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Past exam of the medicine course of the University of Oxford / 1867 / Second examination / Materia medica / 2 / Solution 2026-10-31

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Prepare quinine sulfate by acidic extraction of cinchona bark, alkaline precipitation of the quinine base, conversion into the sulfate, and purification by crystallization. “Quinia” is the historical name for quinine.

The 1867 process starts with one pound of yellow cinchona bark in coarse powder, three fluid ounces of hydrochloric acid, four pints of solution of sodium hydroxide, and sufficient distilled water and diluted sulfuric acid. The hydrochloric acid is diluted with ten pints of distilled water. The bark is first moistened with diluted sulfuric acid, macerated for twenty-four hours with occasional stirring, and then exhausted in a percolator with the diluted hydrochloric acid. The historical endpoint is disappearance of the bitter character of the emerging liquid. This describes the period's test, not an instruction to taste an unidentified medicinal material.^[ref]

The soluble alkaloid salts enter the percolate. Adding solution of sodium hydroxide liberates the much less water-soluble quinine base. After settling, the liquid is decanted and the precipitate collected on a filter and washed with cold distilled water until the washings cease to be coloured. The precipitate is then treated warm with distilled water and diluted sulfuric acid, added gradually to obtain a neutral liquid with nearly all the precipitate dissolved. The hot solution is filtered through paper, the filter washed with distilled water, and the filtrate concentrated until a surface film appears. It is then set aside for crystallization; the crystals are dried on filter paper without applied heat. This is a separation of a naturally occurring alkaloid, not a synthesis of the quinine skeleton.

Writing

B

for the quinine base, the central changes are:

B H^{+} + O H^{-} ⟶ B ↓ + H_{2} O, 2B + H_{2} S O_{4} ⟶ (BH)_{2} S O_{4} .

(1)

The successive changes in solubility explain the apparatus: a percolator washes soluble salts out of the powder; a filter retains the liberated base; a heated porcelain vessel dissolves the crude base as its sulfate; cooling then recovers the less soluble crystals. Other cinchona alkaloids can accompany the product, which is why appearance and bitterness are insufficient evidence of purity.

The 1867 “Characters and Tests” combine identity and purity checks. They must be read as a group:

Appearance and solubility: filiform, silky, white crystals, sparingly soluble in water, with the characteristic bluish appearance of the solution. A little dilute acid increases solubility. The visible blue phenomenon is associated with quinine fluorescence.
Sulfate identity: barium chloride gives a white precipitate insoluble in nitric acid. The precipitate is barium sulfate.
Quinine identity: treating the solution first with chlorine water and then with Ammonia gives an emerald-green colour, the thalleioquin test. A positive colour reaction establishes a characteristic chemical response, rather than proving that no other alkaloid is present.
Strong-acid colour test: dissolution in pure sulfuric acid gives only a faint yellowish tint, with no further colour change on gentle warming. Marked colouring suggests foreign organic matter, such as sucrose or other readily altered material.
Acid dissolution and liberation of the base: ten grains with ten minims of diluted sulfuric acid and half a fluid ounce of water form a complete solution; Ammonia then gives a white precipitate. Material insoluble under the acid conditions would fail the dissolution requirement.
Ether separation: the precipitated base must pass into diethyl ether on shaking the whole with half a fluid ounce of diethyl ether, without crystalline matter remaining at the boundary between the two liquid layers. Less ether-soluble cinchona alkaloids, particularly cinchonine, can leave material at this boundary. This is a period screening test, not a modern quantitative determination of every related impurity.
Water of crystallization: twenty-five grains lose 3.6 grains on drying at $212^{\circ} F$ , equivalent to $100^{\circ} C$ . This checks excessive moisture or a materially different degree of hydration.

The historical quantity and endpoint requirements belong to the 1867 monograph.^[ref] Its formula represents the then-described heptahydrate; the current “Quinine sulfate” instead specifies a dihydrate and modern tests for related cinchona alkaloids. The modern specification explains that distinction; the exam's historical tests remain those listed above.^[ref]

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