The samples were fixed immediately after sample collection on-site to prevent reduction of DO by microbes.

What acid did you fix the water samples with?

No acid was used. Winkler’s A (Manganous sulphate) and Winkler’s B (alkali-iodide azide) reagentswere used to fix the water samples. titration was later done with standard sodium thiosulphate solution.

why don't you directly measured using DO meter (in laboratory if Insitu probe not applicable) and was hence conversion to %DO for Water Quality Index

The easiest way to directly measure the DO with a probe in the lab, its easy, and repeatable. There is the option of fixing the water sample the use standard titration methodology to determine the DO but this is time consuming and not overly consistent age of chemicals and type of chemicals used to fix the samples can effect the results.

On-site measurement using a calibrated probe is the best. Waiting until the sample is back in the lab usually results in samples that exhibit lower than usual DO values. Changes in pH or temperature will effect DO values.

thank you colleagues. I thought the same way that fixing method is becoming obsolete. I think on-site determination of DO with meter probe is the best. Results from titrametry may lack precision.

No there is no any acidification.use dissolve oxygen analyzer if availabble(model jpb 607).calibrate it with electrolyte calibration solution supplied from the manufacturer,and fallow the manual .

I understand your concerns, but titrimetry was the only option available at the time of study. Thank God we've been able to acquire a water quality multimeter which in addition to the accuracy of the results obtained, its a less tedious option.

As researchers its important to remember that when the desirable is not available, the available becomes desirable. Furthermore titrimetry is a classical method and has produced trustable results in the past when it was the only option and thus should not be despised because of recent 'exotic' methods.

@victor, you are right by your submission.i agree with you that when the desired isnt available, the available would be desired.

We have 4-5 publications in this regards. I can send those as a reference for your work. Please send me mail at "[email protected]"

My previuos organization International Water Managment Institute (IWMI) had done a great work on water quality in Pakistan, India and Sri Lanka. You can also visit website www.iwmi.pak.org

I am sure you will get lot of helping material.

@muktar, i really appreciate that gesture. I have sent what you demanded.

Surely friend i send you tonight after sorting my publications and also some of my colleages working on water quality.

But you must visit IWMI website also.

It is my great pleasure to help you!!

Thank you mukhtar. Wil mail you later to get acquittance.

Winkler's Method is the best and most acceptable to determine the DO in water

On spot reading taking is best approach.

Oh yes Mukhtar, i agree with you. Taking the reading on-site reduce contamination.

I agree. The differences we have found in DO measures are around 2.0 mg/L when we read on site vs. reading in the lab, after carrying the sample.

I agree with Adriana. I found that the difference between the DO reading in situ and in the lab is impacted by the types of pollutants you have in your sample. Thus, DO change can be quite unpredictable. I usually rely on the use of probes, which don't always have to be very expensive and are quite reliable as long as you calibrate frequently.

Depending on how much biological activity is happening in your sample, DO can go up or down within the sample after it is taken.Photosynthesis can increase DO, while respiration (microbial activity) can decrease DO. In fact, this change in DO is often used in very productive systems to measure primary production and respiration. It is best to measure DO immediately at the site with a dissolved oxygen probe and meter, if available. The Winkler Method involves small glass bottles and a set of chemical reagents (available in single-use packets for use in the field) that convert the dissolved oxygen into a measurable chemical reaction After the first couple of reactions are completed in the field, the sample is "fixed" and the analysis can be completed later in the lab, if you do not have enough time to complete the analysis in the field.

Dear friends,

DO is very important parameter for water quality and its concentration can vary due to photosynthesis or respiration or way of sampling. Best method of DO measure is to use Winkler method at spot or sampling point.

Or if you want to analyze the sample in lab, then first take the samples in 300 ml white and black BOD bottle, fill properly and after that fix DO at spot.

Now, the samples are ready to carry the lab. This way you will find very little fluctuation in DO concentration after fixing and that fluctuation may be calculated by P-R.

thanks and regards

I usually work in Mediterranean rivers. Some of these rivers are of intermittent flow and measuring summer DO in situ, is not always easy nor is to be trusted. For example with the use of an YSI 55 oxygen-meter, I sometimes measure DO saturation >200%. This is due to the way the field apparatus is calibrated. Usually oxygen meters are calibrated with atmospheric oxygen which accounts for about 21% of the air. Therefore, when I place the electrode into a still pool of a river full of green algae at midday, the photosynthesis occurring will produce oxygen molecules, not oxygen diluted from the air. When this happens my readings in the measurement will be unacceptable. In those cases -I agree with Narendra Kumar- Winkler's Method is the best, although not at hand in situ.

Yorgos is correct in that % saturation can be tricky to interpret. However, isn't that why we report both DO in terms of concentration (mg/L) and saturation (%). Most YSI probes have that option.

Dear Rasha I am afraid that the same goes for the concentration. In such extreme cases, if the electrode is calibrated with the atmospheric oxygen it will provide readings like >20mg/L.

Yorgos,

There's nothing invalid (therefor unacceptable) about a super-saturated reading. It's an indication that photosynthesis is producing oxygen faster than it can diffuse away (either into the air above or the water below). Winkler method -fixing the value in the field- should confirm the same (or similar) value.

A clarification: While a reading >100% isn't necessarily a sign of instrument malfunction, we're taught that calibrations should span the range of observed values. If we calibrate between zero and 100% and then observe a value of 200%, we're assuming extrapolation from 100% to 200% is valid, which may not necessarily be true.

colleagues, Thank so much for all the contributions.

I want to believe Winkler method is still acceptable for DO determination but most reputable journals these days are not accepting this method. They prefer the quick-quick method of probing for DO determination. Perhaps, the two methods should be combined in any DO determination.

just try out Winklers Iodometric method, for reference you can check out APHA book