recently we found variation in results we obtained by 2 test methods whether 
 we acidify the sample or not.
 normally we discharge our effluent slightly alkaline (Ph 7.5 to 8.5).The normal
procedure for analysis is collect sample filter in through whatman filter paper
take to AAS for analysis.when we acidified the sample after filteration we found
copper indicated to be vary high ( showing 50 to 100 % higher value)

so we did an experiment and used 2 different methods.one AAS (flame) and Neocupione method .the neoprione method is described below :
 50ml of sample in a separting flask
add 5 ml hydrooxyamine-hydrochloride (200gm in 1000ml watere)
add 10 ml sodium citrate solution (250gm in 1000ml water)
add 10 ml neocuprione solution (1 gm dissolved in  500ml isopropylalcohol then made up to 1000ml with water)
add twice20 ml chloroform,shakefor 1 minute allow layer to separate and collectextracts in 50 ml flask and make up volume to 50 ml with IPA.
check absorbance at 457 nm and from standards similarly prepared and extracted
plot calibration curveand find value.

  
SAMPLE EFFLUENT TAKEN AT PH OF 8.4

           EXPERIMENT                             BY AAS        BY NEOCUPRIONE


 1)  sample filter through 0.45micron
     absolute filter                               0.4           0.62

  2) filtered sample of (1) acidify
     to ph of 1.5 to 2                             0.68           1.6

  3) acidified sample of (2),raise
     ph to 8.5 allow for15 minutes
     filter through 0.45 micron                    0.39           0.65


all analysis were done by same analyst on same day andstandards prepared from certified copper standrd.

. since we have filtered through 0.45 micron filter only dissolved copper should be present in the sample.then why copper increases when we acidify the sample by both methods.what is the correct ph or method for analysis.

when samples have to be preserved for regulatory requrements, the samples are acidified and preserved. with this high variation between acidified and non acidified samples it can be make or break with the maximum permissible limits

can any technetter throw light? thanks in advance
 

  

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