Hi All, I will appreciate if anyone could share his/her experience or any info about this matter with me. I am doing COD test on some industrial waste samples. to make it economic, i make reagents solution by myself by following the standard method. after all sample and standards preparation, two hours digestion in oven , and cooling down to ambient temperature, when i was gonna transfer samples from cod vials to spectrophotometer cuvette to read the absorbency, the problems with floating particles in cuvette comes out (sedimentation). no one has faced this matter and if you know the reason or anyway that can fix it. Actually i am not using the ready COD vials with solution which is in market (like Hach) and i dont have COD spectrophotometer that be able to put COD vial directly inside to read the absorbency. i just have a general use spectrophotometer that just support its own cuvette.
Thanks Dr Azizul, the floating particles form when i transfer the solutions to cuvette, before that they are totally clear. I tried to read it after one day and just take 2ml of each sample with pipette without moving the COD tubes.I Could read the absorbancy then. but am afraid about changing the COD after one day and am curious about the reason. do the compositions inside samples interfere with the reading or any other reason?
Dear Tahereh Jafary,
COD values do not change after the digestion is over. So, even if you measure the absorbance after one day, the reading would be same as that of the correct one of previous day. About appearance of suspended particle, check whether you are transferring the solution too fast or whether the cuvette contains some solids from before. Make sure the cuvette is totally clean. Transfer the solution slowly, using dropper/ pippette, make sure there is no bubble formed and no solution drips in the side.
Hi there, I am using chemical for preparation of COD reagents (APHA method, close reflux) and not using Hack reagent for COD measurement. But not transferring digested solution to spectrophotometer cuvette, you may use HACH spectrophotometer. If Hach instrument not available, use quartz cuvette and let solution to cool down to room temperature.
Dear Dr Younesi, Actually hatch spect is not available there, I let the samples cool down before reading. Thanks for your advice.
Dear Raka, actually i did the same yesterday and transferred by pipette, better than before. I did all cautions to transfer slowly , but still problem was there. if COD as u mentioned doesn't change after time, that would help then. Thanks again.
Dear Tahereh Jafary
We have been carrying out COD tests with accuracy of less than 1% most of the time. Some time it so happens that the error is above 1 % ( may be up to 5%). I believe only Chrome acid reflux method with a standard and a blank along with samples. Regarding spectrophotometric method the test COD is prone for errors with unknown reasons.
Regarding solids, yes it forms. If the sample is from the industrial effluent, then prefilter it to find COD (COD of dissolved matter). Even then there is possibility for floating (SS matters). So find the composition of the Industrial effluent, especially for nitrates, amines-ammonia or acid reacting chemicals.pollutants etc. try to neutralize the Industrial effluent before filtration and COD measurement.
Conclusion if please follow reflux methods where in possibility for complete digestion (2 to 3 hrs) is possible. The gases formed during COD/ chromic acid oxidation may form floating matter which may hinder COD test by absorbance test.
Thanks Dear Udaya, Deep and warm thanks for the time you read and answer my question patiently. I will appreciate if you refer me to any reference regarding to Chrome acid reflux method that you just mentioned.
I am not sure about the problem because i never face this type of problem...I am also determine the COD of industrial effluent with the help of close reflux method in this method i ti-trate the solution with FAS after 2 hour digestion, u can try this to follow APHA.
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