Fluoride Determination in Pickling Solution of Stainless Steel by Ion Selective Electrode

The Fluoride Ion Selective Electrode is an ion-selective sensor and will quickly and accurately measure fluoride ion activity rather than concentration in aqueous solutions. Fluoride ion activity depends on the solution total ionic strength and pH, and on fluoride complexing species. Adding an appropriate buffer provides a nearly uniform ionic strength background, adjust pH, and breaks up complexes so that, in effect, the electrode measures concentration. Pickling removes a thin surface layer from the stainless, using an acid solution, which is usually a hydrofluoric acid (HF) and nitric acid (HNO3), also called mixed acid. In this study, fluoride ion strength has been measured of pickling waste water from stainless steel by ion selective electrode. The results met the specification.


Introduction
Acid Cleaning (Pickling): Stainless steel is composed of iron (Fe), nickel (Ni), chromium (Cr) and several other minor components. Stainless steel is not resistant to chemical or physical attack. The corrosion resistance of stainless steel depends on the formation of a 'passive surface film' composed of nickel and chromium oxides (Cr 2 O 3 &NiO). When stainless steel is newly made it is cleaned of oils and greases used in the fabrication process.
Pickling is the most common chemical procedure used to remove oxides and iron contamination. Besides removing the surface layer by controlled corrosion, pickling also selectively removes the least corrosion-resistant areas such as the chromium-depleted zones. Pickling normally involves using an inorganic acid mixture containing nitric acid (HNO 3 ), hydrofluoric acid (HF) and, sometimes, also sulfuric acid (H 2 SO 4 ). Owing to the obvious risk of pitting corrosion, chloride-containing agents such as hydrochloric acid (HCl) must be avoided [1]. Pickling solution also remove contaminants such as ferrous and ferric oxide particles.
Pickling involves metal removal and a charge or dulling in the visual brightness of the metal. This solution marketed commercially under the trade name TSIAB [4]. Sufficient buffer for 15-20 determinations can be prepared by mixing with stirring 57 mL of glacial acetic acid, 58 g of sodium chloride, 4 g of cyclohexylaminedinitrilotetraacetic acid, and 500 mL of distilled water in a 1-L beaker. Cool the contents in a water or ice bath, and carefully add 6 M sodium hydroxide to a pH of 5.0 to 5.5. Dilute to 1 L with water, and store in a plastic bottle [5] [11]. 5. Standard fluoride solution (working solution): Dilute 100 mL stock fluoride solution to 1000 mL with distilled water; 1.00 mL= 10.0 µg F -6. Serial Fluoride standards: Into 100 mL volumetric flasks, put V mL) of 10 mg/L fluoride standard, quantitatively fill with D. I. water to obtained series standards, mg F -/L (Table2) [3] [8]. 7. Deionized water, on the day of use. Water was purified using a Millipore Milli-Q system via a pure water device marked Purelab Option-Q7BP. All fluoride standard solutions should be stored in high density polyethylene bottles at 4°C [6], [7].

Installation-Electrode Check
Remove the protective end cot covering the electrode tip rubber insert covering the filling hole of the reference electrode. Fill the combination electrode with the 4 N KCl filling solution. Connect the electrodes to the proper terminals of the meter [9] [10].
Check the operation of the electrode as follows: measure 30.0 mL of buffer solution (pH=5.32) in a 100 mL graduated cylinder. Add DI water to bring to volume to 99.0 mL. Pour the solution into a 400 mL plastic or Teflon beaker. Set the beaker on a magnetic mixer, put in a stirring bar, and start the mixer. Stir a moderate. Stir both standards and samples at the same rate. Set the meter in the mV display mode. Immerse the electrode tip in the solution and turn on the meter. Using a new pipet tip add 1.0 mL of Standard Fluoride Solution (100 mg/L). Measurements should always be made going from a more dilute to a more concentrated solution. Allow 15 minutes for the meter reading to stabilize. A reading that holds for 3-4 minutes is considered stable. Record the meter reading as mV 1 . Add second 1.0 mL portion of Standard Fluoride Solution. When the reading stabilizes record it as mV 2 . Subtract mV 2 from mV 1 to get ∆E. Look up the mV difference on the Table 5  . .

Sample Preparation and Analytical Procedure for Analysis
For accurate measurement, the standards and the samples should be at the same temperature. Add DI water to a 1 liter volumetric flask to within ½ inch from the mark. Accurately pipet 1.0 mL of Pickling -Acid Cleaning Solution into the volumetric flask. Add DI water to the mark and mix. Pipet a 10.0 mL of the sample solution into a Teflon beaker, add 30.0 mL of buffer solution, 60 mL of DI water. Set the beaker on the mixer, immerse the electrode, stir well and record the stable reading as mV 1 . Add 1.0 mL portion of Standard Fluoride Solution in excess. When the reading stabilizes (about 3-4 minutes) record the meter reading as mV 2 . Subtract mV 2 from mV 1 , to get ∆E. Look up the mV difference on the Table 6 to get mg F - [2,9].

Result and Discussion
The fluoride electrode is a solid state electrode can be used with a standard calomel reference electrode and almost any modern pH meter having an expanded millivolt scale. For the best results, the instrument was calibrated every 1-2 hours. Two standard solutions have been used to bracket the concentration range of interest.
Before use, the fluoride ion-selective electrode has been calibrated by measuring a series of known standard solutions. For a full calibration, 100 mL of solutions containing 0.2, 0.5, 1.0, 2.0, and 4.0 ppm Fmust be prepared [9].

Calculations
The volume of sample taken for measurement is 1.0/1000 x10.0= 0.010 mL, therefore the F-content of the original sample in mg/L is mg F-x 100,000. The equivalent weight of Fis 19.0 g or 19,000 mg. The F-normality of the original pickling sample is then:  Table 4, and Figure 1.

Conclusions
The Ion-Selective Electrode Method has been used for determining fluoride in the pickling (Acid Cleaning) Solution of Stainless Steel. The result have been displayed as ppm (mg/L), and mole/L in solution. The concentration of F-in Pickling (Acid Cleaning) Stainless Steel solution was 0.43 N. The specification range is 0.4 to 0.6.
The average spiked recovery of fluoride content in pickling solution was 98.4%, RPD was 0.39% Table 4.