Dear Sir. Concerning your issue about the measurement of dissolved oxygen in saline solutions. The colorimetric method offers a basic approximation of dissolved oxygen concentrations in a sample. There are two methods designed for high-range and low-range dissolved oxygen concentrations. These methods are quick and inexpensive for basic projects, but limited in scope and subject to error due to other redoxing agents that may be present in the water. The traditional method is the Winkler titration. While this method was considered the most accurate and precise for many years, it is also subject to human error and is more difficult to execute than the other methods, particularly in the field (Crone, T. (2004, November). The Basic Sediment Transport Equations Made Ridiculously Simple. In OCEAN/ESS 410 Marine Geology and Geophysics. Retrieved from http://www.ocean.washington.edu/courses/oc410/reading/sedtrans_2004.pdf). The Winkler method now exists in seven modified versions which are still used today.  Salinity can be measured with a conductivity/salinity sensor and automatically compensated for, or approximated and manually input as Calibration and operating procedures can vary between models and manufacturers. An instruction manual should be referenced during the measurement and calibration processes (Gao, Q. (2006, November). Correlation of Total Suspended Solids (TSS) and Suspended Sediment Concentration (SSC) Test Methods . In New Jersey Department of Environmental Protection Division of Science, Research, and Technology . Retrieved from http://www.state.nj.us/dep/dsr/soils/tss%20vs%20ssc%20test%20methods.pdf). I think the following below links and the attached file  may help you in your analysis:

http://www.fondriest.com/environmental-measurements/equipment/measuring-water-quality/dissolved-oxygen-sensors-and-methods/

https://www.fondriest.com/pdf/ysi_do_handbook.pdf

http://www.ott.com/download/ldo-white-paper/

Thanks

There are now other oxygen sensors that you could use, notably those that rely on luminescence decay (e;g. from PreSens, www.presens.de); they have the advantage that they do not consume oxygen during the measurement, contrary to Clark electrode, and are more temperature resistant.

You might actually have had a misstep with the calibration as the probe might be calibrated relative to saturation levels against air at normal pressure, not actual oxygen concentrations, and automatically correct for salinity.

And you've got the order of magnitude wrong, it is not 8 g/L, only 8 mg/L.

What both the optical and the Clark cell sensors measure is the partial pressure of oxygen in your sample. Since the partial pressure of oxygen is identical in both samples (both are 100% saturated) the sensor's output signal is the same in both the freshwater and seawater media. For example, assuming that the atmospheric pressure is 760 mm Hg, it follows that at equilibrium PO2 = 0.21 x 760 = 160 mm Hg and dissolved oxygen % saturation is (160/160) x 100% = 100%

At equilibrium, freshwater and seawater are 100% saturated and will thus give the same output readings, even though O2 concentrations are different since freshwater can dissolve more O2 as you pointed out.

Now, the degree of saturation can be expressed as % sat O2 = ([O2] / NAEC) x 100  where NAEC is the normal atmospheric equilibrium concentration for O2 and [O2] is the in situ O2 concentration. The important point here is that NAEC depends on temperature and salinity. Therefore a knowledge of salinity is required to convert your sensor's output signal from PO2 (or % sat O2) to [O2].

From your results, it looks as if your instrument is assuming that salinity is zero in both samples. Are you entering the sample's salinity value at some point during the measurement? Depending on the probe you are using, the salinity value is either (1) automatically measured by a conductivity sensor and factored into the instrument's calculation of [O2] in mg/L or (2) measured beforehand and entered manually when prompted by the instrument.

I hope I have managed to explain more or less clearly why you are getting essentially the same reading in both freshwater and seawater media.

Best wishes

Thank you for your answers! This helped! 

@Catherine M C G Renard: Yes that was a typo, I meant 8 mg/L. Thanks for spotting!

@Isam Eldin Hussein Elgailani: Thank you for the info on Winkler titration!

@François L L Muller: Thank you! This was exactly what I wasn't aware of: that it measures partial pressure and not absolute conc. I am a metallurgist so oxygen probes are very new to me. My experiment needs to measure salinity vs. D-O2 conc. So I guess I can't use Clark or optical. I guess I should look for Winkler method.

Thank you all once again!

I suggested that you can do it analytically with titration  using winkler method

it is favery famous and accurate for sea water analysis and I think it will help you for your saline solutions

here you can find the summary of the method:

https://www.researchgate.net/profile/Mohamed_Hassaan2/publications?pubType=method

regards

For a classic paper, see:

http://pubs.acs.org/doi/abs/10.1021/ja02216a012

The dissolved oxygen concentration (DO) is often measured with a Clark polarographic oxygen electrode, which calibration and conversion from percent saturation to molarity is addressed at § 2.2.1 (pp. 11-12) at the signalled reference:

Thesis Controlo do Oxigénio Dissolvido em Fermentadores para Minimi...