Science Behind pH Indicator Strips

What is pH?

colour chart for asphirate strips

The self-ionisation of water produces two ions, the hydronium ion, H3 O+ (or more simply H+) and the hydroxide ion, OH—. The concentration of H+ defines whether that solution is acidic, neutral or basic. The most convenient way of looking at these concentrations is in terms of pH.

The pH scale runs from 0 — 14, with a pH of 7 considered “neutral” (where the concentrations of H+ and OH— are equal). When the concentration of H+ ions increases, the pH of the solution will fall below 7 and be classed as “acidic”. Conversely, the pH of the solution will increase above 7 and be classed as “basic”, when the concentration of H+ ions decreases.

Chemicals that dissolve into water will all have some effect on the pH of the resultant solution. This will depend on how easily they ionise to form H+ and/or OH— ions. Examples of chemicals that ionise very easily and therefore have a strong impact on the pH are hydrochloric acid and sodium hydroxide.

What are buffered solutions?

A buffer is a solution that can resist small changes in pH. Buffers are made of two component parts, a weak acid plus its conjugate base or a weak base plus its conjugate acid. Examples of these are citric acid (C6 H8 O7 ) and sodium citrate (Na3 C6 H5 O7 ) as well as sodium carbonate (Na2 CO3 ) and sodium bicarbonate (NaHCO3 ). Strong acids and bases do not make good buffer components as their relative acidity/basicity is too high. Buffers resist changes to pH by ‘soaking up’ any H+ or OH— that may be added to the solution. This of course will only happen up to a certain concentration of additional H+ or OH—, as there is only a finite level of buffer (known as its buffering capacity).

Why does the pH strip have to be adjusted for water?

In terms of the NHS, water is sometimes used to flush the tubes used for nasogastric feeding. If the strip were to read a pH of 5 or below, simply because it has come into contact with water rather than gastric aspirate, then a false-positive reading may be made. By setting the pH of the strip to 7, then any accidental reading of water will not result in a false positive as, the pH reading of the strip will be 7 and therefore outside the critical range.

Are all manufacturers the same?

Universal Indicator Paper Reel

The same issues regarding measurements of water and colour chart calibration exist for all manufacturers. However, manufacturers will differ depending on which pH indicators are selected for the pH range required and, therefore, it is not uncommon for different colours to be observed for the same pH range.

Why are the colours different to that of the pH wheel?

The colour spectrum would infer that the colours associated with pH would follow a logical pattern from red (acidic) through to blue (basic), via orange—yellow—green. Although this can be achieved with universal indicator, the common misconception is that this colour change comes from one pH indicator. This is not the case as universal indicator is typically made up of a range of different pH indicators.

There are several hundred different pH indicators available to scientists and they usually only exhibit a colour change over 1 — 2 pH units. Careful consideration is taken to both pH working range and colour combinations when designing a pH strip, which in turn determines what the resultant colours may be.

What is IVD?

IVD stands for “in vitro diagnostic” and refers to tests carried out on samples that have been taken from the human body like gastric aspirate, blood or urine.

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