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Incoming links: Alkaline solution
Alkali 2026-10-31
Glucose can be obtained by hydrolysis of starch and detected as a reducing sugar by a heated alkaline copper(II) test. A positive copper test alone does not uniquely identify glucose.
Suspend starch in water, warm it to make a paste, and heat with a dilute acid, such as dilute sulfuric acid, to cause hydrolysis of the glycosidic bonds. The chemical reaction passes through shorter carbohydrates; sufficient hydrolysis ultimately gives glucose. The usual repeat-unit accounting isThis idealized polymer formula omits end-group details. The acid catalyses cleavage and is not consumed in the displayed equation. Merely using amylase from malt should not be described as necessarily producing pure glucose; it can leave maltose and shorter polysaccharides.
When sulfuric acid is used, neutralization with calcium carbonate removes the acidity and much of the sulfate as calcium sulfate. Filter out the insoluble residue, then concentrate the chemical solution gently and crystallize the glucose; further recrystallization is required to distinguish a reasonably pure chemical substance from a crude syrup. Overheating and unnecessarily prolonged strong acid treatment degrade the sugar. Hydrolysis of sucrose is a less selective preparative answer because it gives fructose alongside glucose.
For the historical copper test, use freshly mixed Fehling's reagent: an aqueous copper(II) sulfate portion and an alkaline tartrate portion. The tartrate keeps copper(II) in coordination complexes instead of allowing immediate bulk precipitation of copper(II) hydroxide. Add a suitably prepared sample to a small test portion and warm in a hot water bath. A yellow-orange to brick-red precipitate of copper(I) oxide indicates reduction of the copper(II) reagent. Run a known glucose sample and a water blank alongside it. In a simple sample, a visible difference between these controls helps distinguish a true positive from a deteriorated reagent or contamination.[ref]
Estimation of Glucose by Amrita Vishwa Vidyapeetham. Theory, uses of Fehling's solution and formation of red copper(I) oxide
The key redox accounting on the copper side isThe required electrons come from oxidation of the sugar. This half-equation deliberately omits the changing tartrate ligands and does not assign a unique organic product. The often printed claim that hot alkaline glucose simply becomes gluconate in this test is too simple: competing isomerization, oxidation and fragmentation make the organic chemistry more complicated.[ref]
The Iodine Test for Reducing Sugars by Holger Fleischer. §2.3, interpretation of copper tests and product complexity
Fructose, lactose, maltose and various other reducing substances can also give a positive Fehling's test. In particular, fructose can form aldoses by tautomerization in alkaline solution; an initial ketone functional group does not make it reliably negative.[ref] Fermentation by suitable yeast, with carbon dioxide formation, and optical rotation can supply further historical evidence, but neither is individually specific to glucose. These are anticipated diagnostic observations, not measurements supplied by the paper. In present laboratory practice an appropriately validated enzymatic glucose assay can establish glucose more selectively; the old reducing-sugar test is not by itself a diagnosis of diabetes.[ref]
Diagnosis of Diabetes in PDB-101. Monitoring: Diagnosis, blood tests and glucose-oxidase assay explanation
The chief historical examples are rhubarb, senna, santonin and logwood for urine; iron preparations, bismuth preparations and charcoal for darkening the faeces. There are two different processes: a substance absorbed and subsequently excreted can colour a secretion, while material remaining in the intestinal lumen can colour the stools without demonstrating systemic drug absorption.
- Rhubarb colours urine yellow to yellow-brown; the colour becomes redder in an alkaline solution. The contemporary account also reports coloration of sweat and breast milk. This is a historical observation about passage of colouring matter, not a claim that every constituent passes unchanged into every secretion. [ref]The Essentials of Materia Medica and Therapeutics by Alfred Baring Garrod (1865). Labiatae, continued, Rheum: colouring constituent, alkaline solutions, and coloration of urine, perspiration and milk
- Senna can give urine a yellow-brown or red-brown colour. The effect is still recognised in current medicinal information. [ref]Senna by the NHS. Side effects, coloration of urine
- Santonin, used historically as an anthelmintic, can make urine conspicuously yellow. This must be distinguished from the drug's separate effect on colour vision: a yellow-looking visual field is not itself excretion of a yellow pigment. [ref]A Manual of Practical Therapeutics by Edward John Waring (1865). §2416, physiological effects and yellow urine
- Logwood can give urine a pink or reddish colour, particularly when it is an alkaline solution; the historical description notes that addition of Ammonia brings out the colour in a previously acid specimen. [ref]The Essentials of Materia Medica and Therapeutics by Alfred Baring Garrod (1865). Cisalpinae, Haematoxylum, urine coloration
- Iron preparations can darken faeces; bismuth preparations also cause dark or black stools. In these examples the result need not be a pre-existing vegetable dye simply passing unchanged through the body. Current information confirms black stools with oral iron and with the modern bismuth medicine bismuth subsalicylate; the latter is corroboration of the colour effect, not an identification of that modern formulation with the 1864 bismuth articles. [ref]Bleeding from the bottom by the NHS. Dark stools and iron tablets[ref]Pepto-Bismol by the NHS. Side effects: black stools and black tongue
- Charcoal supplies a black material directly to the bowel contents. Its appearance in the stools does not require its absorption and subsequent renal excretion.
Colour alone does not identify either the drug or a disease. A medicinal pigment can resemble blood or a bile pigment; pH, timing after administration and appropriate chemical or microscopic examination distinguish possibilities. Conversely, a history of taking a colouring medicine does not prove that an observed specimen contains no blood. These examples describe pharmacological and historical observations, not results from an unseen specimen.