What is the purpose of dilution and instructions for performing a dilution?

Document ID

Document ID TE1595

Published Date

Published Date 03/19/2020
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Question
What is the purpose of dilution and instructions for performing a dilution?
Summary
Dilution explanation
Answer
Dilutions can be important when dealing with an unknown substance.
A dilution can be performed not only to lower the concentration of the analyte that is being tested, so that it is in range, but also to help eliminate interferences from other substances that may be present in the sample that can artificially alter the analysis.
An analyte is the compound in the sample that is desired to be tested.  An example is if performing a chlorine test, chlorine would be the analyte.
An interference is a compound that can add to, or subtract from the result of the analysis. For example, oxidized manganese (Mn +6 ) acts as an interference to the chlorine test. If the Mn +6 is present in the sample it will add to the value of the chlorine test due to the way the it reacts to the chlorine test reagents. By performing a dilution on a sample it may reduce the interfering substance to a point where it no longer interferes with the test.

When performing a dilution the following equation is often cited:

C 1 V 1 = C 2 V 2

C 1 is the initial concentration
V 1 is the initial sample volume before dilution 
C 2 is the final concentration after dilution
V 2 is the final volume after dilution

Example: To prepare a 2 mg/L Cl 2 standard by dilution of a 50 mg/L Cl 2 primary standard using a 100 mL volumetric flask, below is the calculation to determine the volume of primary standard to add to the volumetric flask. 

C 1 = 50 mg/L Cl 2
V 1 = Calculated variable in mL
C 2 = 2 mg/L Cl 2
V 2 = 100 mL

50*V 1 =2*100
V 1 =(2*100)/50
V 1 =200/50
V 1 =4 mL

So in this example if 4 mL of the Primary 50 mg/L Cl 2 standard would be added to a 100 mL volumetric flask and then diluent added to the 100 mL graduation, then the solution in the volumetric flask would be 2 mg/L Cl 2 .

This equation can be used to solve for any one of the four variables. For best accuracy use volumetric glassware for volume measurements.  

 For more information and additional examples, please see Chemical Analysis the section of the Water Analysis Handbook

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