Weird Results from Refractometer/Conductivity Meter

[AwesomeusPrime]

Hi Guys,

I was calibrating my Apex Conductivity meter last night, using the Neptune 53,000 microsiemens/cm solution. When I finished the calibration, the probe read 35.6ppt in the solution. I placed the probe in the tank and it read 33.2ppt, lower than I thought because my refractometer was reading 35ppt.

I then Calibrated my refractometer to 35ppt using the 53,000 microsiemens/cm solution and tested the reef water, which then read 43ppt.

I repeated the process 3 times with 3 different solutions of the same brand of 53,000 microsiemens/cm solution and got the same results every time.

In the past I've calibrated my refractometer using the conductivity solution and never had an issue. Why would this solution not calibrate correctly on my refractometer?

EDIT* This solution is a solution of KCl and RODI water
Second EDIT* Solution was at 78.3 F

 

[chipmunkofdoom2]

Was the solution specifically designed to be measured using refractive index? It doesn't sound like it if it claims to be KCl solution with a known conductivity.

I can't speak as to why this may have worked in the past, but I wouldn't expect to get proper calibration on a refractometer using a conductivity standard. In Randy's article on DIY salinity standards, he lists different recipes for refractive index and conductivity because of the way these measurement techniques differ.

 

[HOOPDEEZ]

+1 conductivity is not the same as refraction, and Neptune says the solution will not work for calibrating refractometers on their website. Just grab a solution specific to refract. They last a long time if you keep the cap on.

 

[AwesomeusPrime]

Ah, well that solves that. This is why I didn't become a chemist I suppose.

 

[Randy Holmes-Farley Verified Community Expert]

Correct.

A 53 mS/cm solution of KCl, which is perfect for calibrating a conductivity meter (assuming it is made correctly) is not going to match the refractive index of 35 ppt seawater. It's refractive index matches seawater at much lower salinity than 35 ppt, so it gets calibrated to read too high, and that explains the 43 ppt reading.

 

[daelie]

Correct.

A 53 mS/cm solution of KCl, which is perfect for calibrating a conductivity meter (assuming it is made correctly) is not going to match the refractive index of 35 ppt seawater. It's refractive index matches seawater at much lower salinity than 35 ppt, so it gets calibrated to read too high, and that explains the 43 ppt reading.

Just out of curiosity, what’s the recipe for a 53 mS KCl solution?

 

[AwesomeusPrime]

Just out of curiosity, what’s the recipe for a 53 mS KCl solution?

32.44 grams of KCl in 1 kg of RODI water @ 59F if you aren't using a temp probe to compensate for temperature.

 

[Randy Holmes-Farley Verified Community Expert]

Just out of curiosity, what’s the recipe for a 53 mS KCl solution?

I have published a recipe for 53 mS/cm sodium chloride (as well as 35 ppt for refractometers and hydrometers, all are different):

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2004-06/rhf/index.htm

In a quick look online, I could not find KCl to match 35 ppt seawater at 25 deg C (only 15 deg C). I expect the error is minimal, however, and that value is about 32.4358 grams of KCl added to 967.5642 grams of pure water (to make 1 kg total).

http://ieeexplore.ieee.org/document/1145440/

 

[Randy Holmes-Farley Verified Community Expert]

32.44 grams of KCl in 1 kg of RODI water @ 59F if you aren't using a temp probe to compensate for temperature.

I think that is 1 kg total, not 1 kg RO/DI.

 

[AwesomeusPrime]

I think that is 1 kg total, not 1 kg RO/DI.

Yeah that's correct, 967.56 grams of RODI not 1kg of RODI

 

[Stigigemla]

I believe the conductivivty of salt water is increasing with temperature. About 2 or 2,1% per degree celcius.
See here http://www.fondriest.com/environmen...ters/water-quality/conductivity-salinity-tds/

 

[Randy Holmes-Farley Verified Community Expert]

I believe the conductivivty of salt water is increasing with temperature. About 2 or 2,1% per degree celcius.
See here http://www.fondriest.com/environmen...ters/water-quality/conductivity-salinity-tds/

Yes, all salt solutions increase in conductivity as the temp rises, and nearly all conductivity meters automatically correct for this effect. There are small differences in the temp effect on conductivity of different salt mixtures, but I think that effect is not significant enough to be a huge impact for reefers. Some fancier meters allow for different types of corrections, or even to program your own preferred correction.

If you make either my NaCl standard or the KCl standard, it should be adequate at most any temp and the conductivity meter will make appropriate corrections. For folks interested, you can easily measure the conductivity of your standard or tank water at different temps to see how well it is working. If you think it is not working well enough, just make sure that the temp that you calibrate/check the conductivity standard and the tank water you measure are at the same temperature.

 

[rtolz]

Randy,
First, let me thank you for your many, many (should I add another "many") years of service to the hobby. I've been doing this since the mid-eighties, and your elucidation of what's really necessary has made it possible over time for me to feel like I could repopulate the coral reefs if there's a natural disaster.
Anyway, I've studied your article on calibration solutions, bought myself some glassware, a suitably precise scale, and then put together both a conductivity and a refractometer calibration solution using a store-brand salt.
The problem is that there was white residue at the bottom of both mixtures, even after several days and occasional agitation. This led me to purchase some KCl to try the conductivity solution again. This one looks much better and I expect to stick with it.
Is it possible to do a refractometer solution with KCl? I see nothing in your article or later postings that address this. Your article points out that I need a 3.65% NaCl solution for a refractometer calibration. Logic (which may or may not apply here) would suggest that I should adjust that percentage by the same ratio (3.24/3.29) that is used in making the KCl vs. NaCl conductivity solution. If I do the math, that means that using KCl for a refractometer calibration solution, I should aim for 3.59%.
Before I waste my time, I admit that this logic may not apply in this case. And it might be inappropriate to use KCl altogether. Maybe I just need purer NaCl that doesn't leave residue.
Your thoughts and guidance are welcome.