Abnormalities in the determination of fluoride ions using concentrated phosphoric acid
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When I’m producing concentrated phosphoric acid, I need to determine the F content in it. However, when conducting the tests in the company’s analysis laboratory, different concentrations of F ions are obtained, yet the potential difference measured remains the same (I will explain in more detail later that the equipment is being used properly and according to the correct procedures). I’m not sure what could be causing this issue; I hope those who know the answer can offer some guidance. Thank you first. The concentration of phosphorus pentoxide in the concentrated solution I prepare is around 45%, while the fluoride content is approximately 1%-4%. The laboratory uses the standard addition method with a fluoride ion selective electrode for measurement. The specific steps are as follows: Weigh 0.2–0.3 grams of concentrated phosphoric acid and place it in a 100-milliliter beaker; add 10 milliliters of hydrochloric acid (it seems to be 1+1), let the mixture stand for 30 minutes while stirring every ten minutes, and then adjust the volume to 250 milliliters ； A 10-milliliter sample was transferred using a pipette and placed in a 50-milliliter volumetric flask. 5 drops of citric acid-sodium citrate buffer, 2 drops of bromocresol green were added, followed by 200 g/L of sodium hydroxide solution to give the sample a blue-green color. Nitric acid (1+15) was then added to turn the solution yellow, and 20 milliliters of citric acid-sodium citrate buffer were added to bring the volume up to 50 milliliters. After cooling to room temperature, the entire sample was transferred into a beaker to determine its potential value; once the potential value stabilized, the data was recorded. Then, 0.5 milliliters of a 1 mg/L fluoride ion solution was added, and after stabilization, the potential value again was recorded. The difference between these two values represents the potential difference we measured. Under identical other conditions, when 5 milliliters of sample are taken from the initially prepared 250-milliliter sample, the resulting potential difference is equal ； After that, we used the standard curve method for determination, and the potential difference measured for 10 milliliters and 5 milliliters of the sample solution was still the same. I don’t know where we went wrong. If anyone knows, please give us some advice. Thank you in advance
Let’s check whether it’s a problem with the potentiometer or the F-ion selective electrode
Note that when the phosphate concentration is high and the fluoride concentration is also high, a small amount of fluorophosphate is formed. This could be the reason why your test results are incorrect. Increase the dilution factor of the sample and apply appropriate heating during the hydrolysis of polyphosphoric acid to promote the hydrolysis of polyphosphates and reduce the tendency to form fluorophosphates with fluoride ions. During the measurement process, instead of using the standard curve method, the standard addition method is employed, either with a single or multiple standard additions (the specific calculation methods can be found online or in some specialized books on electroanalytical chemistry); this helps to eliminate the influence of the sample background on the measurements. Additionally, the pH value of the solution should be adjusted as accurately as possible; if possible, use a pH meter to monitor the pH adjustment range.