How to prepare a iodine standar solutions in the range of micrograms per liter accurately?
Do not attempt to accurately weigh small quantities of iodine. It is impossible to do due to sublimation and analytical balances don’t like the vapours. Start with an iodine solution of about 0.05M. Most analytical labs will have that available but if you need to make it up use about 12.7g analytical grade iodine crystals and dissolve in a solution of 20g analytical grade KI in about 40ml deionised water. Make this to 1lit and analyse it by titration against a suitable standard eg standard thiosulphate solution. From this you will get the conc of the iodine in mg/lit. Simply calculate the dilution factors for the much more dilute solutions you require. Avoid massive dilutions in one step. Always pipette at least 10ml and make up to no more than 1lit (100 fold dilution). For higher dilutions use several steps. If you need exactly 20, 40 etc micrograms/lit you will need to calculate the required volume of concentrate and use a burette but I would guess that these could be approximate as
Suggest create an amount of stock iodine standard by measuring out 1 L diluent, removing 1000 micrograms and replacing with 1000 micrograms iodine. Use a pipette. 1000 micro-gram / L stock. Then prepare dilutions: Say you require 20 mL of each dilution. 10 mL stock + 10 mL diluent gives you 20 mL 500 micro-gram/L, 8 mL stock + 12 mL diluent gives you 20 mL 400 micro-gram/L, 6 mL stock + 14 mL diluent gives you 20 mL 300 micro-gram/L, 4 mL stock + 16 mL diluent gives you 20 mL 200 micro-gram/L, 2 mL stock + 18 mL diluent gives you 20 mL 100 micro-gram/L, 0.8 mL stock + 19.2 mL diluent gives you 20 mL 40 micro-gram/L, 0.4 mL stock + 19.6 mL diluent gives you 20 mL 20 micro-grams/L. If you need say 100 mL of each dilution, then multiply mL stock and diluent by 5.
