Determination of fluoride ion concentration in concentrated hydrochloric acid

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Determination of fluoride ion concentration in concentrated hydrochloric acid: This article details the step-by-step procedures for using the HC-800 fluoride ion analyzer, which is based on ion-selective electrodes, to measure the fluoride ion content in strong acids in a fast and accurate manner. I. Experimental equipment: 1 HC800 fluoride analyzer, 36.5% concentrated hydrochloric acid (with a density of 1.2 g*cm-3), a 100 ml volumetric flask, a glass stirring rod, a 50 ml beaker, a dropper, deionized water, buffer solution, pH test strips (1–14), 5 mol/l sodium hydroxide solution, a glass graduated cylinder, and a suction bulb. II. Experimental steps: 1. Calculate the mass concentration of the sample substance as = (1000*1.2*36.5)/36.5 = 12 mol/L. 2. Transfer to a clean beaker; add a small amount of deionized water first, then pour in 10 ml of the sample slowly using a glass rod while stirring gently. Bring the volume up to 100 ml in a volumetric flask, thereby achieving a dilution factor of 10. (The diluted concentration is 1.2 mol/L.) 3. Take a small amount of the diluted solution and test it with a pH strip; it remains a strong acid, so secondary dilution is carried out for neutralization. 4. Place 10 ml of the diluted sample in a beaker, then add 2.4 ml of sodium hydroxide solution for neutralization (the amount of sodium hydroxide required was calculated using the formula C_base * V_base = C_acid * V_acid, resulting in 2.4 ml). 5. Continue to add 20 ml of buffer to the beaker, stir well using a glass rod, and then add deionized water to bring the volume up to 100 ml in a volumetric flask. 6. Take a portion and proceed with testing directly on the machine (3 minutes; the printing results are displayed immediately). 7. The instrument’s printed result is F=Xmg/L multiplied by the dilution factor of 100, which gives the actual concentration. III. Experimental Instructions 1. Two dilution steps were carried out in the above procedure, which is suitable for measuring high-concentration samples. If it is known that the sample contains a small amount of F ions, the previous dilution step can be omitted; simply take 10 ml of the solution to be tested and add 24 ml of sodium hydroxide solution for neutralization. 2. There are small amounts of heavy metal ions in the solution; a buffer is added to eliminate interference and ensure accurate measurements. 3. Since the optimal pH for measurement by the instrument is between 5 and 7, sodium hydroxide needs to be added for neutralization and fine adjustment; after neutralization, a pH test strip shows that the value is essentially consistent. 4. When pouring concentrated hydrochloric acid into water, stir slowly continuously during the process to prevent localized overheating during dilution, which could cause the solution to boil and lead to splashing and burns. 5. If the concentration of the acid is unknown, sodium hydroxide can be added slowly during neutralization, with the pH being checked using a pH test strip at each addition; adding sodium hydroxide should be stopped once the pH falls between 5 and 7. IV. Measurement advantages: 1. Wide measurement range, fast speed, fully automatic and easy to learn. 2. Compared to other semi-automatic measuring instruments, it is cumbersome to operate and has large errors. 3. It is cheaper and more economical compared to methods such as colorimetry. 4. Automatic fast printing to display the results. 5. It can also be connected to a computer for real-time online monitoring and management, etc.