Hanna PO4 Colormeter Users, Info Needed

[Aquatic Image]

Tonight I tested my tank water and got some strange readings. I did 5 tests back to back. All the water was taken from the same location just below the surface on the main system. These are the readings in the order I got them.
.06, .03, .04, .02, .01
Seems strange to me. I also tested my RO/DI water and got .01 3 times back to back.

I dont understand how I can take 5 tests from my tank and get 5 different reading and take samples from my RO and get the same reading 3 times in a row.
Anyone have any idea what could be going on?

[Aleksander]

What chemicals are you using? With such low concetrations it is critical to have totaly clean cuvettes.

[Aquatic Image]

Chemicals? What do you mean? Tank additives or the reagents for the test?

[Orion76]

0.01 is essentially a negative because it's the minimum value and will never show 0, so you probably got the same value three times in a row because there was no reaction.

The fact that the reading is a step lower after each test makes me think the cuvette is dirty and it's becoming cleaner the more tests you do. The cuvettes need to be very clean, try washing them with a little detergent and rinse them well in RO water after. Residue from previous tests may be interfering.

[GTR]

I store the glass tubes in a cup of RO/DI water when not in use. This keeps them from forming any hard to get off water spots, inside or out. Also I use a cloth to hold the tube while adding the reagent and shaking so I don't get oils or finger prints on the outside.

SteveU

[Aquatic Image]

I appreciate the infor here. I am not worried about the reading from the RO unit but that from my tank. I guess they could have been dirty but I rinse them well and dry them each time also making sure there are no residues on the outside of the cuvette. Maybe the inside became a bit cleaner after each test. Maybe from now on we will store them in RO water. I can not wait until I get my own that way we can check the two of them to eachother.

[Aged Salt]

Steve, good points After I obtain my Hanna, I'll ask you to post your protocol step by step, thnx. Bob

[GTR]

No problem Bob. I use the "Alexander Method"

SteveU

[Aleksander]

I store the glass tubes in a cup of RO/DI water when not in use. This keeps them from forming any hard to get off water spots, inside or out. Also I use a cloth to hold the tube while adding the reagent and shaking so I don't get oils or finger prints on the outside.

SteveU

Storing in a cup of RO/DI water is not good enough. That will not prevent forming thin film. Use some neutral detergent, rinse several time and dry cuvettes.

[Aged Salt]

Aleksander, thnx. for the information--I believe you mentioned the company sells a curvette cleaner for this purpose, which makes good sense. Bob

[wasp]

I think part of the problem is that the Po4 levels we are aiming at are actually very close to the bottom of the range that the Hanna tests for.
So when the reagent is added there is almost no color in the water, certainly none that I can see, so there is scope for the reading to be out by a point, or a few points, perhaps due to a slight fingermark or speck of dust.
If water was tested that was around the middle of the range that the Hanna tests for, the few points difference between tests would be much less significant.

[GTR]

I know this is old but I'll add that IMO I think it's important to be consistent in the method of testing to get reproducible results. I believe that the time you shake the test sample after the reagent is added can change the results considerably. At the time a first got mine I talked to Hanna direct for some guidance and I use a random 45 second countdown to mix.
The accuracy of the test is fully dependent on the time the reagent is in contact with the test sample. (The longer it sits the higher the reading.) To be honest I'm not sure if this timing is correct but if I use that as a standard every test at least I'll be sure of the trends I'm seeing from test to test, from week to week,

SteveU

[TJcop]

I haven't got mine yet (hopefully it should arrive soon!), but what about storing the glass in some (rubbing) alcohol instead of water? They should dry spot free and not have any films develop.
No, not this kind of alcohol.

[G.Alexander]

We should also remember the photometer has a relative wide tolerance for its accuracy. I do not think this is necessary in the range we measure. RO water works fine for me.

G.Alexander