Taking the density changes into account the procedure based on mixing 81.53 g of KNO₃ with 500 mL of water produces 0.94 ppm of NO₃^- in the final solution.

For 1 ppm you need to dissolve KNO3 in a somewhat lesser amount of water and the fill it up to 500 mL.

Mistaking volume of solvent with the final volume is a very common error.

Thank you both for your replies. I eventually found out that I was calculating the wrong values. I was supposed to calculate the NO3 value, not the N value. Doing that brings me to the expected change of 1 ppm and leads me to believe that now I finally have the correct K value. Below I will share the calculation I have now. Basically, the result should be accurate in 2 decimals. @Borek does your explanation then change anything?

1ppm = 1mg/L, thus 1 ppm increase = 100mg/100L

K = 39,1 g/mol
NO3 = 14 + 3*16 = 62 g/mol

Total = 101,1 g/mol

Mass NO3 in powder 62/101,1 = 0,6133 | 81,53 * 0,6133 = 50g = 50000mg
Mass K in powder 39,1/101,1 = 0,387 | 81,53 * 0,387 = 31,54g = 31540mg

Concentration of N in the solution when adding 1 mL to 100L of demineralised water should then make 50000mg/500ml = 100mg/ml = 1ppm. Concentration of K 31540mg/500ml = 63,08mg/ml = 0,63ppm

Hi evreybody, I am synthetisizing two kind of bodipy molecules . In the last step I functionalize with a triethoxysilane (aminoethylt trymethoxy silane or APTES) group the dyes by the following proces (see also the file attached)
 

BODIPY-COOH + APTES + TEA + COMU  BODIPY-CONHCH₂CH₂Si(OEt)₃

The COOH group is linked through an alkyl chain to the meso position of the bodipy.

After the reaction the solvent is removed and the raw material is purified on silica gel by chromatography with petroleumether/AcOEt 50%/50% to 100% AcOEt as eluent.

Simmetric Bodipy (1,3,5,7 methyl 2,6 ethyl substituted     R=Et in the file attached) is an orange solid stable after the purification
Asimettric Bosipy  (1,3,5,7 methyl 2 ethyl substituted   R=H in the file attached)  is not stable and decompose in a dark red brownish insoluble solid the day after the purification and solvent removal. It happens evrey time I try to isolate it. Anyone knows why does this hanppen?

Thank you very much

We have been putting this off in order to pursue some other syntheses, but it is time to move forward.  One of my colleagues uses 90%, and he gave us a protocol recently.  That is a good starting point for us to develop conditions.

Yes you got it.