how to prepare ammonia buffer solution for hardness | how do you make ammonia buffer solution? 

But in a some book is If the magnesium salt of EDTA is unavailable, dissolve 1.179 g disodium salt of ethylenediaminetetraacetic acid dihydrate (analytical reagent grade) and 780 mg magnesium sulfate (MgSO4 7H2O) or 644 mg magnesium chloride (MgCl2 6H2O) in 50 mL distilled water. Add this solution to 16.9 g NH4Cl and 143 mL conc NH4OH with mixing and dilute to 250 mL with distilled water. To attain the highest accuracy, adjust to exact equivalence through appropriate addition of a small amount of EDTA or MgSO4 or MgCl2

Why we take 16 . 9 gm of ammonium chloride Is there any logic to take specific quantity

What if we want to make it in small quantity

Ammonium chloride 16.9 gram and ammonium solution 143 ml not 14.3ml for 250ml

thanks

Btw adding water makes down the Ph ,... Why would you add them