How to accurately dilute test samples to compensate for high range phosphate values

[BigJohnny]

My tank is currently above 1ppm phosphate. I am trying to reduce phosphate slowly because I tend to have problems when i do it too quickly, however I have no way of knowing how fast I am reducing it because my test kits dont test above 1ppm and I am not sure where I am at before and after GFO use.

For example, my NYOS phosphate test kit only goes up to 1ppm. When i test my water it shows it as darker than 1ppm, but thats it. I dont know how much higher and I can't monitor subtle color changes like that so I figured id use my Hanna ULR Phosphorous, however that only tests up to 200ppb which is about .6ppm phosphate. My first thought was, dilute the sample 50% with RODI and multiply the results by 2, but I am concerned that using 50% RODI will mess up the accuracy since the other 50% is saltwater and I know the Hanna doesn't test phosphate accurately in ultra pure water. If that is the case then I guess i could dilute with 50% fresh saltwater at the same s.g.? Id rather just use RODI if possible.

If anyone knows the answer i would appreciate it.

Thanks

 

[Retro Reefer]

I doubt that will work.. testing kit manufacturers typically will give you a method of testing for lower ranges if possible in the instructions that are included with kit.

 

[BigJohnny]

I doubt that will work.. testing kit manufacturers typically will give you a method of testing for lower ranges if possible in the instructions that are included with kit.

Yea I get what your saying but this is an ULR (ultra low range) checker so it wouldnt make sense for them to include high range dilution instructions. My red sea nitrate test kit says do 16ml tank water for low range or 15ml RO 1ml tank water for high range, but I don't think that test kit has the same issue of trying to detect nitrate in ultrapure water like the hanna phosphorus does. OR adding 1/16th salt water is all that is needed to make test perform properly. Maybe the hanna would work just fine as long as it's not 100% rodi.

Im almost 100% positive it would work if I diluted with fresh mixed saltwater of matching s.g. because that'd be no different than testing a sample with a lower phosphate concentration, but I'd rather just use RODI if possible.

@Randy Holmes-Farley
@JimWelsh

Do either of you know what prevents the hanna ulr phosphorous from being accurate in RODI water and how salty it would have to be to test accurately? I feel like a 50/50 dilution would still work but I'm not sure how much accuracy I'd lose since the machine was probably calibrated at NSW levels.

 

[BigJohnny]

@Hanna Instruments

I forgot you were on Reef2reef! Can I dilute a saltwater sample 50/50 with RODI and still maintain reasonable accuracy with the Hanna ULR Phosphorous?

 

[JimWelsh]

This document merely says, "dilute the sample and re-run the test". I would interpret that to mean RODI.

 

[BigJohnny]

This document merely says, "dilute the sample and re-run the test". I would interpret that to mean RODI.

Wow, I briefly scanned that doc earlier and missed that. Probably because its under the "errors and warnings" section. I assume they mean RODI as well. Thanks for posting.

 

[Hanna Instruments]

@Hanna Instruments

I forgot you were on Reef2reef! Can I dilute a saltwater sample 50/50 with RODI and still maintain reasonable accuracy with the Hanna ULR Phosphorous?

Theoretically yes that should work however the absorbance curve for the HI736 is based off of a seawater sample matrix and thus the accuracy statement is formulated for full strength saltwater. We cannot say for certain if the accuracy statement will be unchanged from a 50% dilution with deionized water.

 

[BigJohnny]

Theoretically yes that should work however the absorbance curve for the HI736 is based off of a seawater sample matrix and thus the accuracy statement is formulated for full strength saltwater. We cannot say for certain if the accuracy statement will be unchanged from a 50% dilution with deionized water.

Thanks for posting. I think I'll try and test the same sample cut 50/50 with rodi and 50/50 with fresh saltwater and see if there is a significant difference in the results.

 

[Jpcoral]

If the process is similar to the Nitrate dilution method it calls for "nitrate free salt water". So that would mean fresh mixed salt water made with 0 TDS RODI water. If you do 50% tank Water and 50% fresh mix salt water. Then you would multiply the Hanna reading by 2 and that should be your result. Not sure but that is my thinking on the dilution method for Phosphate ULR.