Solutions, colloidal Graham

To a chemist, Thomas Graham, belongs the credit of being the first to recognize and describe in detail the properties common to colloids. Although the two classes of solutions, colloidal and crystal-... 

Graham s definitions were expanded, and the concept of a colloidal state of matter evolved. According to this view, a substance could occur in a colloidal state just as it could occur under various conditions as a gas, liquid, or solid. If a colloidal solution was, at that time, defined as a solution in which the dispersed particles were comprised of large molecules, the ascertion would have been more acceptable. 

It was a Scot, Thomas Graham, who explained colloids, in 1862. He noticed that some solutions passed through parchment paper, others didn t. lie discovered that most of those that filtered through were of chemicals that formed crystals — he called them "crystalloids. The others he called colloids — from Greek kollodes, glue-like. 

Thomas Graham s investigations of diffusion 11861) led hun to characterize as crystalloids substances, such as inorganic salts which in water solutions would diffuse through a parchment membrane and as colloids... 

In 1861. Thomas Graham first used the term syneresis to describe the phenomenon of exuding small quantities of liquid hy gels. By definition, syneresis is the spontaneous separation of an initially homogeneous colloid system into lwo phases—a coherent gel and a liquid. The liquid is actually a dilute solution whose composition depends upon the original gel. When the liquid appears, the gel contracts, but there is no net volume change. Syneresis is reversible if the colloid particles do not become ton coagulated immediately after their formation. 

Graham [3] defined a colloidal dispersion in 1861 and he also defined the range of size for colloidal particles by two observations. The lower limit was determined by the slow diffusion of colloids in a dispersion, which indicated a particle size larger than 1 nm. The upper limit of the colloidal size is given by the slow rate of sedimentation under normal gravity. His conclusion was that molecules could not be larger than a 1 / n. Today particles which have a size, at least in one dimension, of 1 nm-10 /im are called colloids. Colloidal solutions can be divided into three classes [1,8]. 

Colloidal Molybdic Acid.—When solutions of sodium molybdate (1 molecule) and hydrochloric acid (4 molecules) are warmed together, or when a solution of molybdic acid dihydrate is evaporated over sulphuric acid under diminished pressure at 20° C., colloidal molybdic acid is formed. From the solution of the hydrosol so obtained, molybdic acid is precipitated by electrolytes. Graham considered that, by dialysis of a solution of sodium molybdate in hydrochloric acid, he obtained colloidal molybdic acid. ... 

See also Sabaaieeff, Chem. Zentr., 1891, i., 10 UUik, Annalen, 1867, 144, 329 1870,153, 373. Rosenheim and Davidsohn (loc. cit.) consider that the solutions previously-prepared by Graham s method, since not precipitated by electrolytes, in all probability contained no colloidal acids. 

Graham, Thomas (1805-1869). Studied diffusion of solutions through membranes established principles of colloid chemistry. 

Dialysis first was reported io 86l by Graham.1 who used parchment paper as a membrane. His experiments were based on the observations of a school teacher. W. G. Schmidt, that animal membranes were less permeable to colloids than to sugar or salt.1 Over the next 100 years, dialysis became widely used as a laboratoiy technique for the purification of small quantities of solutes but, with minor exceptions, it realized no large-scale industrial applications. In the last 20 years, development of dialysis for die treatment of kidney failure has brought about a resuigence of interest in dialysis fora wide range of separations. 

T. Graham was not able to establish the existence of osmotic pressure in colloidal solutions. Based on this fact he has classified solutions as either colloidal or true. Further studies, such as those carried out by A. Sabaneev with protein solutions, revealed that in fact osmotic pressure can be observed in colloidal systems, but its magnitude is very small due to the relatively large size of the particles in comparison with molecular dimensions, and, consequently, their low concentration. Indeed, in ideal systems the osmotic... 

Dialysis (5) as a unit operation considerably antedates gas and liquid permeation. Membrane dialysis was used by Graham in 1861 to separate colloids from crystalloids. The first large industrial dialyzers, for the recovery of caustic from rayon steep liquor, were installed in the United States in the 1930s. Industrial dialysis units for recovery of spent acid from metallurgical liquors have been widely used since 1958. In dialysis, bulk flow of solvent is prevented by balancing the osmotic pressure, and low-molecular-weight solutes are recovered by preferential diffusion across thin membranes having pores of the order of 10 cm. Frequently diffusion is enhanced by application of electric fields. 

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