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. _. 4.2 Gla ectrode. ._ <br /> 4.3 Reference electrode: A silver-silver chloride or <br /> electrode o constant potential may be used, other reference <br /> NOTE: Combination electrodes incorporating both measuring and referen <br /> functions are convenient to use and are available w ced <br /> gel- <br /> type filling materials that require minimal maintenance. solid. <br /> 4.4 M2netic stirrer and Teflon-coated stirring bar. <br /> 4.5 Thermometer or temperature sensor for automatic Compensation. <br /> 5.0 REAGENTS <br /> 5.1 Primar standard buffer salts are available from <br /> of Standards Specia Pub ication 260 and should be used m the National Bureau <br /> extreme accuracy is necessar ed in situations where <br /> salts requires some special precautionsaandnhandling,of esuchnce sa lows from these <br /> dilution water, drying ovens, and carbon-dioxide-frees low-conductivity <br /> solutions should be replaced at least once each monthpurge gas, These <br /> 5.2 Secondary standard buffers may be prepared from <br /> Purchased as so utions from commercia vendors, m NBS salts or <br /> solutions have been validated by comparison with standard available <br /> recommended for routine use. s and are <br /> 6.0 SAMPLE COLLECTION, PRESERVATION, AND HANDLING <br /> 6.1 All samples must be collected using a sa lin plan <br /> the considerations discussed in Chapter Nine of this m g P that addresses <br /> manual . <br /> 6.2 Samples should be analyzed as soon as possible. <br /> 7.0 PROCEDURE <br /> 7.1 Calibration:.. <br /> 7.1.1 Because of the wide variety of pH meters <br /> and detailed operating procedures cannot be incorporated into accessories, <br /> Each analyst must be acquainted with the o eratio his method. <br /> familiar with all instrument functions. Special attention each system and <br /> electrodes is recommended, on to care of the <br /> 7.1.2 Each instrume.gr.,! ir-fr <br /> minimum of t�„_ ,�,• e~-�-• ode system must be calibrated at a <br /> �s�.:e:ts treat bracket the expected pH of the samples and are <br /> approximately* three n}i units Or <br /> may involve use of " more apart. Various instrument designs <br /> adjustment, as Outlined in the pmanufacturer'sbalance" orNsinstruction or a slope <br /> • instructions. Repeat <br /> 9040 — 2 <br /> Revision 0 <br /> Date September1986 <br />