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<br />once in the control and the high, medium, and <br />low toxicant concentrations. <br />11.9.2.2 Toxicant Measurements-The con- <br />centration of toxicant in the test chambers <br />should be measured as often as practical during <br />the test. At a minimum, the concentration of <br />toxicant must be measured in (a) each chamber <br />concurrently at least once during the test, pref- <br />erably near the beginning of the test; (b) except <br />for the control treatment, each test chamber <br />(especially for those toxicant concentrations <br />closest to the LC50 or EC50) at least one <br />additional time during the test, on a schedule <br />designed to give reasonable confidence in the <br />concentration of toxicant in the test chambers, <br />taking into account flow rate, and the number <br />of independent toxicant-metering devices; and <br />(c) at least one appropriate chamber whenever <br />a malfunction is detected in any part of the <br />metering system. For each treatment, the high- <br />est of all the measured concentrations obtained <br />during the test divided by the lowest should be <br />less than 1.5. If the ratio is greater than 1.5, the <br />metering system should be checked, and addi- <br />tional samples from the proper chambers <br />should be analyzed to determine the precision <br />of the sampling or analytical methods. In ad- <br />dition, if the measured concentration of toxi- <br />cant in any chamber is more than 30 % higher <br />or lower than the concentration calculated from <br />the composition of the stock solution and the <br />calibration of the metering system, identifica- <br />tion of the cause may provide useful informa- <br />tion about the operation ofthe metering system <br />or the properties of the toxicant. Measurement <br />of the concentration of toxicant in the solution <br />flowing into the test chamber will indicate <br />whether the cause is in the metering system or <br />in the test chamber. If the concentration in the <br />test chamber is too high, the stock solution may <br />have been prepared incorrectly or the metering <br />system may not have been calibrated correctly. <br />If the concentration is too low, additional pos- <br />sible causes are volatility and degradation. If <br />the cause is degradation, the test organisms are <br />probably being exposed to significant concen- <br />trations of degradation products. Measure- <br />ments of the degradation product(s) and use of <br />a faster flow rate may be desirable. <br />11.9.3 All Tests-Temperature must be re- <br />corded at least hourly throughout acclimation <br />and throughout the test in at least one test <br />chamber, except that for systems whose tem- <br /> <br />~~l~ <br /> <br />E 729 <br /> <br />perature is controlled by a constant-tempera- <br />ture area or water bath, the temperature of the <br />area or bath may be recorded at least once <br />every 6 h, although these may vary more than <br />test temperature. <br />11.9.4 Methodology: <br />11.9.4.1 For measurement of dissolved or <br />suspended toxicant or both, water samples <br />should be obtained by pipetting or siphoning <br />through glass or fluoroplastic tubing from a <br />point midway between the top, bottom, and <br />sides of the test chamber and should not include <br />any surface scum or material stirred up from <br />the bottom or sides. For measurement of total <br />toxicant in static tests, the whole volume of <br />solution in a test chamber should be (a) used <br />as the sample or (b) treated appropriately (for <br />example, by adding acid, base, or surfactant <br />and mixing thoroughly) to uniformly distribute <br />the toxicant before a sample is taken. For mea- <br />surement of total toxicant in flow-through tests, <br />a large volume of the solution flowing into the <br />test chamber should be collected and used as <br />the sample or treated appropriately to uni- <br />formly distribute the toxicant before a sample <br />is taken. Samples of test solutions must be <br />handled and stored appropriately to minimize <br />loss of toxicant by microbial degradation, hy- <br />drolysis, photodegradation, chemical reaction, <br />volatilization, or sorption. <br />11.9.4.2 Chemical and physical data should <br />be obtained using ASTM standards whenever <br />possible. When an ASTM standard does not <br />exist or is not sensitive enough, methods should <br />be obtained from other reliable sources (24). <br />The concentration of un-ionized ammonia may <br />be estimated from pH, temperature, and the <br />concentration of total ammonia (25). <br />11.9.4.3 The analytical method used to mea- <br />sure the concentration of toxicant in test cham- <br />bers must be validated before beginning the <br />test. The accuracy of the method must be mea- <br />sured by the method of known additions using <br />dilution water from a tank containing test or- <br />ganisms; on each of two separate days three <br />samples must be analyzed at the lowest or the <br />next to the lowest toxicant concentration that <br />will be used in the toxicity test. The accuracy <br />and precision of the analytical method should <br />be checked using reference or split samples, <br />interlaboratory comparisons, or alternative <br />(preferably reference or corroborative) meth- <br />ods of analysis. The accuracy of standard so- <br /> <br />15 <br />