METHOD 9040B pH ELECTROMETRIC …

1 CD-ROM9040B - 1 Revision 2 January 1995 METHOD 9040 BpH ELECTROMETRIC SCOPE AND 9040 is used to measure the pH of aqueous wastes and thosemultiphase wastes where the aqueous phase constitutes at least 20% of the totalvolume of the corrosivity of concentrated acids and bases, or of concentratedacids and bases mixed with inert substances, cannot be measured. The pHmeasurement requires some water pH of the sample is determined electrometrically using eithera glass electrode in combination with a reference potential or a combinationelectrode. The measuring device is calibrated using a series of standardsolutions of known glass electrode, in general, is not subject to solutioninterferences from color, turbidity, colloidal matter, oxidants, reductants, ormoderate (< molar solution) error at pH levels >10 can be reduced or eliminated by usinga low-sodium-error of oily material or particulate matter can impairelectrode response.

2 These coatings can usually be removed by gentle wiping ordetergent washing, followed by rinsing with distilled water. An additionaltreatment with hydrochloric acid (1:10) may be necessary to remove any effects on the ELECTROMETRIC determination of pH arisefrom two sources. The first is caused by the change in electrode output atvarious temperatures. This interference should be controlled with instrumentshaving temperature compensation or by calibrating the electrode-instrument systemat the temperature of the samples. The second source of temperature effects isthe change of pH due to changes in the sample as the temperature changes. Thiserror is sample-dependent and cannot be controlled. It should, therefore, benoted by reporting both the pH and temperature at the time of AND meter: Laboratory or field model.

3 Many instruments are commer-cially available with various specifications and optional - 2 Revision 2 January electrode: A silver-silver chloride or other referenceelectrode of constant potential may be : Combination electrodes incorporating both measuring andreferenced functions are convenient to use and are available withsolid, gel-type filling materials that require minimal stirrer and Teflon-coated stirring and/or temperature sensor for automatic grade chemicals shall be used in all tests. Unlessotherwise indicated, it is intended that all reagents shall conform to thespecifications of the Committee on Analytical Reagents of the American ChemicalSociety, where such specifications are available.

4 Other grades may be used,provided it is first ascertained that the reagent is of sufficiently high purityto permit its use without lessening the accuracy of the Primary standard buffer salts are available from the NationalInstitute of Standards and Technology (NIST) and should be used in situationswhere extreme accuracy is necessary. Preparation of reference solutions fromthese salts requires some special precautions and handling, such as low-conductivity dilution water, drying ovens, and carbon-dioxide-free purge solutions should be replaced at least once each Secondary standard buffers may be prepared from NIST salts orpurchased as solutions from commercial vendors. These commercially availablesolutions have been validated by comparison with NIST standards and arerecommended for routine COLLECTION, PRESERVATION, AND HANDLING All samples must be collected using a sampling plan that addressesthe considerations discussed in Chapter Nine of this Samples should be analyzed as soon as Because of the wide variety of pH meters and accessories,detailed operating procedures cannot be incorporated into this analyst must be acquainted with the operation of each system andfamiliar with all instrument functions.

5 Special attention to care of theelectrodes is Each instrument/electrode system must be calibrated at aminimum of two points that bracket the expected pH of the samples and areapproximately three pH units or more apart. (For corrosivity characteri-zation, the calibration of the pH meter should include a buffer of pH 2CD-ROM9040B - 3 Revision 2 January 1995for acidic wastes and a pH 12 buffer for caustic wastes; also, forcorrosivity characterization, the sample must be measured at 25 1EC if thepH of the waste is above ) Various instrument designs may involveuse of a dial (to "balance" or "standardize") or a slope adjustment, asoutlined in the manufacturer's instructions. Repeat adjustments onsuccessive portions of the two buffer solutions until readings are pH units of the buffer solution the sample or buffer solution in a clean glass beaker usinga sufficient volume to cover the sensing elements of the electrodes and to giveadequate clearance for the magnetic stirring bar.

6 If field measurements arebeing made, the electrodes may be immersed directly into the sample stream to anadequate depth and moved in a manner to ensure sufficient sample movement acrossthe electrode-sensing element as indicated by drift-free readings (< pH). the sample temperature differs by more than 2EC from the buffersolution, the measured pH values must be corrected. Instruments are equippedwith automatic or manual compensators that electronically adjust for temperaturedifferences. Refer to manufacturer's rinse and gently wipe the electrodes prior to measuringpH of samples. Immerse the electrodes into the sample beaker or sample streamand gently stir at a constant rate to provide homogeneity and suspension ofsolids.

7 Note and record sample pH and temperature. Repeat measurement onsuccessive aliquots of sample until values differ by < pH units. Two or threevolume changes are usually to Chapter One for the appropriate QC must be thoroughly rinsed between analysts in twenty laboratories analyzed six syntheticwater samples containing exact increments of hydrogen-hydroxyl ions, with thefollowing results: Accuracy as Standard Deviation Bias BiaspH Units pH Units % pH Units + + + + REFERENCESCD-ROM9040B - 4 Revision 2 January Bureau of Standards, Standard Reference Material Catalog 1986-87,Special Publication - 5 Revision 2 January 1995 METHOD 9040 BpH ELECTROMETRIC measurement