APPENDIX I 



THE COMPOSITION OF SOLUTIONS 



EXPRESSED AS PERCENTAGES: CONVENTIONS 



ADOPTED IN THIS BOOK 



It would be a convenience if the composition of solutions were 

 generally expressed in terms of gram formula- weight per litre, or 

 of normality in the case of acids. At present, however, it is usual 

 in biological laboratories to express concentrations as percentages. 



When percentages are expressed by the symbols w/w, w/v, 

 v/w, and v/v, the meaning of the numerator is the weight (w) in g, 

 or volume (v) in ml, of the solute. The denominator denotes the 

 weight (lOO g) or volume (lOO ml) of the solution (not of the 

 solvent). Thus, for instance, the expression 'formaldehyde, 4% 

 w/v' means that in 100 ml of the solution there are 4 g of formalde- 

 hyde, and the expression 'acetic acid 5% v/v' means that in 100 

 ml of the solution there are 5 ml of glacial acetic acid. 



As a general rule, solids should be dissolved in such a way that 

 the percentage composition can at once be given as w/v, and 

 liquids so that it can be given as v/v. In other words, a particular 

 weight of a solid or a particular volume of a liquid should be 

 made up to a particular volume by the addition of solvent. In 

 biological laboratories the effect of the solute on the volume of the 

 solution is often overlooked, a certain weight of substance being 

 dissolved in a particular volume of water or other solvent'. The 

 solution, which contains x grams of solute to 100 ml of solvent, 

 differs from an x% w/v solution, but many of the solutions com- 

 monly used in microtechnique are not concentrated enough to 

 cause the difference to be of practical importance. 



For certain particular purposes (p. 144) it is desirable to express 

 the composition of a solution in terms of the number of g or ml of 

 solute in 100 g of water (or other solvent). When this is done, the 

 denominator may be written with a capital letter, to draw attention 

 to the unusual way of expressing the concentration. Thus, a 

 solution of 5 g of mercuric chloride in 100 g of water may be called 

 a 5% w/W solution. It is convenient to remember that W here 

 means 100 g of water (or other solvent). 



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