#
Development of in-House Industrial Fluosilicic Acid Certified Reference Material: Certification of H_{2}SiF_{6} Mass Fraction

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{2}SiF

_{6}mass fraction in this by-product, method validation is required, which needs a certified reference material (CRM) with its traceability to the International System of Units (SI). This work describes the development of a certified reference material of fluosilicic acid, which is commercially unavailable. Details of all steps, such as sample preparation, homogeneity and stability studies, value assignment, establishment of metrological traceability, and uncertainty estimation of the certified reference material, are fully described. The H

_{2}SiF

_{6}mass fraction in the CRM was quantified by two analytical methods, i.e., UV-VIS as a primary method of analysis and flame mode atomic absorption spectroscopy (AAS) as a second method. It is worth noting that the results obtained from each method were in good agreement. The CRM certified value and corresponding expanded uncertainty, obtained from the combined standard uncertainty multiplied by the coverage factor (k = 2), for a confidence interval of 95%, was (91.5 ± 11.7) g·kg

^{−1}. The shelf life of the developed CRM is determined to be 1 year, provided that storage conditions are ensured. The developed CRM can be applied to validate analytical methods, improve the accuracy of measurement data as well as to establish the meteorological traceability of analytical results.

## 1. Introduction

_{4}and hydrofluoric acid HF with the gas stream [2]. These fluoride gases are washed in scrubbers and are transformed into fluosilicic acid [3]. This research work is carried out in order to be in agreement with environmental regulations which continue to limit chemical processing emissions. For this reason, phosphate plant operators are required to neutralize fluoride gas [3]. It is more environmentally friendly to capture these fluorinated gases in the form of fluosilicic acid by water absorption in scrubbers [3]. Interestingly, H

_{2}SiF

_{6}by-product is considered to be an important source of silica and fluorine, which have several uses in many chemical industries [4,5,6].

_{2}SiF

_{6}mass fraction must be determined during all the scrubbing cycles. Therefore, the validation and quality assurance of H

_{2}SiF

_{6}analysis are of great importance.

_{2}SiF

_{6}mass fraction was performed using two independent analytical methods based on UV-VIS and flame mode AAS. The results obtained by both methods were in good agreement by taking into account their uncertainties. The assessment of homogeneity testing and stability testing, the assignment of reference value and uncertainty evaluation were carried out in accordance with ISO 35 Guide requirements [9].

## 2. Materials and Methods

#### 2.1. Sampling and Preparation of Candidate CRM Sample

#### 2.2. Analytical Methods Used for Characterization

_{2}SiF

_{6}mass fraction, two indirect analytical methods were used for the characterization of the candidate material, i.e., UV-VIS and flame AAS. They are based on the analysis of the silicon mass fraction, which is proportional to that of H

_{2}SiF

_{6}.

#### 2.2.1. Determination of the H_{2}SiF_{6} Mass Fraction by UV-VIS

_{2}SiF

_{6}mass fraction.

^{−1}). After that, a 5 mL aliquot of the solution was diluted to 100 mL. The complexation of the silicon, reduction of the complex, and measurement were carried out in accordance with the NF T90-007 standard method [10]. The spectrophotometer calibration was carried out by diluting a 1 g·L

^{−1}silicon standard reference material (SRM) from the National Institute of Standards and Technology (NIST).

#### 2.2.2. Determination of the H_{2}SiF_{6} Mass Fraction by Flame AAS

^{−1}boric acid. Calibration was performed by using mono-elemental silicon standards, prepared from a 1000 ppm Si standard reference material from the National Institute of Standards and Technology (NIST). The measurement was carried out in a reducing flame of nitrous oxide-acetylene. The operating parameters of the instrument are summarized in Table 1. The results of silicon analysis are then converted to (g·kg

^{−1}) H

_{2}SiF

_{6}mass fraction.

#### 2.3. Homogeneity Study

_{min}and N

_{prod}indicate the minimum number of units used for the homogeneity study and the total number units of the candidate material (100 units), respectively.

^{®}(2016).

_{2}SiF

_{6}mass fraction of these ten stratified random samples was analyzed using the UV-VIS method. Homogeneity testing was evaluated using the analysis of variance (ANOVA) on triplicate results. Figure 1 summarizes the scheme of this study.

#### 2.4. Stability Study

_{2}SiF

_{6}mass fraction in the samples using the UV-VIS method. In this work, the long-term stability was studied throughout one year at twelve different times. Each month, one bottle randomly chosen from the sampling batch stored at room temperature (about 25 °C) was analyzed on duplicate. The results were taken as the first point (t = 0 month) and were evaluated statistically using linear regression analysis.

#### 2.5. Confirmation of the Metrological Traceability

^{−1}silicon standard reference material (SRM) from the National Institute of Standards and Technology (NIST). The results obtained were compared with a second analytical method by flame AAS (Perkin Elmer Pin AAcle 900T).

#### 2.6. Results Validation

_{2}SiF

_{6}mass fraction in analytical fluosilicic acid was in the range between 33.5% and 35% (w/w).

#### 2.7. Assignment of Reference Value and Estimation of Uncertainty

_{2}SiF

_{6}mass fraction.

_{char}), homogeneity (u

_{hom}), and stabilities (u

_{lts}and u

_{sts}), was calculated using Equation (2) according to the ISO 35 guide [9].

_{2}SiF

_{6}mass fraction value (U

_{CRM}) was calculated using a coverage factor k = 2 with 95% level of confidence.

## 3. Results and Discussion

#### 3.1. Homogeneity Assessment of CRM

_{2}SiF

_{6}mass fraction in the candidate certified reference material.

_{2}SiF

_{6}mass fraction of aliquots taken from each sample together with the corresponding expanded uncertainty values, and the average value (red line) was determined as the average of the H

_{2}SiF

_{6}mass fraction for all the analyzed units.

_{2}SiF

_{6}mass fraction in the aliquots taken from the bottle are within the range given for the mean value of the analyzed package. The calculated intervals for the H

_{2}SiF

_{6}mass fraction in the samples have a common part in the interval corresponding to the average value.

_{2}SiF

_{6}mass fraction, which also indicates that the material was regarded to be sufficiently homogeneous.

_{between}) was higher than the mean square within bottles (M

_{within}), which means that the method of measurement has good repeatability. In this case, the standard uncertainty associated with between bottle variance (u

_{bb}) was calculated based on the ANOVA results by using Equations (3) and (4) (number of replicates n

_{0}= 3 and degrees of freedom of mean square within bottles v

_{Mwithin}= 20) [9]. However, the higher u

_{bb}value (Equation (3)) was chosen for calculating the u

_{CRM}[18].

_{bb}) was 1.0% for the H

_{2}SiF

_{6}mass fraction.

#### 3.2. Stability Assessment of CRM

#### 3.2.1. Long-Term Stability

_{2}SiF

_{6}mass fraction in industrial CRM fluosilicic acid was assessed over a time interval of 12 months. Two subsamples of the material from one bottle, selected randomly, were analyzed at each time point (each month). The time at which the homogeneity was assessed was taken as day 0.

_{2}SiF

_{6}mass fraction of the candidate material with the elapse of storage time at laboratory temperature (25 °C).

_{2}SiF

_{6}mass fraction in industrial fluosilicic acid was shown to be stable over a period of at least one year.

_{0}) is expected to be equal to the value obtained in the characterization, while the slope (b

_{1}) should not be significantly different from zero [19]. Table 4 summarizes the analytical results of the long–term stability test.

_{crit}), in which the candidate CRM was analyzed each month throughout one year.

_{5 %}). Figure 4 shows the distribution of residuals of the regression line.

_{lts}has been assessed in accordance with ISO Guide 35 [9]. u

_{lts}was estimated based on the following Equation (5):

_{lts}= t × s(b

_{1})

_{1})” represents the standard error of the slope. Thus, the “u

_{lts}” value calculated for an expiry date of 12 months was used as the standard uncertainty due to the long-term instability during storage. The calculated value was 1.13 g·kg

^{−1}.

#### 3.2.2. Short–Term Stability

_{2}SiF

_{6}mass fraction in the material were 92.0 g·kg

^{−1}(s = 0.6 g·kg

^{−1}) and 91.3 g·kg

^{−1}(s = 0.8 g·kg

^{−1}) at 35 °C and room temperature, respectively.

_{2}SiF

_{6}mass fraction at both storage temperatures, which proves that the material is stable and without significant changes in its chemical composition. Therefore, the uncertainty associated with the short-term stability (u

_{sts}) was estimated as zero according to ISO Guide 35 [9].

#### 3.3. Confirmation of Metrological Traceability

_{2}SiF

_{6}mass fraction, expressed in g·kg

^{−1}and determined according to the Standard NF T90-007 [10], is calculated using the following Equation (6):

_{2}, in milligrams per liter, in the test sample solution; V

_{t}is the volume of the test sample in milliliters (500 mL); V is the volume of dilution (100 mL); v is the aliquot intake for dilution (5 mL); m is the mass of the sample in grams; and M

_{H2SiF6}and M

_{SiO2}are the molar masses of H

_{2}SiF

_{6}and SiO

_{2}, respectively.

_{1}is the slope of the calibration curve; and B

_{0}is the intercept of the calibration curve.

_{2}in the test sample solution obtained from the calibration curve (C), the volume of the test sample (V

_{t}), the volume of dilution (V), the volume of the aliquot (v), the mass of the sample (m), the molar mass of H

_{2}SiF

_{6}, and the molar mass of SiO

_{2}. In order to establish the traceability of the result, it is necessary to establish the traceability of these influence parameters.

_{0}), and the slope (B

_{1}) associate the concentration (C) of SiO

_{2}in the sample solution by Equation (7) to the concentration of the calibration solutions, establishing traceability to the values of the calibration solutions. These calibration solutions were obtained by diluting the reference solution of (999.4 ± 3.4) mg·L

^{−1}of silicon (Si) in water. The concentration of the reference solution is traceable to a NIST SRM solution (NIST SRM No 3150) according to the manufacturer’s certificate. The dilution steps were carried out using volumetric glassware, whose manufacturer specifies the value of the volume and its tolerance. The calibration solutions were measured using a molecular absorption spectrophotometer at (650 ± 10) nm, then the absorption values and the concentration of the calibration solution were used to calculate the intercept (B

_{0}) and slope (B

_{1}) of the calibration curve by least square linear regression. Photometric accuracy of the spectrophotometer was determined by comparing the difference between the measured absorbance of the reference standard materials and the established standard value [24]. Neutral density glass-filters F2-666, F3-666, and F4-666 certified by DKD (Germany) of various transmittance values at 440, 465, 546.1, 590, and 635 nm were used for this verification. The wavelength accuracy of the spectrophotometer was evaluated by measuring a known wavelength of a holmium oxide filter standard reference [24], certified by DKD (Germany). The results of these two tests were satisfactory. These activities achieve traceability for the concentration of SiO

_{2}(C), which has the largest contribution to the overall uncertainty.

_{2}SiF

_{6}and SiO

_{2}were calculated from the IUPAC atomic weights of the elements [25].

_{crit 5%}) and a Student’s t-Test for Equal Means from two samples (t < t

_{crit 5%}) [26] demonstrated that the differences were not significant. Therefore, the mass fractions determined using both protocols with very different principles were in good agreement. In addition, the relative deviation between the two methods is low compared to the uncertainty of the certified value (Section 3.5), which confirms the traceability of the certified value.

#### 3.4. Accuracy of the Analytical Method Used for the Characterization

^{−1}, while the mean of the measured values under intermediate precision conditions [28] was 339 g·kg

^{−1}(s = 2.01 g·kg

^{−1}), and it was in the range of the uncertainty of the assumed value. Therefore, no significant difference was observed between the measured and assumed values.

#### 3.5. Assignment of Certified H_{2}SiF_{6} Mass Fraction Value and Its Uncertainty

_{2}SiF

_{6}mass fraction in industrial CRM fluosilicic acid is the mean of the analytical results obtained by molecular spectrophotometry [10] of the ten randomly selected samples analyzed on triplicate. The calculated value was found to be 91.5 g·kg

^{−1}.

_{2}SiF

_{6}mass fraction (u

_{CRM}) can be obtained following Equation (2) by combining u

_{char}(the standard uncertainty due to characterization), u

_{hom}(the standard uncertainty due to inhomogeneity), and u

_{lts}and u

_{sts}(the standard uncertainties due to long-term and short-term stabilities, respectively).

_{2}SiF

_{6}mass fraction (u

_{char}), the uncertainties from each component in Equation (6) were combined according to the ISO Guide 98-3 (law of propagation of uncertainties approach) [14] using the following Equation (8).

_{2}SiF

_{6}mass fraction are presented as a cause-and-effect diagram in Figure 5.

_{2}(C in mg·L

^{−1}) is calculated using a calibration curve. The linear least squares fitting procedure used assumes that the uncertainties of the values of the x-axis are considerably smaller than the uncertainty of the values of the y-axis [15]. Consequently, the usual uncertainty calculation procedures for C only reflect the uncertainty due to random variation in the absorbance and not the uncertainty of the calibration standards, nor the inevitable correlations induced by successive dilution from the same stock solution [15]; however, the uncertainty of the calibration standards is sufficiently small to be neglected. Thus, the estimated standard uncertainty of C is 0.044 mg·L

^{−1}with an average of 0.763 mg·L

^{−1}. The uncertainty associated with the mass of the sample is estimated, using the data from the calibration certificate and the manufacturer’s recommendations on uncertainty estimation, as being U = (0.75 + 0.0034 × m) in mg (coverage factor k = 2). This contribution has to be counted twice, once for the tare and once for the gross weight. This gives for the standard uncertainty of the masse (u(m)) a value of 0.00143 g.

_{2}SiF

_{6}and SiO

_{2}have been calculated by combining the uncertainty of the atomic weights of their constituent elements published in the current IUPAC table [25]. The estimated standard uncertainties are 0.0006 g·mol

^{−1}for the molar mass of H

_{2}SiF

_{6}and 0.0007 g·mol

^{−1}for the molar mass of SiO

_{2}.

_{char}) has been estimated at 5.32 g·kg

^{−1}.

_{MRC}) was based on the combination of u

_{char}, u

_{hom}(Section 3.1), u

_{lts}(Section 3.2.1), and u

_{sts}(Section 3.2.2), as follows in Equation (2), and was calculated to be 5.84 g·kg

^{−1}. As observed, the major contribution to the overall combined uncertainty (Table 6) came from the uncertainty of method characterization (u

_{char}), while the contribution from the uncertainty of long-term stability (u

_{lts}) was less significant.

_{MRC}) was calculated to be 11.7 g·kg

^{−1}by multiplying the combined standard uncertainty (u

_{MRC}) by the coverage factor k (k = 2).

## 4. Conclusions

_{2}SiF

_{6}mass fraction of the developed CRM was determined by UV-VIS as a primary method of measurement. The accuracy of the results that were obtained by the primary analytical method (UV-VIS) was evaluated by comparing the results with those obtained with a second analytical method (AAS). As demonstrated, both results were in good agreement. In addition, a pure 34% (w/w) fluosilicic acid was analyzed with the first method and presented satisfactory results. The developed CRM showed good homogeneity and high stability for at least one year. The uncertainty of the certified value was estimated by combining the uncertainties due to the analytical method, homogeneity, and stability. The certified value of CRM developed in this study is traceable to the International System of Units (SI). Since there is no available CRM fluosilicic acid in the market, this material will be useful for routine testing in the laboratory, especially for the validation and/or verification of internally developed analytical methods, for analytical instrument calibration, or for quality control.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Scheme of the homogeneity study. (A) batch of fluosilicic acid; (B) sampling; (C) stratified samples contributing to the between-bottle variation; (D) aliquots contributing to within-bottle variation; (E) preparation; (F) measurement.

**Figure 2.**Within (wb) and between (bb) bottle homogeneity of the H

_{2}SiF

_{6}mass fraction of the candidate CRM. Blue, green, and orange bars show the results of first, second, and third analysis of each sample, respectively.

**Figure 3.**H

_{2}SiF

_{6}mass fraction during the long-term stability test. Points and bars show the mean values and standard deviations, respectively.

**Figure 5.**Cause and effect diagram with uncertainty sources of the H

_{2}SiF

_{6}mass fraction characterization.

Element | Si |
---|---|

Wavelength (nm) | 251.61 |

Slit (nm) | 0.2 |

Acetylene flow rate (L.min^{−1}) | 8.3 |

Nitrous oxide flow rate (L.min^{−1}) | 6.0 |

Lamp current (mA) | 40 |

Background correction | Yes |

Repetition times | 3 |

Criteria Observed for: | |
---|---|

The smallest average | 1.195 |

The highest average | 0.991 |

Limit for 1% risk | 2.564 |

Limit for 5% risk | 2.355 |

Source of Variation | Sum of Squares | Degrees of Freedom | Mean Square M | F-cal | p-Value | F-crit |
---|---|---|---|---|---|---|

Between bottles | 50.89 | 9 | 5.65 | 1.74 | 0.14 | 2.39 |

Within bottles | 64.87 | 20 | 3.24 | |||

Total | 115.76 | 29 |

Slope (b_{1}) | Intercept (b_{0}) | Slope Standard Deviation (s_{b1}) | p-Value (95%) | Student’s t-Value | Student’s t_{crit} | |
---|---|---|---|---|---|---|

H_{2}SiF_{6} (g·kg^{−1}) | 0.0185 | 91.3129 | 0.0935 | 0.8469 | 0.198 | 2.228 |

No of Sample | H_{2}SiF_{6} Mass Fraction (g·kg^{−1}) Obtained by UV-VIS | H_{2}SiF_{6} Mass Fraction (g·kg^{−1}) Obtained by AAS | Relative Deviation |
---|---|---|---|

09 | 93.20 | 91.20 | 2.1% |

13 | 90.43 | 90.09 | 0.4% |

21 | 92.80 | 90.37 | 2.6% |

40 | 92.67 | 90.54 | 2.3% |

48 | 89.57 | 90.08 | −0.6% |

56 | 91.07 | 90.70 | 0.4% |

70 | 89.80 | 91.70 | −2.1% |

76 | 89.53 | 92.97 | −3.8% |

86 | 92.83 | 90.60 | 2.4% |

95 | 93.17 | 89.47 | 4.0% |

Source of Uncertainty | Symbol | Absolute Uncertainty (g·kg^{−1}) | Relative Standard Uncertainty |
---|---|---|---|

Analytical method | u_{char} | 5.3 | 5.8 |

Homogeneity | u_{hom} | 2.1 | 2.3 |

Long-term stability | u_{lts} | 1.1 | 1.2 |

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## Share and Cite

**MDPI and ACS Style**

Kounbach, S.; Ben Embarek, M.; Chemaa, A.; Boulif, R.; Benhida, R.; Beniazza, R.
Development of in-House Industrial Fluosilicic Acid Certified Reference Material: Certification of H_{2}SiF_{6} Mass Fraction. *Minerals* **2021**, *11*, 92.
https://doi.org/10.3390/min11010092

**AMA Style**

Kounbach S, Ben Embarek M, Chemaa A, Boulif R, Benhida R, Beniazza R.
Development of in-House Industrial Fluosilicic Acid Certified Reference Material: Certification of H_{2}SiF_{6} Mass Fraction. *Minerals*. 2021; 11(1):92.
https://doi.org/10.3390/min11010092

**Chicago/Turabian Style**

Kounbach, Said, Mokhtar Ben Embarek, Abdeljalil Chemaa, Rachid Boulif, Rachid Benhida, and Redouane Beniazza.
2021. "Development of in-House Industrial Fluosilicic Acid Certified Reference Material: Certification of H_{2}SiF_{6} Mass Fraction" *Minerals* 11, no. 1: 92.
https://doi.org/10.3390/min11010092