F.6 chemistry Q

1. The properties of the chemical required to a primary standard solution :
- chemical stability over reasonable time
- not photosensitive
- low level of contamination from surrounding (e.g. atmosphere)
- low level of contamination casued by the apparatus that comes into contact with the sample
in other words, primary standard solution will remain standard after a reasonable time elapsed (e.g. 24 months in sealed condition)
Standard solution can be standardized with primary standard solution.
2. Standardize is the process to find out the exact content (composition) of the standard solution. Exact value for calculation of other values you want to know. Exact value is needed to reduce the error induced if you use the guess concentration of solution.
3. Sodium hydroxide will react with carbon dioxide in the atmosphere to form sodium carbonate and this will decrease the number of OH- ions in the solution with time, so NaOH is not a good standard solution. If the sodium hydroxide solution is not standardized just before you perform the any titration, you will never know its exact concentration at that particular moment and how can you perform your calculation for exact values of other chemicals?
OH- + CO2 ---> HCO3-
HCO3- + OH- ---> (CO3) 2- + H2O
or
NaOH + CO2 ---> NaHCO3
NaHCO3 + NaOH ---> Na2CO3 + H2O

4. The solubility of iodine in water is extremely low and is almost insoluble in water. In aqueous iodine, the electrons in iodine molecule interact with the electrons in water molecule. Such electron interactions make the solution very pale yellow.
However, iodine is very soluble in KI solution, and the soluble is deep brown due to the presence of (I3)-ions.
I3- and I2 + I- is in equilbrium, so the exact amount of iodine can be obtained.
I hope this can help your understanding. :)

In order to be a standard solution, it must maintain standard for prolong period with no special care needed. Necessity to clean the pipette by standard solution is to increase the consumption of standard solution and thus the renewal rate of the standard solution.