I think you have to elaborate and verify your measurement. Such difference is not a stochastic error but it is a systematic error resulting from the calibration process. It is good if you report about the repetition experiment.

Best wishes

Hello Behrad;

you don't get used to our laboratory apparatus yet ?????

in my opinion, maybe, the calibration curves should be drawn again.

hi mohammad Javad!

yes, I am used to many problems in our lab but this is a new one!

right! i"m gonna have to redraw my calibration curve but the thing is my last readings totally corresponded with the calibration curves available on the internet. maybe the tungsten lamp is depreciated. i also had made many 10 ppm samples before which gave me the same result and i triple checked the new reading to rule out any testing errors!

anyways thanks for commenting!

Dear Mohammad Javad Bardi

If we suppose that the calibration of the instrument is correct during the two measurements. It might point out that the dissolving procedure and the process solubility of methylene blue did not yet complete when you measured last year. However, now after one year, it is sure that the methylene blue got dissolved fully in DI water. Therefore, it is expected to get more absorbance with fully dissolved methylene blue.

Regards

I think you have to elaborate and verify your measurement. Such difference is not a stochastic error but it is a systematic error resulting from the calibration process. It is good if you report about the repetition experiment.

Best wishes

to solve this problem many thing must take into consideration such how you store your samples , temperature , concentration, the calibration curve of the system.

At an absorbance about 2 you are at about 1%T, which means that more than 99% of available light is being blocked .