Microbiological residual chlorine check

A forum for discussion of small laboratory concerns within TNI.

Moderator: eaturner

Microbiological residual chlorine check

Postby dchristopher » Tue Jun 07, 2011 1:16 pm

In NELAC 2003, (a)(3), Sample Receipt Protocols, it states

"sufficient sodium thiosulfate is added to each container to neutralize at a minimum 5mg/L of chlorine for drinking water and 15mg/L chlorine for wastewater samples"

I would like to know how other labs are determining that the 15mg/L has been neutralized ie: testing the sample on a spec and requiring the result to be below a certain level; if so what level, claculating a percent reduction or simply using a test strip for the abscence of chlorine.
Posts: 1
Joined: Mon Jun 06, 2011 12:06 pm

Re: Microbiological residual chlorine check

Postby kchapman1 » Fri Jun 10, 2011 1:08 pm

Here is the procedure that is used at my lab:

Check of effectiveness of Sodium Thiosulfate for chlorine residual neutralization each autoclave batch.

1. Prepare concentrated CL2 solution by adding 2.0 mL of household bleach to 1 Liter distilled water. (Determine concentration by making a 1:100 solution and measuring on meter. It should be approx 100 ppm)
2. Determine the appropriate volume of concentrated CL2 solution to add to the 250 ml check bottle in order to have at least 15 ppm Cl2 by the following Calculation:

Calculation: 15.0 mg/L X (volume of bottle, ml) / B = C

Where B = Concentration of Stock CL2 Solution
C = mLs of Concentrated Stock CL2 Solution added to check bottle

3. Shake well.
4. Measure Free CL2 Residual using the HACH DPD powder pillow method.
5. Result should be < 0.05 mg/L.
6. If > 0.05 mg/L, check another bottle.
7. If still > 0.05 mg/l re-sterilize bottles with fresh thiosulfate.
Posts: 24
Joined: Tue May 26, 2009 1:21 pm

Return to Small Laboratory Advocacy Group