I read this at this webpage: http://www.whfoods.com/genpage.php?dbid=90&tname=nutrient#impactcookingstorageprocessing
How do cooking, storage, or processing affect potassium?
Potassium losses from cooking of high-potassium foods can be significant. In the case of spinach for example, potassium levels have been shown to drop from 6.9 to 3.0 grams in 3 and 1/2 ounces of spinach after blanching for several minutes (a loss of about 56%).
I don't understand this. How can potassium disappear after cooking ?