Ca++ Calibration Solution

[NoFeedback]

I am returning to reefkeeping after a period of relative inactivity (didn't do much but keep the tank alive). In the past, monitoring Ca++ concentrations were always part of my routine and I used the Hach test kit to perform this measurement. I replaced the reagents after finding unusually high readings of mid-500 ppm Ca++ with my tank water. Still getting readings of mid-500 ppm! Fish store testing seems more reasonable in the low to mid 400 range (varying logically with BICB dosing). My Hach kit obviously doesn't agree and I never had problems in the past that I noticed with Ca testing using the Hach test kit.

As an amateur chemist, I decided a calibration solution was in order so I mixed 0.41 g of CaCl2.2H2O into 250 ml of distilled water which, in theory, should give me a 450 ppm Ca++ calibration solution. Hach kit read 336 ppm Ca++ with this solution. I always dilute my test sample 50/50 to reduce EDTA use with the Hach kit.

As I said, "amateur chemist". Does anyone have an idea what might be going on with my methods?

Thank you,

Mike

 

[JimWelsh]

I am returning to reefkeeping after a period of relative inactivity (didn't do much but keep the tank alive). In the past, monitoring Ca++ concentrations were always part of my routine and I used the Hach test kit to perform this measurement. I replaced the reagents after finding unusually high readings of mid-500 ppm Ca++ with my tank water. Still getting readings of mid-500 ppm! Fish store testing seems more reasonable in the low to mid 400 range (varying logically with BICB dosing). My Hach kit obviously doesn't agree and I never had problems in the past that I noticed with Ca testing using the Hach test kit.

As an amateur chemist, I decided a calibration solution was in order so I mixed 0.41 g of CaCl2.2H2O into 250 ml of distilled water which, in theory, should give me a 450 ppm Ca++ calibration solution. Hach kit read 336 ppm Ca++ with this solution. I always dilute my test sample 50/50 to reduce EDTA use with the Hach kit.

As I said, "amateur chemist". Does anyone have an idea what might be going on with my methods?

Thank you,

Mike

Your CaCl2.2H2O is a hydrated salt, and may have more water bound up than the theoretical, although 336 ppm vs. 450 ppm is a very large discrepancy. A more accurate Ca standard can be prepared using dried CaCO3, and dissolving it in just enough HCl to complete the dissolution, prior to the final dilution to volume (be careful to wash down any splattered material from the CO2 bubbling that ensues). Also, how old is your Hach kit? If it's old (you've had it for the "period of relative inactivity"), then perhaps you just need fresh reagents.

 

[Dana Riddle]

I am returning to reefkeeping after a period of relative inactivity (didn't do much but keep the tank alive). In the past, monitoring Ca++ concentrations were always part of my routine and I used the Hach test kit to perform this measurement. I replaced the reagents after finding unusually high readings of mid-500 ppm Ca++ with my tank water. Still getting readings of mid-500 ppm! Fish store testing seems more reasonable in the low to mid 400 range (varying logically with BICB dosing). My Hach kit obviously doesn't agree and I never had problems in the past that I noticed with Ca testing using the Hach test kit.

As an amateur chemist, I decided a calibration solution was in order so I mixed 0.41 g of CaCl2.2H2O into 250 ml of distilled water which, in theory, should give me a 450 ppm Ca++ calibration solution. Hach kit read 336 ppm Ca++ with this solution. I always dilute my test sample 50/50 to reduce EDTA use with the Hach kit.

As I said, "amateur chemist". Does anyone have an idea what might be going on with my methods?

Thank you,

Mike

Which Hach method - titration? The newer colorimetric method (usually recommended for freshwater only.) Other? Just curious.

 

[NoFeedback]

Yes, titration. And I did replace all the reagents in the past month.

Also, I am really happy to see both you and RHF still very active in the hobby. I doubt you remember but I was originally active in the FishNet era and met you guys at MACNA IX in Chicago.

Thank you!

Mike

 

[Dana Riddle]

Yes, titration. And I did replace all the reagents in the past month.

Also, I am really happy to see both you and RHF still very active in the hobby. I doubt you remember but I was originally active in the FishNet era and met you guys at MACNA IX in Chicago.

Thank you!

Mike

Always a pleasure to hear from someone from the FishNet days!

 

[Randy Holmes-Farley Verified Community Expert]

Yes, always good to see folks are still around!

I’ll just add that I’m not a huge fan of a freshwater standard for a seawater test as it doesn’t say if the kit is properly distinguishing calcium from similar compounds such as magnesium. That can not explain a low result for calcium, but it points out the whole idea of matrix effects. So unless the test says it is ok for fresh or salt, I wouldn’t rely on it.