In the formula above we can input the carbonate hardness and the amount of CO₂ in mg/l to calculate the corresponding pH value.Īs example, for a carbonate hardness of 8° and an amount of 10mg/l CO₂ a pH-value of 7,4 is present in the tank. Therefore the pH-value can also be expressed with the carbonate hardness: pH = 6.35 + log(15.664 * dKH / CO₂) The hydrogencarbonate HCO₃- is exactly the salt that defines the german carbonate hardness dKH. The relationship with carbonate hardness (dKH) PH-values above 6.35 result from more hydrogencarbonate HCO₃- than H₂CO₃ and vice versa. If the concentration of and is of equal order the formula can be solved to: pH = 6.35 + log( / ) = 6.35 + log(1) = 6.35 Since the variation is only small regarding temperatures changes, we can assume in a typical aquarium a pK of 6.35. For H₂CO₃ the values are shown in the following table: The value pK is dependet on the acid and temperature. The pH value of such a system can be calculated with pH = pK + log( / ) With, and as the concentrations of the Ions respectively. Since both parts are in equilibrium this state is represented with an arrow in both directions: H₂CO₃ ⇔ H+ + HCO3-Īs discussed the system is in equilibrium and thus there is a fixed ratio between these two sides: Therefore a part is present as H₂CO₃ and another part ionized as H+und HCO₃. In contrast to strong acids the dissolved H₂CO₃ is a weak acid and only partially ionized. In our aquarium we have dissolved CO₂: H₂O + CO₂ ⇒ H₂ CO₃ In chemistry this is emphasized with an arrow to the right side: HCl ⇒ H+ + Cl. For strong acids like HCl a complete ionization is happening leading to separation into H+ and Cl. If you want to use an already existing calculator you can look here: pH-KH-Calculator Introduction: Acid chemistry in a nutshellĭissolving an acid in water will ionize the acid. Within this article we will discuss the mathematics between these relations. In our tanks a complex equilibrium between the water parameters CO₂, KH and pH is present.
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