Spectrophotometric determination of trace mercury in water based on hydrogen peroxide catalyzing the oxidation reaction of nickel-palatine chrome black complex Rheological Behaviors and Secondary Networks of Silica In-situ Composits of Polyacrylamide Hydrogel 催化过氧化氢氧化镍-铬蓝黑R络合物光度法测定水体中痕量汞白炭黑原位复合聚丙烯酰胺水凝胶的流变行为与次级网络