Salkowski reaction

Nencki and Sieber (634) in 1882 described a beautiful red color obtained on adding concentrated hydrochloric acid to certain pathological urines. Much confusion exists in the older literature between this Nencki-Sieber reaction, the Salkowski reaction (417, 745) (a red color with hydrochloric acid and dilute ferric chloride), and the urorosein reaction (a red color with hydrochloric acid and nitrite), but the same chromogen is probably mainly... 

Salkowski reaction Dissolve 1-2 mg of the sample in 1 mL chloroform and add 1 mL concentrated sulfuric acid, forming two phases, with a red or yellow color indicating the presence of sterols and methylated sterols. 

Inhibition of tomato and barley plants growing in soils infested with Centaurea repens (knapweed) was reported by Fletcher and Renney (38). A toxic component was isolated in highest concentration from the foliage of knapweed. The inhibitor was considered to be an indole alkaloid or auxin precursor because of its ultraviolet absorption spectrum and the positive reactions obtained with Salkowski and Ehrlich reagents. The presence of the inhibitor was considered to explain partially the rapid establishment of Centaura spp. in almost pure stands. 

In an attempt to identify the chromophoric systems resulting from reactions of steroids with strong acids ( colour reactions Kober, Allen, Oertel, Talbot, Salkowski, and Liebermann-Burchard reactions) a detailed study has been made, with use of all the usual spectroscopic techniques. Visible colours are attributed generally to charge-transfer between the steroid, as donor, and a delocalized carbonium ion, generated from the steroid in acidic media, acting as acceptor. The structures of the steroidal carbonium ions are discussed. 

Both the Salkowski reagents (Nos. 104 and 108) yield reaction products with differentiating colours many of the products fluoresce... 

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