Weird PO4 Reading

[geoffrey]

Hi All,

Tested my water yesterday and PO4 reading seems weird. I am using Sera and the reading exceeds 20ppm (like dark blue ink). Since I don't have distill water handy, I diluted the sample water with tap water and it still gives me dark blue!

1) I feed sparingly with 2~3 heapful of tetra bits using MacDonald's stirrer once a day for my 20 fishes (2.5ft tank) and used potting soil with JBL as base. Any reason why PO4 so high.

2) I know the instruction asked for distill water, but using tap water shouldn't give much diversion right? Potable water in CCK is <0.1ppm PO4 only.

3) How do I decipher the expiry date under the box of my kit? If I have it correct, expiry is 2 years later. Could I be wrong?

4) The white reagent in the kit looks lumpy and damp. Anything wrong with it.

I am very comfortable with Sera's NO3 and Fe kits but not its PO4. Thought of switching to Dupla. Anyone has any experience with Dupla's PO4 test kit? Does it need dilution for high reading?

[geoffrey]

Forget to add this, the kit measured 0.5ppm PO4 last week.

[bclee]

I uses Sera PO4 kit. It is quite good.

Do you shake the solution after each reagent you add?

IME, if you do not, the solution will immediately turn dark blue when you add the reagent 3.

It is important that you add the reagent in sequence and shake to mix thoroughly after adding each reagent.

If I am not wrong, the top of the scale for undiluted samples is 2.0ppm and not 20ppm.

BC

[geoffrey]

Oop, sorry, 2ppm.

No, I swirled gently as instructed when I add reagent 1 and 2. As soon as I add 3, it turns blue. And I only shake after adding reagent 3. I seem to have this intermittent high reading with Sera's PO4 kit once in a while.

Based on your feedback, it seems like an application problem. Will try again tonight, this time, shake for 1 and 2.

Thanks BC.

[Allen_1971]

PO4 test kits are quite notorious for being inaccurate... in fact I've read in some places that one of the reagents commonly used oxidises thus giving some inaccuracies. It could be that your test kit is spoilt. Try testing your tap water and see if it still gives you the same readings?