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1 <br /> To make a measurement df pH, the solution is <br /> i saturated, or nearly so, with quinhydrone This <br /> establishes an equimolar mixture of quinone and <br /> hydroquinone The potential of this redox couple is <br /> 1 expressed in mV by <br /> Eh = 699 4 — 59 16 pH (al 250C) <br /> The potential of this half-cell is measured by dipping <br /> 1 a clean gold electrode and reference electrode into <br /> the solution which has been saturated with <br /> quinhydrone The voltage. in mililvolls at all tempera- <br /> tures and Including the reference electrode potential <br /> iwill be given by <br /> E = E0 --E' — 01984T pH <br /> where E Is the observed potential in millivolts <br />' E0 — E' is the standard voltage of the whole <br /> system and is approximately 453 millivolts if a calomel <br /> reference is used - <br /> T is the absolute temperature (TOG -t 273 16 or "K) <br /> A buffer saturated with quinhydrone may be used to <br /> calibrate the system If a buffer close to the pt1 of the <br />' sample is used, the sail effect In this system is <br /> minimized (see Table 5-4) <br /> The quinhydrone system is not usable above pH 85 <br /> due to the dissociation of hydroquinone it is also not <br /> necessary at high pH since HF is dissociated and [lie <br /> F— Ions do not strongly attack glass <br /> The presence of strong oxidants or reductants will <br /> interfere For best results, samples should be held at <br /> constant temperature Solutions of high salt or acid <br /> content can also cause errors <br /> f <br /> r - <br /> i <br /> r <br /> GOLD REFERENCE <br />