Potassium reacts with sodium tetraphenylborate to form a precipitate. Under defined conditions this turbidity can be used for concentration measurements. Turbidities interfere and have to be filtered prior to the test. Good reproducibility is obtained for drinking, surface and ground water. For polluted waste waters low potassium values are measured. 

The natural potassium content in ground water is generally about 1 – 2 mg/l K. Higher values may indicate faecal contaminations, but can also originate from potassium fertilisers. For the growth of plants and animals potassium is an essential factor. Especially in agriculture the determination of potassium therefore gains increasing importance.