We want to set up a different scenario here that asks the question: what does a difference of 2 \u00b0C actually signify in instrumental analysis? In a thought experiment, we therefore remove all means of temperature control from the analysis devices in the laboratory and look to see how big \u2013 or indeed how little \u2013 an impact an ambient temperature increase from 20 \u00b0C to 22 \u00b0C has on the measurement result. <\/p>\n\n
The warming has a most dramatic impact on enzymatic reactions. Almost every clinical laboratory uses enzymes to identify glucose, cholesterol and other relevant biomolecules. The enzyme reactions largely follow what\u2019s known as the Q\u2081\u2080 rule<\/strong>: this states that enzyme activity doubles with every ten degree increase in temperature. <\/p>\n\n This relationship between reaction speed and temperature is also known as van’t Hoff’s Law, after Jacobus Henricus van \u2019t Hoff, the Dutch Chemist who formulated it in 1884 (expanded in 1889 by Svante Arrhenius to become the Arrhenius equation).<\/p>\n\n A two degree rise in temperature increases enzyme activity by almost 15 percent.<\/strong> This has consequences for medical tests, such as establishing blood sugar levels. A glucose assay that gives a correct reading of 100 mg\/dl at 20 \u00b0C, for instance, would measure as high as 115 mg\/dl at 22 \u00b0C. In diabetes diagnostics, this could be the difference between \u201chealthy\u201d and \u201cin need of treatment\u201d. <\/p>\n\n The pH value is also sensitive to temperature changes. Buffer solutions that are intended to keep pH values stable show that even a change of just a few degrees can cause a temperature-related drift. <\/p>\n\n The temperature dependency of the pH value is primarily down to the self-ionisation of water<\/strong>. This shifts towards the ionic product as an endothermic reaction when temperature rises<\/strong>, causing the pH value to fall<\/strong> (increase in H\u207a concentration). <\/p>\n\n H\u2082O \u21cc H\u207a + OH\u207b<\/p>\n\n Buffer systems are also temperature-dependent because of the dissociation constant<\/strong> Ka<\/sub>, as evidenced by the van’t Hoff equation:<\/p>\n\n dln(Ka<\/sub>)\/dT = \u0394H\u00b0\/(RT\u00b2)<\/p>\n\n As well as temperature-dependent dissociation of the buffer and the self-ionisation of the water, the ionic activity is also subject to temperature dependency.<\/p>\n\n As per the equation for activity coefficient \u03b3,<\/strong> the following applies: All these influences are very labour-intensive to calculate in detail. For small temperature changes, however, they can get close in linear terms, which in practice is indicative of a simplified equation of the temperature dependency: <\/p>\n\n pH(T) = pH(T\u2080) + \u03b1 \u00d7 (T – T\u2080)<\/p>\n\n Where \u03b1 is the temperature coefficient that is calculated specifically for a buffer system (normally specified by the manufacturer). Consequently, the change in pH for a typical phosphate buffer at a temperature change of 2 \u00b0C is as follows: <\/p>\n\n pH(20 \u00b0C) = 7,0000<\/p>\n\n pH(22 \u00b0C) = 7,0000 + (-0,0028\/\u00b0C \u00d7 2\u00b0C) = 6, 9944<\/p>\n\n which in turn gives<\/p>\n\n H\u207a concentration at i 20 \u00b0C: 10\u207b\u2077 mol\/l = 1,000 \u00d7 10\u207b\u2077 mol\/l<\/p>\n\n H\u207a concentration at 22 \u00b0C: 10\u207b\u2076’\u2079\u2079\u2074\u2074 mol\/l = 1,013 \u00d7 10\u207b\u2077 mol\/l<\/p>\n\n This equates to a percentage change of +1.3%.<\/strong><\/p>\n\n A phosphate buffer with a pH of 7.0 at 20 \u00b0C drops to pH 6.994<\/strong> at 22 \u00b0C. Though this may sound very little, it signifies a 1.3 increase in the concentration of hydrogen ions<\/strong>, determined by the logarithmic interplay of the pH scale (changing the pH value by 1 results in the hydrogen ion concentration changing by a factor of 10). The high measurement requirements of certain analyses means even a deviation such as this could have consequences, for example if it is a precipitation reaction or a biochemical reaction that is pH-sensitive. <\/p>\n\n IN THE SPOTLIGHT: acidification of the oceans In high-performance liquid chromatography<\/strong> (HPLC), a temperature difference of just 2 \u00b0C causes a notable shift in retention times.<\/strong> This is primarily a result of the partition equilibrium between the stationary and mobile phase, which changes with the temperature. The van’t Hoff equation<\/strong> describes this as follows: <\/p>\n\n ln(k’) = -\u0394H\u00b0\/(RT) + \u0394S\u00b0\/R + ln \u03a6<\/p>\n\n Where k’ is the retention factor, \u0394H\u00b0 the standard enthalpy of the analyte partition between the phases, \u0394S\u00b0 is the corresponding entropy change, R is the gas constant, T the absolute temperature and \u03a6 the phase ratio of the columns.<\/p>\n\n The formula shows a reverse temperature dependency<\/strong>: that is to say, higher temperatures lead to lower retention factors and, in turn, shorter retention times. As a rule of thumb for small analyte molecules, the retention reduces falls by two percent for every degree the temperature increases. [7] <\/strong><\/p>\n\n Parallel to the partition equilibrium, the viscosity <\/strong>of the solvent also changes: it falls by 2.2 percent, which reduces the column pressure and influences the subsequent separation. For a pharmaceutical company checking the purity of medication, variations such as these would have a profound impact. Quantitative assessments would be systematically incorrect and the method validation would have to be completely revised. <\/p>\n\n The electrical conductivity<\/strong> of solutions increases in a linear relationship with temperature \u2013 by approximately two percent per degree.<\/strong> Consequently, a two-degree warming means an increase of four percent.<\/strong><\/p>\n\n A saline solution of 1000 \u00b5S\/cm at 20 degrees suddenly shows 1040 \u00b5S\/cm at 22 degrees. In water analysis<\/strong>, for example this would lead to false readings for ion concentration. Drinking water limit values<\/strong> could appear exceeded, when in actual fact nothing in the composition has changed. <\/p>\n\n Not even electrochemistry <\/strong>is spared the effects. The Nernst equation<\/strong> \u2013 at the heart of all electrochemical measurements \u2013 incorporates the temperature in its figures: E = E\u2080 + (RT\/nF) \u00d7 ln(Q).<\/strong> <\/p>\n\n The Nernst factor<\/strong> RT\/F increases from 58.85 mV per decade at 20 \u00b0C to 59.16 mV at 22 \u00b0C. That equates to an increase of 0.53%<\/strong> \u2013 enough for pH electrodes to systematically display false values. <\/p>\n\n The thought experiment shows: temperature is a critical quality factor in many areas of analysis. Even a difference of just two degrees Celsius can result in significant change<\/strong>, whether in terms of enzyme reactions, pH values or other temperature-dependent factors, such as electrical conductivity, viscosity or ion solubility.<\/p>\n\n In laboratory work, precise temperature measurement, adjustment calculus in the systems and exact temperature control are necessary to ensure analytical results are always reproducible. This is why it is important to have measuring devices regularly maintained and to check their calibration or recalibrate them. <\/p>\n\n Sources:<\/strong><\/p>\n\n [2] Woods Hole Oceanographic Institution: Scientists identify how ocean acidification weakens coral skeletons – https:\/\/www.whoi.edu\/press-room\/news-release\/scientists-identify-how-ocean-acidification-weakens-coral-skeletons\/<\/a><\/p>\n\n [3] Ocean Acidification ICC: Why ocean acidification is called climate change’s evil twin – https:\/\/news-oceanacidification-icc.org\/2024\/12\/25\/why-ocean-acidification-is-called-climate-changes-evil-twin\/<\/a><\/p>\n\n [4] NCBI PMC: Pteropod shell thinning in natural CO\u2082 gradients – https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC7814018\/<\/a><\/p>\n\n [5] Coast Adapt: Ocean acidification and its effects – https:\/\/coastadapt.com.au\/ocean-acidification-and-its-effects<\/a><\/p>\n\n [6] European Environment Agency: Ocean acidification indicators – https:\/\/www.eea.europa.eu\/en\/analysis\/indicators\/ocean-acidification<\/a><\/p>\n\nA shake-up for pH values<\/h3>\n\n
Chromatography precision declines<\/h3>\n\n
Conductivity measurements drift away<\/h3>\n\n
When reference electrodes give false values<\/h3>\n\n
Conclusion<\/h3>\n\n
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