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Why is controlling pH necessary in complexometric titration?
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Kurt Wagner
Why is controlling pH necessary in complexometric titration?
There are two parts to this. A complexometric titration works by a complexing agent (EDTA) clustering around a metal ion. Sometimes there is a second auxiliary complexing agent, generally ammonia. The endpoint is determined by an indicator.
Firstly, because EDTA is a weak polybasic acid, its degree of dissociation varies with pH, which can affect the exact complex formed.
Secondly, the indicators usually work within a limited pH range. Thus if you are titrating Fe using salicylic acid, it needs to be in the acid range at about pH 2 to 3. On the other hand to use eriorchrome Blue Black R for Zn, it should be alkaline pH 8 to 12.
There are two parts to this. A complexometric titration works by a complexing agent (EDTA) clustering around a metal ion. Sometimes there is a second auxiliary complexing agent, generally ammonia. The endpoint is determined by an indicator.
Firstly, because EDTA is a weak polybasic acid, its degree of dissociation varies with pH, which can affect the exact complex formed.
Secondly, the indicators usually work within a limited pH range. Thus if you are titrating Fe using salicylic acid, it needs to be in the acid range at about pH 2 to 3. On the other hand to use eriorchrome Blue Black R for Zn, it should be alkaline pH 8 to 12.
Every titration requires all independent variables but one to be kept constant. The one remaining variable is almost always the volume of added titrant. Temperature, pH, … are all maintained, unless one of them is the dependent variable that you will measure. In acid-base titrations, the pH usually is the dependent variable to be measured; in complexometric titrations, however, we use the formation of a complex as an end point indication.
Any change in pH can influence the outcome of the complex formation. Thus, when we perform complexometric titrations with, for example, EDTA, we must always operate in a buffered solution lest we include a second variable, rendering our data output confusing and, for practical purposes, useless.
Every titration requires all independent variables but one to be kept constant. The one remaining variable is almost always the volume of added titrant. Temperature, pH, … are all maintained, unless one of them is the dependent variable that you will measure. In acid-base titrations, the pH usually is the dependent variable to be measured; in complexometric titrations, however, we use the formation of a complex as an end point indication.
Any change in pH can influence the outcome of the complex formation. Thus, when we perform complexometric titrations with, for example, EDTA, we must always operate in a buffered solution lest we include a second variable, rendering our data output confusing and, for practical purposes, useless.
There are two parts to this. A complexometric titration works by a complexing agent (EDTA) clustering around a metal ion. Sometimes there is a second auxiliary complexing agent, generally ammonia. The endpoint is determined by an indicator.
Firstly, because EDTA is a weak polybasic acid, its degree of dissociation varies with pH, which can affect the exact complex formed.
Secondly, the indicators usually work within a limited pH range. Thus if you are titrating Fe using salicylic acid, it needs to be in the acid range at about pH 2 to 3. On the other hand to use eriorchrome Blue Black R for Zn, it should be alkaline pH 8 to 12.
There are two parts to this. A complexometric titration works by a complexing agent (EDTA) clustering around a metal ion. Sometimes there is a second auxiliary complexing agent, generally ammonia. The endpoint is determined by an indicator.
Firstly, because EDTA is a weak polybasic acid, its degree of dissociation varies with pH, which can affect the exact complex formed.
Secondly, the indicators usually work within a limited pH range. Thus if you are titrating Fe using salicylic acid, it needs to be in the acid range at about pH 2 to 3. On the other hand to use eriorchrome Blue Black R for Zn, it should be alkaline pH 8 to 12.
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Every titration requires all independent variables but one to be kept constant. The one remaining variable is almost always the volume of added titrant. Temperature, pH, … are all maintained, unless one of them is the dependent variable that you will measure. In acid-base titrations, the pH usually is the dependent variable to be measured; in complexometric titrations, however, we use the formation of a complex as an end point indication.
Any change in pH can influence the outcome of the complex formation. Thus, when we perform complexometric titrations with, for example, EDTA, we must always operate in a buffered solution lest we include a second variable, rendering our data output confusing and, for practical purposes, useless.
Every titration requires all independent variables but one to be kept constant. The one remaining variable is almost always the volume of added titrant. Temperature, pH, … are all maintained, unless one of them is the dependent variable that you will measure. In acid-base titrations, the pH usually is the dependent variable to be measured; in complexometric titrations, however, we use the formation of a complex as an end point indication.
Any change in pH can influence the outcome of the complex formation. Thus, when we perform complexometric titrations with, for example, EDTA, we must always operate in a buffered solution lest we include a second variable, rendering our data output confusing and, for practical purposes, useless.
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