In this technique22966~93.94 the pH of the blood is measured before and after equilibration with two gas mixtures of different but known Pc02. The pH of the two equilibrated blood samples is plotted against the log Pcoz of the equilibrating gases and the points joined by a straight line. This is the buffer line of the blood. The pH of the blood as drawn from the patient is then interpolated in this line and the Pcoz read from the ordinate. Since the equilibration technique ultimately depends on only three blood pH determinations it is important to make these measurements as precisely as possible (see section on pH measurement). Astrup et a166 and Siggaard-Andersen et a122 have described a micro-equilibration unit,* comprising two H-shaped thermostatted glass chambers, which allows two samples of the blood to be equilibrated with each gas simultaneously. The whole unit is shaken mechanically at about 2,500 reciprocations per minute.The gases used for equilibration of the blood are usually 4 % and 8 % C02 in oxygent, but unless a certificate of analysis has been supplied by the manufacturers the user must check the composition by analysis. For this purpose the Haldane apparatus is commonly used; for accurate work the calibration of the analysis burette should be checked by mercury displacement.For clinical work the Campbell-Haldane analyser 123 is adequate. When the Haldane type of apparatus i s used the first few analyses will yield a
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