KIESELGUHR OR DIATOMITE

Large deposits of diatomite are found in Aberdeenshire in the UK, Virginia and California in the USA, Germany and North Africa. The crude product contains about 65–87% of SiO₂, together with organic matter, clay, iron oxide and about 5–15% of water. The silica is mainly amorphous, being present in the siliceous walls of minute, unicellular plants belonging to a number of families of the Bacillariophyceae. A much smaller percentage of silica occurs in the walls of spicules of siliceous sponges and, in a crystalline form, as sand. Depending on the geographical origin of the diatomite, the diatoms may be either freshwater or marine forms.

The material is dried and crushed, ignited to remove organic matter, boiled with dilute hydrochloric acid to remove impurities such as iron, washed with water and dried. It is then sifted or ‘air-blown’, the finest grades used in face powders being obtained by the latter method.

Characters

Purified kieselguhr is a fine, white or pale-buff odourless powder. For microscopical examination it may be mounted in cresol or olive oil. In the latter medium the amorphous silica of the diatoms becomes almost invisible, while the crystalline particles of sand remain clear. Only small amounts of sand (Fig. 34.1H) should be present.

Fig. 34.1 Diatomite (various sources) showing the shells of diatoms and other constituents. A, Diatom skeletons of the Fragilariaceae (e.g. Synedra, Fragilaria); B, entire or broken portions of Coscinodiscus spp.; C, shells of the Naviculaceae (e.g. Navicula); D, various forms belonging to the Biddulphioideae (Biddulphia, Trinacria, etc.); E, fragments of Asterionella spp.?; F, silicoflagellates; G, sponge spicules; H, sand particles.

The diatoms (Fig. 34.1) consist of two halves or valves which fit together like a pill-box. The two positions from which they may be studied are known as the valve-view and the girdle-view. The valves show considerable variation in shape, some samples of kieselguhr showing numerous discoid types resembling that of the Arachnoidiscus found in agar, while other samples consist largely of pennate forms. A mixture of both types is usually most suitable for filtration. In many diatoms a median cleft is found in the valves, known as the raphe. The valves also show dots and lines, which vary in the different species and are due to minute cavities in the wall.

Kieselguhr is insoluble in all acids except hydrofluoric, but is soluble after fusion with alkalis. It is used for the filtration of oils, fats, syrups, etc., and in the form of the Berkefeld filter for sterilization. Highly purified material is used as an inactive support in column, gas and thin-layer chromatography; the powder will hold up to its own weight of water and still retain its powdery consistency. Diatomite is also employed in face powders, pills, polishing powders and soaps, and to absorb nitroglycerin in the manufacture of dynamite; it is a component of the BP (Vet) pyrethrum dusting powder.

Extant species of diatoms form an important component of plankton and are involved in the food chains of seas and rivers. (For a wide-ranging illustrated introduction to these single-celled plants see The Diatoms; Biology and Morphology of the Genera by F. E. Round et al. (1990), Cambridge University Press.)

PREPARED CHALK

Chalk is a whitish or greyish rock which is widely distributed in north-western Europe. It consists mainly of the shells of unicellular animals known as the Foraminifera. Chalk as quarried often contains about 97 or 98% of calcium carbonate, the remainder being largely siliceous and therefore insoluble in acids. The impure chalk is finely ground with water and freed from most of the heavier siliceous impurities by elutriation. The coarser product is sold as ‘whiting’ and the finer elutriated product is allowed to settle and while still pasty is poured into a funnel-shaped trochiscator. The latter is tapped on a porous chalk slab and ejects the chalk to form ‘cones’, which are allowed to dry giving Prepared Chalk BP. These cones (‘crab’s eyes’) may be powdered.

Characters

For examination chalk should be mounted in cresol, warmed and examined microscopically (Fig. 34.2). Most of the foraminiferous shells have been broken but a number of whole ones usually remain. The whole shells may be concentrated in a small bulk by removing the broken ones by elutriation and examining the residue. Note the following:

Fig. 34.2 Shells from prepared chalk. Globigerina in (A) water, (B) cresol (×200). C, Textularia in cresol (×200). D, coccoliths (×400).

Globigerina. In these the shell is of calcite and is perforated by large canals. Each consists of a few lobular chambers arranged in a plane or helicoid spiral. The size varies from about 35 to 80 μm.

Textularia. In these the shell is composed of grains of sand cemented together by calcareous matter. They are usually conical or cuneiform in shape and are composed of numerous chambers in two alternating parallel series. The size varies from about 50 to 180 μm.

Remains of fossil algae. Small rings or discs about 4–9 μm in diameter, termed coccoliths or morpholites.

Prepared chalk is assayed by acid–alkali back-titration; there are pharmacopoeial limits for heavy metals and arsenic; limits for aluminium, iron, phosphate and matter insoluble in hydrochloride acid are determined gravimetrically.

Precipitated chalk

Precipitated chalk (calcium carbonate BP/EP is made by the interaction of a soluble calcium salt and a solublecarbonate. The precipitate varies considerably with the method of preparation. When precipitated at about 0 °C, the product is very light and almost entirely amorphous; at about 30 °C a denser precipitate of minute rhombohedra is formed, and if boiling solutions are used, the precipitate consists of prismatic rhombohedra with a higher specific gravity than either of the previous forms. The BP assay involves a complexometric titration of calcium; there are limits for various metals, etc.

Uses

Chalk is used as an absorbent and antacid.

GELATIN

Gelatin is a mixture of reversible gel-forming proteins derived from certain animal tissues, particularly skin and bones, with hot water. The process converts insoluble collagens into soluble gelatin, the solution of which is then purified and concentrated to a solid form.

The initial stages of the preparation vary with the starting material, bones, for example, being defatted with an organic solvent and sometimes decalcified by treatment with acid. Two types of gelatin are characterized in the BP/EP—type A is obtained by partial acid hydrolysis of animal collagen and type B by partial alkaline hydrolysis; mixtures of both types are also permitted.

Gelatin sponge

Gelatin sponge can be conveniently mentioned here as an absorbable, water-insoluble haemostatic material. It may be prepared by whisking a warm solution of gelatin to a foam of uniform porosity and drying. After cutting into pieces it is sterilized by dry heat. Note the BP standards for this material. It is used in a similar manner to oxidized cellulose.

FISH BODY OILS

The oils expressed from the bodies of a number of ‘oily’ fish of the families cited on p. 43 contain esters of omega-3 fatty acids. As such, they have become important dietary supplements and two such oils are included in the BP/EP. For an explanation of the structural representation of the various acids, as cited below, see Chapter 19, ‘Fatty Acids’.