[Procyon]

To determine the sulfate levels in water I'm planning to use the turbidimetric method using BaCl2 and spectrophotometer. The problem is that there are some differences between the methods used by EPA (method #375.4) and the method for determining sulfate levels in a sulfate reducing column (which MAY be part of the difference).

The EPA method uses an absorbance of 420 nm; is recommended for a sample with no more than 40 ppm  (mg/Lt) of sulfate ; but do not specify the grs of BaCl2 needed and uses a conditioning reagent, reason which requires a sample volume of 100ml. I only have 5 ml of sample as maximum, and even  if I dilute it to 100 ml, the sulfate levels will be below the minimun detectable limit.

The second method uses an absorbance of 890 nm, it just uses 1,4 ml of sample and do not uses the conditioning reagent. Nevertheless this method was uses to measure levels of 200 to 600 ppm (mg/L) of sulfate.

So why the differences of absorbance, volumes of samples required and use of the conditioning reagent?
 I don't know which variant will work for the levels I expect (30 - 70 mg/L), and with low volumes (1ml).

[Arkcon]

What is the conditioning reagent?  What is its name, so we can try to figure out what it does?  I guess if both methods are equivalent for the EPA you can choose whichever one matches your sample size and expected sulfate level.

[Procyon]

The conditioning reagent is:  30ml conc. HCl  + 300 ml distilled water    +     100 ml of 95% ethanol or isopropanol    +    75 gr NaCl    +    50 ml.  I must add 5 ml of this solution to 100 ml of water sample and stirring at constant speed.

One method matches the sample size but not the expected sulfate level... and the other viceversa.

I suppose I can do the EPA method using volumes one less order of magnitude. That is to dilute my sample to 10 ml instead of 100ml. Does this bring an inconvienence?