1 g TSP dodecahydrate (MW 380) in 100 mL RODI is 2500 ppm PO4. Dilute 4.8 mL of this solution to 20 mL RODI to make it less concentrated so as to avoid forming a precipitate when adding it to 6 L of saltwater. Stir the 6 L while adding this diluted solution. The PO4 should be 2 ppm.
Wouldn’t it be more advisable to have a lower ppm concentration, for a better accuracy with the test kit?
For what it’s worth, the ELOS test kit (with Salifert Alkalinity vial) reads the solution at 2ppm (diluted by 50% because the maximum on the test is 1ppm), and the Salifert reads it at 1 ppm which is quite obviously wrong (I even bought a new fresh kit, which still reads 1 ppm ), lol. This confirms my belief that Salifert has always been reading low, and now I’ve got a 2 ppm standard ( thanks Dan ), that has been confirmed.
Since we are taking the difference of the initial and final PO4 reading, we need to have the difference much greater than the standard deviation of the measurement. The propagation of error makes the variation in the difference greater than in either measurement. By using 2 ppm, we should see roughly a 1 ppm drop.
I recast the adsorption data (plot 1} and corresponding desorption data (plot2) into the plot below which shows what happens to the amount of PO4 adsorbed to the same sample of sand after 24 hours adsorbing PO4 and 96 hours after desorbing PO4. Only 4 samples were involved in this study. As you correctly pointed out, the sand is not equilibrating. In fact, almost a constant amount of PO4 desorbs from the sand regardless of content. Wild, right?
After 24 hrs by Salifert, nothin has really gone down, still reading near 1 ppm, I’m thinking perhaps the colour change above 1 ppm is just too subtle.I recast the adsorption data (plot 1} and corresponding desorption data (plot2) into the plot below which shows what happens to the amount of PO4 adsorbed to the same sample of sand after 24 hours adsorbing PO4 and 96 hours after desorbing PO4. Only 4 samples were involved in this study. As you correctly pointed out, the sand is not equilibrating. In fact, almost a constant amount of PO4 desorbs from the sand regardless of content. Wild, right?
I am thinking about cycling the same sample of sand several times through adsorption-desorption to observe what happens to its ability to adsorb PO4.
I recast the adsorption data (plot 1} and corresponding desorption data (plot2) into the plot below which shows what happens to the amount of PO4 adsorbed to the same sample of sand after 24 hours adsorbing PO4 and 96 hours after desorbing PO4. Only 4 samples were involved in this study. As you correctly pointed out, the sand is not equilibrating. In fact, almost a constant amount of PO4 desorbs from the sand regardless of content. Wild, right?
I am thinking about cycling the same sample of sand several times through adsorption-desorption to observe what happens to its ability to adsorb PO4.
I am going to repeat a 24 h adsorption measurement with much higher mixing, suspending the sand. Poor mixing seems like the only factor that could be giving much lower 24 h adsorption than Millero’s.
Sounds good. The rate of binding is likely affected by how much the sand-water mixture is being stirred. The drop might continue for some time in your system.
So it appears, halved again today to 0.25 ish.
Has this sand been washed to remove dust? I discovered the “fines” could adsorb PO4 10-20 times better than the sand. A little dust can go a long way in binding PO4
It was initially yes, in fresh saltwater, but I wasn’t at all thorough, just got the vaste majority out. I think my “gentle” stirring may be producing some as it does seem to clear a little slower.
Let’s maybe wait for your final results before deciding on making you do more work. But it is an interesting idea and parallel’s JDA’s experiment where he used lumps of araginite in a reactor. I am slowly talking myself into saying yeah do it :-) but let’s wait for data.
Adsorption appears to have stopped at 0.25ppm, been the same for the last three days (Salifert and ELOS). Should I redose another 2ppm to see if it can suck up more?
