Potassium Test Kit Instructions
Ok, thanks to big help from Rick and Dan - here's the procedure for the test method. (Calculation equation will follow soon, but a chart is provided as a peek to how the checker PO4 to potassium ppm mapping will look)
"Modified Red Sea K - Recipe 2.3"
The test is literally only one reagent (Red Sea Reagent B - tetraphenylborate solution), but the procedure sounds almost comically detailed because the amount of scattering that you measure is very dependent on the way in which the mixing is done. Shaking, splashing, vigorous stirring all affect the way the precipitate forms and - because it’s hydrophobic - how the precipitate grabs onto bubbles and water surface. What follows is the procedure that we found gives tight repeatability and avoids frustrating inexplicable outliers.
1. In a Red Sea Vial, add 1.00mL of the sample water and 3.00mL of distilled water to make 4.00mL total. Swirl 15 seconds to mix. This is the Diluted sample.
All liquid additions to the hanna reaction cuvette should be done by turning the cuvette at a 45 degree angle, slowly adding the liquid to the side of the glass and letting it flow smoothly down to the bottom. Add all liquids to the same side of the glass cuvette.
2. To a hanna cuvette, add 0.200mL of Red Sea Potassium Reagent B (Tetraphenylborate solution) using the above procedure.
3. To the hanna cuvette, add 0.500mL of the Diluted sample from step 1.
4. Swirl the cuvette smoothly for 10 seconds at a modest rate - aim for 3 swirls per second.
5. Let it sit for 4 minutes.
6. Add 10.0mL distilled water to the hanna cuvette using the same procedure above.
7. Cap and invert the cuvette slowly and smoothly 10x. One invert and back upright every second.
8. Use a different hanna cuvette with clear water (distilled / tap) as the blank “C1”.
9. After the reacted cuvette has sat for 1-2 minutes, measure in the hanna PO4 checker.
(Optional) 10. The sample turbidity is stable for at least an hour, so it is possible to invert the reacted cuvette a couple more times, let it sit 1-2 minutes and remeasure (can take the average of triplicate measurements). There might be a tiny improvement in lowering variability this way, or can be used as a check against outlier measurement weirdness, but with this procedure it doesn’t seem strictly necessary.
Here’s a preliminary look at how the procedure performs…
My data using the hi774 checker in 35 ppt NaCl-water is in the X’s, and using the more complex Millero recipe (includes all major elements) is the diamonds. Rick’s data using a different PO4 checker hi713 is the stars. (Points plotted are the average of 3 re-measurements.)
It is hoped that the agreement will be better using the same model of hanna checker, but this is still good enough to demonstrate that the recipe is usable.
