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1 2 3 4 5 SILICA, DISSOLVED Method 370.1 (Colorimetric) STORET NO. Dissolved 00955 Scope and ApphcatlOn 1 1 ThIS method IS apphcable to dnnkmg, surface and salme waters, domestic and mdustnal wastes 1 2 The workmg range of the method IS approXImately 2 to 25 mg slhca/1 The upper range can be extended by takmg sUltable ahquots, the lower range can be extended by the addltlOn ofammo-naphthol-sulfomc aCId solutlOn, as descnbed m (6 8) Summary of Method 21 A well-mIxed sample IS filtered through a 0 45 u membrane filter The filtrate, upon the addltlOn of molybdate lOn m aCIdIC solutlOn, forms a greemsh-yellow color complex proportlOnal to the dIssolved slhca m the sample The color complex IS then measured spectrophotometncally 22 In the low concentratlOn modlficatlOn the yellow (410 nm) molybdoslhclC aCId color IS reduced by 1-ammo-2-naphthol-4-sulfomc aCId to a more mtense heteropoly blue (815 nmor650nm) Interferences 31 ExceSSIve color and/or turbIdIty mterfere Correct by runmng blanks prepared wIthout addItionoftheammomummolybdatesolutlOn See(67) 32 Tanmn mterference may be ehmmated and phosphate mterferences may be decreased wIth oxahc aCId 33 Large amounts of Iron and sulfide mterfere 34 Contact wIth glass should be mlmmlzed, slhca free reagents should be used as much as possIble Ablankshouldberun Apparatus 41 Platmum dIshes, 100 ml 42 Colonmetnc eqUlpment-one of the followmg 4 2 1 Spectrophotometer for use at 410 nm, 650 nm and/or 815 nm wIth a 1 cm or longer cell 422 FIlter photometer wIth a vlOlet filter havmg maXImum transmIttance as near 410 nm as possIble and a 1 cm or longer cell 4 2 3 Nessler tubes, matched, 50 ml, tall form Reagents 5 1 Use chemIcals low m slhca and store m plastIc contamers 52 Sodmm bIcarbonate, NaHC03, powder 53 SulfuncaCId,H2S04, 1N Approved for NPDES Issued 1971 Edltonal reVlSlOn 1978 370 I-IPDF Image | Methods for Chemical Analysis of Water and Wastes
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