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Water is the most important solvent and a very weak electrolyte
.
Very small part of the water molecule dissociation generates H + (hydronium ions in water to H .
When the dissociation reaches equilibrium, H 2 O, H + and OH - satisfy the relationship
KW [Theta] = [H + ] [OH - ]
Since Kw Θ is the product of ion concentration, it is called the ion product constant of water
.
Solutions from water is very small, equilibrium system [H + ] and [OH - ] are very small
The dissociation reaction of water is an endothermic reaction, so KwΘ will increase with the increase of temperature, but since this change is not obvious, it is considered that Kw Θ =1.
0×10 -14 within a certain temperature range
.
Also note that the Kw [Theta] is the standard solution of water from the reaction equilibrium constant, so the expression of [H + ] and [OH - ] concentration is relative, i.
e.
Kw [Theta] expression should
For the convenience of writing, the standard form of concentration is often omitted, and concentration is used instead of relative concentration
.
In this chapter and some subsequent chapters, this method of replacing relative concentration with concentration is adopted, but it must be noted that the expression of the standard equilibrium constant should be the relative concentration
For any aqueous solution system, H 2 O, H + and OH - are always in equilibrium, no matter whether the system is acidic, alkaline or neutral, it always satisfies the relation Kw Θ =[H + ][OH - ]
.
Acidic solution, [H + ]> [OH - ]; the basic solution [OH - ]> [H + ]; solution of [H + ] = [OH - ], the solution is neutral
In an aqueous solution, the acidity and alkalinity of the solution can be expressed by the concentration of H+ and OH- in the solution
.
If the temperature does not change, the value of Kw Θ does not change, so when the concentration of one ion in H + and OH - is known , the concentration of the other ion can be calculated, and then the acidity and alkalinity of the solution can be judged.
Usually use pH to indicate the strength of the solution's acidity and alkalinity
.
Here p represents an operation, that is, take a negative logarithm for a value of dimension 1
pH=-lg[H + ]
= -LG the pOH [OH - ]
At room temperature
Kw Θ =[H + ][OH - ]=1.
0×10 -14
So there is
pK w Θ =pH+pOH=14
At room temperature, when the solution is neutral, [H + ] = [OH - ],.
7 = the pH
.
Since Kw Θ is not greatly affected by temperature, in general, when pH=7, the solution is neutral; when pH>7, the solution is alkaline, and when pH<7, the solution is acidic