Past exam of the medicine course of the University of Oxford / 1867 / First examination / Chemistry / 9 / Solution

Combine investigation of the source with several independent analyses; a clear appearance or a single negative chemical test cannot exclude sewage pollution. No sample results are supplied, so the answer is an examination scheme rather than a finding about an actual water sample.

Record where and when the water was collected, recent rainfall, proximity of drains and cesspits, and possible routes from sewage to the supply. Compare with a credible uncontaminated sample from the same geological setting. Use clean, suitably closed vessels and examine promptly; contamination introduced during sampling and chemical changes during storage can both mislead. Separate portions are needed when one analysis alters the sample for another.

For an 1867 investigation, inspect colour, turbidity and sediment, and examine concentrated sediment by microscopy for suspicious organic debris. These observations can motivate further work but cannot identify all disease-causing agents. Determine residue after evaporating a measured volume; carefully heating that residue may reveal charring or a volatile fraction. Loss on ignition is not a specific measure of sewage: mineral constituents can also change on heating, and some organic material can be lost during evaporation itself.

Test a separate aliquot for chloride, for example by silver nitrate precipitation, and compare its quantity with local background. Elevated chloride can accompany sewage but also comes from natural salts. Examine Ammonia or ammonium and oxidized nitrogen, nitrite and nitrate. Organic nitrogen can be converted during decomposition and nitrification, so an old contamination may have little free Ammonia. Conversely, a nitrogen signal alone does not establish its origin.

An oxidant-consumption experiment with a known amount of potassium permanganate, under controlled reaction time, temperature and acidity, supplies another comparison. Residual oxidant is measured against a reagent blank. A large consumption indicates oxidizable material, but reducing inorganic constituents also consume oxidant, while some organic materials react incompletely. It is therefore an operational test, not a direct count of sewage organisms.

A method actually reported in June–July 1867 first distils off free Ammonia, then uses alkaline permanganate to generate further Ammonia from some organic nitrogen. A boiling flask, condenser and successive receiving vessels separate the fractions, whose Ammonia can be compared by colour with standards using Nessler's reagent. The later fraction became known as albuminoid ammonia. It is neither proof of the presence of albumen specifically nor complete conversion of every organic nitrogen compound. Contemporary discussion already disputed how different substances behaved in the preliminary distillation.^[ref]

A present-day assessment adds validated microbiological water testing, including a specific Escherichia coli test using culture or an accepted equivalent, together with quality control and a sanitary survey. Escherichia coli is an indicator of faecal contamination, not a label for all sewage-derived pathogens. The Australian drinking water guideline specifies none detected in a $100\mathrm{mL}$ sample and notes that the indicator does not reliably represent all enteric viruses or protozoa.^[ref]

The justified conclusion comes from the combined evidence and the detection limits. Chemical indicators can support a sewage-contamination hypothesis, but chemical clarity or a negative result in one aliquot is not a certificate of microbiological safety. This modern distinction should not be attributed to an examiner as a fully developed 1867 laboratory protocol.