Percipitate while pH setting of insect ringer - (Apr/29/2015 )

Hi,

I have a big problem: I´m preparing an insect ringer solution (pH 7,4; total volume 100 ml) with those ingredients

15.75g NaCl

2.35ml 2M KCl

3.7ml 2M KH₂PO₄

0.05g NA₂HPO₄

0.37g MgCl₂

All ingredients dissolve excellently. The starting value of pH is app. 4,4. When I was trying to set up 7,4 by NaOH or KOH everything was working fine. But at the point of 7,0 - 7,1 there was percipitation.

Has anybody an idea what could be the reason for that percipitation or what substance I could use to set up the pH instead of NaOH/KOH?

Thanks in advance!

probably magnesium hydroxide.

you should adjust the pH prior to addition of mgcl2.

I did some fast calculation in Minteq and the precipitate might be MgHPO4*3H2O and Mg(OH)2, although the calculations are not completely reliable due to the extremely high ionic strength. I guess you tried to prepare a stock rather than the working solution because ~16% of salts is a lot