152 RESINS, BITTEE PRINCIPLEa, ETC 



Usmc acid, which occurs in yisliow crystals, yiolda a oolonrfess 

 armmoniam salt. 



§168. OrcinandBeta&rclTL Estimation of Orcm.- — Orcin and bela- 

 orcih, .which have alreariy been mentioned as products of the 

 decompositiMi of certain cpustitiients of lichens, and which soroe- 

 times occiir ready formed in plants, can be obtained in colonrlogs 

 acicular crystals soluble in water, alcohQl, and. ether. Exposure 

 to light colours them reddish; alkalies, chloripated lime aiid ferric 

 chloride, violet. By the action of ammonia and air orcin yields a 

 blue colouring matter, whilst, under the same conditions, |>eta^ 

 orcin gradually turns red. Orein mejts at 58% betaorcin at ji 

 temperature above 109'. 



Eeymann estimates orcin in-lichens by titrating with bromine- 

 water,! by which monobromorcin is first produced and' subse- 

 quently converted into trfbromorciB. To the Bolntio© of orcin 

 in a stoppered bottle titrated bromine- water is sidded till the 

 precipitate has assumed a yellowish, colour and excess of broiliine 

 is jH-eseut, which is then estimated by iodide of potassiuja and 

 hyposulphite of soda. The amouul. of orcin present is' calculated 

 from the equations : 



C^HgO^-f- Bra =^ HBr -f-C^HtlBrO, 



and 



CyH7Br02+ 2Br2=v 2HBr+e7H5Br30i 



2- 



TANNlb ACaDS. 



§ 159. ConsiUuiion.^ — In estimating tannic acids an .error kas 

 generally been committed in overlooking too completely the 

 chemical <^J^er««a.? existing betw^ten the various substances, that 

 have received this- name. It has usually been considered sufficient 

 to deterlnine quantitatively the Value Of a reagent in terms of 

 gallotannic acid, the tannin most easily procurable, and to apply 

 the results thii? obtained to the estimation of other tannins. 

 This would be admissible under, the assumption that all tannins 

 possessed approximately ideptical equri'aleat weights and pro- 

 duced nearly identical chemical effects. Bvit it has already been 

 shown in § 52 that such is not the case. M .^sill be suffieiont 

 here to repeat that tannins exist which do not aUcsr of eoai 

 parisoiis witli oil© another, even with regard to their conditulwn. 

 At prbseut many tannic acids may be assumed to be ghmsHta., 



^ Ber. <i. d- Chem Gea. yiii. 790, 1875 (Joiirn. OheSni ^bc kxvii. 1298). 



