[log in to unmask] wrote:
> 
>  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

Response:

It is accepted practice to acidify effluent samples tested for metals
with nitric acid to a pH of less than 2.  The acidification prevents
precipitation of the metal and assures or improves the removal of a
representative aliquot from the sample for testing.  Even so, it is
recommended the sample be shaken vigorously prior to extracting an
aliquiot for testing.

Regarding AAS testing, it is abosolutely imperative the sample is acidic
prior to aspiration into the AAS.  The acidification inhibits
interference resulting from chelators or complexors in the effluent
which may affect the wavelenght of abosorbance of a given species.  The
best method to minimize interference is by "digesting" the sample. 
Digestion involves refluxing the sample at approximately 180 F but below
the boiling point for 1-4 hours.  Digestion reduces interferences from
organic species in the sample via oxidation of such species.

You may also be interested to know there are a number of methods to
improve sensitivity in acetylene-air AAS testing.  While the procedures
are too numerous to discuss in detail in this forum, we would be happy
to provide you with copies of our procudures.  Please feel free to call
Frank Pernat at 612-479-6525 if you would like these procedures.

Regards,
Ted Stern
Circuit Research Corporation

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