Practical analysis and data interpretation for water

Using resources • Earth's resources and potable water

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Give an example of when a difference within the same order of magnitude might be insignificant.

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If measurement uncertainty is ±20% and two concentrations differ by 10%, the difference falls within the same order of magnitude and within error, making it likely insignificant.

Key concepts

What you'll likely be quizzed about

Sampling and safety

Representative sampling requires selection of a sample that reflects the source; improper sampling causes biased results. Use clean, labelled containers and avoid contamination since contact with air, hands or dirty equipment changes measured ion concentrations and pH. Record sample location, time and visible conditions because these factors affect interpretation and comparability. Safety procedures limit risks during analysis. Handling acids, hot apparatus and electrical instruments causes hazards; appropriate personal protective equipment, correct waste disposal and known emergency actions reduce risks and prevent contamination of subsequent samples.

pH measurement and interpretation

pH measures hydrogen ion activity on a logarithmic scale; a change of 1 pH unit represents a tenfold change in hydrogen ion concentration. Low pH (acidic) causes increased corrosion of metals and release of certain dissolved species; high pH (alkaline) causes scale formation and precipitation of metal hydroxides. Accuracy depends on calibration and method. A properly calibrated pH meter gives quantitative values; universal indicator papers provide rapid, approximate values. Temperature and ionic strength affect pH readings, and lack of calibration or unsuitable probes causes systematic error.

Dissolved solids, conductivity and total dissolved solids (TDS)

Dissolved ions increase water conductivity because charged species carry electrical current; higher conductivity causes higher TDS estimates. TDS commonly refers to the mass concentration of dissolved solids and correlates with conductivity via a calibration factor. Limiting factors include the presence of non-ionic solutes and suspended particulates. Conductivity meters cannot differentiate between ion types, so high conductivity indicates quantity but not composition. Gravimetric evaporation gives direct mass of dissolved solids but requires complete evaporation and careful drying to avoid decomposition.

Distillation for purification

Distillation separates volatile water from non-volatile dissolved impurities by heating and condensing the vapor; evaporation removes dissolved salts and many non-volatile contaminants, producing condensate with much lower TDS. Cause: heating causes phase change of water; effect: non-volatile solutes remain in the boiling flask. Limiting factors include volatile solutes that co-distill, formation of azeotropes, losses by bumping or incomplete condensation, and contamination from apparatus. Multiple distillation steps or additional methods (e.g., activated carbon, ion exchange) are necessary when volatile contaminants or dissolved gases remain.

Extracting and interpreting information from charts, graphs and tables

Reading axes and units ensures correct interpretation; misreading a logarithmic scale as linear causes large misestimation of values. Trends indicate cause-and-effect relationships: rising concentration over time suggests a persistent source or accumulation, while abrupt peaks suggest episodic contamination. Tabulated data require attention to significant figures, units and measurement uncertainty. Comparison of datasets requires common units, comparable sampling times and awareness of outliers. Clear labelling of axes and keys prevents misinterpretation and incorrect conclusions.

Orders of magnitude and evaluating significance

Orders of magnitude use powers of ten to compare sizes of quantities quickly. A difference of two orders of magnitude (100×) indicates a far more significant difference than experimental noise and often implies practical importance. Application requires consideration of measurement uncertainty and detection limits. Small differences within the same order of magnitude and within error margins usually lack practical significance. Large differences across orders of magnitude indicate robust trends or major quality differences that merit action.

Key notes

Important points to keep in mind

Collect samples in clean, labelled containers and record time, location and visible conditions.

Calibrate pH and conductivity meters before use; note calibration standards and date.

Treat pH as a logarithmic measure; a unit change equals a tenfold change in [H+].

Use conductivity as a rapid TDS proxy but confirm composition when specific ions matter.

Use gravimetric residue for accurate TDS when volatile solutes are absent.

Expect distillation to remove non-volatile solutes; watch for co-distilling volatile contaminants.

Check axes, units and scale (linear or logarithmic) when reading graphs and charts.

Compare values using common units and consider sampling times to avoid false trends.

Apply orders of magnitude to prioritise action: differences of ≥2 orders typically indicate major significance.

Account for measurement uncertainty and detection limits before drawing conclusions.