Think I’m gonna get a Hanna, which one do folks suggest?.
Which Hanna to buy depends on the lowest concentration you want to measure and the range of concentrations. The Hanna ultra low range (ULR) model measures in the 0 - 0.05 ppm a bit more reliably than the low range (LR) model. The LR model has a maximum measurement of 2.5 ppm where as the ULR model tops out at 0.6 ppm.
@Rick Mathew can add more background to this topic
Checked some sand from a 30 month undisturbed sandbed to see if it leaked anything thus;
Cool experiment. I want to do this to compare to your results. If you are interested in comparing PO4 in sand beds, I added information to help standardize the method
1) Obtained a sample from approximately 1.5 inch depth
This will be tricky to standardize. Ideally we would grab a core sample of the sand bed and then cut out a slice of the core around 1.5 cm. Until, then we we will do our best to scoop out sand at comparable depths
2) Rinsed the sample (2.5 grammes ish) twice in 0ppm new saltwater.
In my sand studies, phosphate is rapidly released in 0 ppm water. It is possible that you rinsed off 10-25% of the phosphate from the sand grain surface in each rinse.
I suggest that we take a level scoop of known volume (we must know the sand volume to compare each others results) and use something adsorbent like a tissue to wick away the excess water in the sand bed sample while it is in the scoop by touching the edge of the tissue to the sand. We will have to live with any detritus in the sand sample.
If you are a detritus hater, we could try quickly rinsing with a minimum amount of tank water and then wicking off the excess. It would be fun and useful to compare the phosphate measurements for sand treated with your original sand sample wash, my suggested no wash and the quick was with aquarium water.
I measure my samples with little chemical scoops. The red Salifert scoop (0.05 mL) delivers ~80 mg of sugar sand. The white Red Sea scoop (0.15 mL) delivers ~220 mg of sugar sand.
3) Added 10mls new saltwater
Good. A known amount of water for the desorption run. I have been able to get a reading with ratios of 12 mL water to 200 mg sand. More sand is better when you need a higher concentration to measure
4) left the sample covered in a wide container for 12hrs, but gently stirred it every 3 ish hours.
Good. Stirring is important. Time is important. 24 hours might be slightly better in ensure equilibrium.
5) Filtered and tested the sample.
Excellent. Very critical. Even a small amount of “dust” will throw off the reading
The result was about 0.1ppm (salifert). This just so happens to be the concentration of the running tank. However this tank has been From zero to 1ppm back down to 0.1 in its lifetime.
pH of the saltwater removed from the same location of the sand was 7.9. Overlying pH of the tank 8.2.
I calculate 1 micro gram was desorbed from 2.5 g of sand, or 0.4 micro gram PO4 per gram of sand @1.05 micro molar PO4…this seems like a reasonable number but we would need to know the phosphate concentration surrounding the sand when sampled to say any more about the number. The 0.4 value suggests it was surrounded by a higher phosphate concentration than 0.1 ppm of the aquarium water. This is reasonable though. Pore water can have a higher concentration of phosphate, ammonia, nitrate, etc.
