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