Hi Matthew, Very interesting text and experiments. Take me back a number of years (60) to Chemistry & Pharmacy courses. I have conducted a few tests since then, but would be quiet and observe as you shared. Thanks, Richard Harris, 56 yr FL Pharmacist -----Original Message----- From: Matthew McCann PE [mailto:mmcc...@franciscan.edu] Sent: Sunday, November 09, 2003 4:18 PM To: silver-list@eskimo.com Subject: CS>measurements using salt test(s)
Hello, Members of the List. A salt test for noble metal colloids traces back to Michael Faraday noticing in 1857 that his gold dispersions would be coagulated by additions of traces of salts. (Faraday, Phil. Trans. Roy. Soc. London, 197, 145, 1857) In 1903, Freundlich described how to apply this precipitation by salts to measure the amount of dispersed noble metal in a hydrosol. (Freundlich; Z. Physik. Chem., 44, 129, 1903) Freundlich's method was succinctly re-stated by Arthur Thomas in 1934. (Colloid Chemistry, McGraw-Hill, p. 179) Thomas wrote: "To a series of carefully cleaned test tubes containing 5cc portions of the hydrosol are added 5cc portions of different concentrations of the electrolytes (i.e. aqueous solutions of the salt.) The electrolyte solution is poured quickly into the hydrosol, the mixture quickly poured back into the empty test tube, and then back into the first test tube in order to effect as complete and rapid mixing as possible. The tubes may then be allowed to stand for two hours. Then they are examined for precipitation. The mean of the concentrations at which precipitation is just complete, as shown by a clear supernatant solution, and the next lowest concentration (where precipitation is not complete) is taken as the precipitation concentration. This concentration is called the "liminal value" (i.e. theshold value.) Naturally all must be done under the same conditions." Freundlich experimented with platinum hydrosols precipitated by sodium chloride and potassium chloride, as well as a number of other substances. Well, it seems to me the necessary equipment is not expensive or hard to find. A test tube rack for 6 tubes can cost $2.50. A 10ml graduated cylinder made of borosilicate glass can cost $2.50. A set of eight borosilicate glass test tubes can cost $2.40. Sodium chloride costs next to nothing. If potassium chloride(KCl) is desired for the sake of a comparison or a control, it can be bought at any supermarket as "sodium- free salt substitute," however it has other additives that will augment the precipitation of the silver. Has anybody had any experience with this measurement technique of Freundlich? Thanks for you input and comments! Matthew
