Yesterday we learned how to make silver colloids (well, re-learned for me). I read through the notebooks of several former students, and concocted the following protocol for making 200 mL of Ag colloid via citrate reduction (essentially one of the Lee & Meisel preps):

Cleaned all glassware with aqua regia (1 part conc HNO₃ : 3 parts conc. HCl)

Rinsed glassware thoroughly with DI water (<4 &mu S/cm)

Boiled 200 mL of DI water with stirring

Added 40 mg AgNO₃ (visually inspected for any cloudiness due to formation of AgCl)

Boiled for 2 min.

Began adding 1% sodium citrate, 1 mL/min (total addition of 6 mL)

Removed from heat and placed in an ice bath when the solution turned greyish-green

The colloids were greyish-green after the 6th addition of citrate. Possibly even after the 5th. Visually, these colloids look better than what we had prepared, which had a milky white color. The extinction spectrum was narrower, but the maximum was a little redder than I’d like (about 450 nm, not 410 nm).