4 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



TYPES OF PRESERVING FLUIDS AND THEIR PRINCIPAL 

 CHARACTERISTICS 



Because of the differences in the chemical behavior of the differen 

 plant pigments, it has been found necessary to modify the compositioi 

 of the preserving fluids to meet the requirements of the various classe 

 of specimens to be treated. Two general types of fluids, consisting 

 primarily of sorbitol borate and sorbitol citrate, have been preparec 

 for natural-color preservation purposes, and the needed modification 

 in pH values, antioxidant properties, and wetting and conditioning 

 action have been worked out for each, in general accordance with dif 

 ferent preservation characteristics of the principal classes of plan 

 material. For specimen material in which conflicting preservatioi 

 requirements exist because of unusual pigment combinations, furtheJ||) 

 experimental modifications may be found necessary in order to obtair 

 satisfactory results. 



The principal types of preserving fluids used in this work may be 

 obtained commercially in prepared form, and the intermediate types 

 can be made from them, as required, by mixing together the propei 

 stock fluids in suitable proportions. 



The stock fluids specified in the supply list, page 3, carry designations 

 which serve to denote their principal characteristics. The letters pre- 

 ceding the numerical portion of the designation indicate the general 

 type of the fluid; e.g., SB is applied to all types formulated with sorbitol 

 borate, and SCX to all sorbitol citrate types. The first two digits indi- 

 cate the specific characteristics of the fluid; those in the 1000 and the 

 2000 series tend to inhibit oxidation but are of a nonreducing nature; 

 those in the 1100 and the 2100 series exert a definite reducing action in 

 addition to their general antioxidant effect. The last two digits in the 

 designations indicate the pH values of the fluids. 



Thus the designation SB-1035 means that the nonreducing sorbitol 

 borate type to which it applies has a pH of 3.5, while SB-1075, for 

 example, is a fluid of similar composition but has its pH value adjusted 

 to 7.5. If it is necessary to add a surface-active and conditioning agent 

 to a preserving fluid, its presence may be indicated by adding A after 

 the numerical designation; e.g., SB-1035-A. 



Preparation of a Graded Series of Preserving Fluids 



In order to facilitate satisfactory preservation of color in the differ- 

 ently pigmented specimens that are ordinarily encountered, it is con- 

 venient to use a working series of preserving fluids that have pH values 

 graded by steps of 1.0 from pH 3.5 to pH 7.5. These fluids can readily 

 be prepared by mixing together high pH and low pH stock fluids in 

 proportions that will provide the desired intermediate values. If for 

 example type SB-1045 fluid having a pH value of 4.5 is desired, a mix- 

 ture should be made which contains 75 percent of stock fluid SB-1035 

 (pH 3.5) and 25 percent of stock fluid SB-1075 (pH 7.5), and conse- 

 quently has a resultant pH value of practically 4.5. The approximate 

 pH of a mixture prepared in this manner can be calculated as follows: 



75 percent of pH 3.5 component = 2.625 

 25 percent of pH 7.5 component = 1.875 



Resultant pH value of the mixture = 4.500 



