Alkali metal hydroxides and alkaline earth hydroxides

Transcription

1 15 Alkali metal hydroxides and alkaline earth hydroxides (lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide) Method number 1 Application Analytical principle Air analysis Ion chromatography Completed in June 2001 Summary The method permits the determination of the alkali metal and alkaline earth hydroxides, sodium hydroxide and calcium hydroxide, when present as particles in a concentration range of 0.1 to 2 times the currently valid threshold limit values [1], and of lithium hydroxide and potassium hydroxide in the same concentration range. For sampling, ambient air is drawn through a quartz fibre filter with a suitable sampling pump. For elution, the quartz fibre filters used for sampling are covered with diluted sulfuric acid and treated in an ultrasonic bath. Quantitative determination is carried out by ion chromatography and conductivity detection.

2 Analytical Methods 16 Characteristics of the method Accuracy: Table 1. Standard deviation s (relative) and mean variation u, n = 10 determinations. Substance Concentration Standard deviation Mean variation s (rel.) u mg/m 3 % % Lithium hydroxide Sodium hydroxide Potassium hydroxide Calcium hydroxide Limit of quantification: Lithium hydroxide: absolute 12.5 ng Li + = mg/m 3 LiOH Sodium hydroxide: absolute 25 ng Na + = mg/m 3 NaOH Potassium hydroxide: absolute 25 ng K + = mg/m 3 KOH Calcium hydroxide: absolute 25 ng Ca 2+ = mg/m 3 Ca(OH) 2 for a sampled air volume of 420 L. Recovery: Lithium hydroxide: % Sodium hydroxide: % Potassium hydroxide: % Calcium hydroxide: % Sampling recommendation: Sampling time: 2 hours Sampled air volume: 420 L Lithium hydroxide (LiOH) [CAS No ] Lithium hydroxide forms white crystals which are readily soluble in water. The molecular weight is 23.95, the melting point 450 8C.

3 17 Lithium hydroxide is used, for example, in batteries and photographic developers, in ceramic products and as CO 2 absorber. At present in Germany there is no threshold value for the substance in air. In Great Britain, for example, the threshold value for lithium hydroxide is 1 mg/m 3. Sodium hydroxide (caustic soda, NaOH) [CAS No ] Sodium hydroxide is available as white hygroscopic granules, lumps, flakes, pellets, pearls or bars. The molecular weight is 40.0, the melting point 318 8C. Sodium hydroxide dissolves very readily in water with the generation of much heat; the solution is strongly alkaline. The solid substance and the solution can cause severe corrosion of the skin, eyes and mucous membranes. Sodium hydroxide and sodium hydroxide solution are used in the chemical industry mainly as neutralizers, for saponification reactions, in the production of sodium compounds and for the digestion of bauxite. In addition, the compound is used, for example, in the soap and detergent industry, for cleaning fats, oils and petroleum, as digestion agent in analytical chemistry, in the synthesis of dyes and in the production of cellulose. The currently valid threshold value in air is 2 mg/m 3 for the inhalable fraction (I); according to TRGS 900, sodium hydroxide is classified in short time exposure limit category =1= [1]. Potassium hydroxide (caustic potash, KOH) [CAS No ] Potassium hydroxide forms colourless crystals, which are sold in the form of dry white bars, flakes, pellets or tablets. The molecular weight is 56.10, the melting point is 360 8C and the boiling point C. In air, the compound actively absorbs water and carbon dioxide. Potassium hydroxide dissolves readily in water with the generation of heat and forms alkaline potassium hydroxide solution. The strongly hygroscopic potassium hydroxide is used as a desiccant and as an absorbing agent for carbon dioxide. In addition, the compound is used, for example, in the production of dyes (alkali fusion), detergent base materials and soaps. At present in Germany there is no threshold value for potassium hydroxide in air. In many other European countries (e. g. Great Britain, Switzerland, Denmark, The Netherlands), however, there is a threshold value of 2 mg/m 3.

4 Analytical Methods 18 Calcium hydroxide (Ca(OH) 2 ) [CAS No ] Calcium hydroxide is sold as colourless crystals or as powder. The molecular weight is Calcium hydroxide dissolves to a small extent in water, but better in glycerol and sugar solution. Ca(OH) 2 irritates skin and mucous membranes. The main use of calcium hydroxide is in the preparation of mortar. In addition, the compound is used, for example, as binder for paint pigments, for waste water purification and in tanning. The currently valid threshold value in air is 5 mg/m 3 for the inhalable fraction (I) [1]. Authors: Examiners: D. Breuer, B. Heinrich R. Hebisch, A. Johnen

5 19 Alkali metal hydroxides and alkaline earth hydroxides (lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide) Method number 1 Application Analytical principle Air analysis Ion chromatography Completed in June 2001 Contents 1 General principles 2 Equipment, chemicals and solutions 2.1 Equipment 2.2 Chemicals 2.3 Solutions 2.4 Calibration standards 3 Sample collection and preparation 4 Operating conditions for ion chromatography 5 Analytical determination 6 Calibration 7 Calculation of the analytical result 8 Discussion of the method 8.1 Precision 8.2 Recovery 8.3 Limit of quantification 8.4 Shelf-life 8.5 Effect of higher humidity 8.6 Sources of error 8.7 Discussion 9 References

6 Analytical Methods 20 1 General principles The method permits the determination of the alkali metal and alkaline earth hydroxides, sodium hydroxide and calcium hydroxide, when present as particles in a concentration range of 0.1 to 2 times the currently valid threshold values [1], and of lithium hydroxide and potassium hydroxide in the same concentration range. For sampling, ambient air is drawn through a quartz fibre filter with a suitable sampling pump. For elution, the quartz fibre filters used for sampling are covered with diluted sulfuric acid and treated in an ultrasonic bath. Quantitative determination is carried out by ion chromatography and conductivity detection. 2 Equipment, chemicals and solutions 2.1 Equipment Pump for personal air sampling, flow rate 3.5 L/min Gasmeter Filter holder, diameter 37 mm, with sampling head for sampling the inhalable dust fraction Quartz fibre filter, diameter 37 mm, e. g. Ederol, from Binzer und Munktel, Battenberg Ion chromatograph with autosampler, column thermostat, suppressor and conductivity detector Data analysis unit Volumetric flasks made of polypropylene, 25 ml, 50 ml, 1000 ml Adjustable pipettors, 10±10000 µl Screw-cap polyethylene vessels, 20 ml Autosampler vials made of polypropylene with screw caps and silicon-ptfe septa Disposable filters for the filtration of aqueous samples, diameter 25 mm, pore size 0.45 µm, suitable for ion chromatography Ultrasonic bath Ultrapure water system 2.2 Chemicals Lithium standard solution: w (Li + ) = 1000 µg/ml, e.g. from Merck, Darmstadt Sodium standard solution: w (Na + ) = 1000 µg/ml, e.g. from Merck, Darmstadt Potassium standard solution: w (K + ) = 1000 µg/ml, e.g. from Merck, Darmstadt Calcium standard solution: w (Ca 2+ ) = 1000 µg/ml, e.g. from Merck, Darmstadt The standard solutions can be stored at a temperature of 4 8C for at least one year. Lithium chloride, anhydrous, e. g. from Fluka, Sigma Aldrich, Taufkirchen

7 21 Sodium chloride, ultrapure, e. g. from Merck, Darmstadt Potassium chloride, analytical grade, e. g. from Fluka, Sigma Aldrich, Taufkirchen Sulfuric acid, c (H 2 SO 4 ) = 2.5 mol/l e. g. Combi Titrisol, from Merck, Darmstadt 2.3 Solutions Eluent: c (H 2 SO 4 ) = mol/l 2 ml sulfuric acid is pipetted into a 1-litre volumetric flask and the flask is filled to the mark with ultrapure water (conductivity > 18.2 MO*cm). 2.4 Calibration standards Stock solution: w (Li + ) = 25 µg/ml w (Na + ) = 50 µg/ml w (K + ) = 50 µg/ml w (Ca 2+ )=50µg/mL 625 µl lithium standard solution and 1250 µl of each of the sodium, potassium and calcium standard solutions are pipetted into a 25 ml volumetric flask and the flask is filled to the mark with ultrapure water. Table 2. Pipetting scheme for calibration standards for a concentration range of 0.8±20 µg/ml. Calibration Volume of the Final volume of Concentration of the solution combined standard the calibration calibration solution solution solution (Li + ) (Na + ) (K + ) (Ca 2+ ) No. ml ml mg/l mg/l mg/l mg/l The volumes of the combined standard solution given in the table are pipetted into polypropylene volumetric flasks; these are filled to the mark with ultrapure water, sealed and shaken. The calibration solutions are transferred to autosampler vials and analysed. The combined standard solution can be stored at a temperature of + 4 8C for at least two months, the calibration solutions can be stored at room temperature for at least two weeks.

8 Analytical Methods 22 3 Sample collection and preparation A filter holder containing a quartz fibre filter is used for sampling. With a pump equipped with a flow regulator, air is drawn through the filter at a flow rate of 3.5 L/min, in accordance with the definition of the inhalable dust fraction. After sampling, the filter holder is closed with the plastic caps provided. For analysis, the quartz fibre filter is placed in a screw-cap vessel and covered with 10 ml elution solution. The vessels are closed, treated for 15 minutes in an ultrasonic bath and then left to stand for 30 minutes. With a disposable syringe, liquid is taken from the prepared sample and filtered into an autosampler vessel through a disposable filter. 4 Operating conditions for ion chromatography Apparatus: HPIC system from Dionex, Idstein Gradient pump GD 40 Electrochemical detector ED 40 Autosampler AS 3500 Chromatography software Peaknet 5.1 Precolumn: IonPac CG14, from Dionex, Idstein Length: 50 mm Internal diameter: 4 mm Column: IonPac CS14, from Dionex, Idstein Length: 250 mm Internal diameter: 4 mm Suppression: Cation self-regenerating suppressor CSRS-II 4 mm, from Dionex, Idstein Column temperature: 40 8C Mobile phase: Sulfuric acid c (H 2 SO 4 ) = mol/l Flow rate: 1.0 ml/min Injection volume: 25 µl System pressure: approx. 10 MPa Detector: Conductivity detector Figure 1 shows a chromatogram obtained under the conditions given above. 5 Analytical determination With an autosampler 25 µl sample solution is injected into the chromatograph and analysed under the conditions given above.

9 23 If the concentrations determined are not within the calibration range, the samples must be diluted appropriately and analysed again. 6 Calibration The calibration solutions described in Section 2.4 are used to draw a calibration curve. Volumes of 25 µl of each of the calibration solutions are injected and analysed as for the sample solutions. The peak areas obtained are plotted against the corresponding concentrations. The calibration functions for the cations are linear in the range from 0.3 mg/l to 3,0 mg/l for Li + and from 0.6 to 6.0 mg/l for Na +,K + and Ca 2+. Figure 2 shows examples of calibration curves for lithium, sodium, potassium and calcium. 7 Calculation of the analytical result The concentrations of lithium, sodium, potassium and calcium hydroxide in the workplace air are calculated using the concentrations of the substances in the solution calculated by the data analysis unit. The data analysis unit uses the calibration functions calculated from the calibration curve. The concentrations of the hydroxides in the workplace air are calculated from the concentrations, taking into account dilution steps and the sampled air volume. The following equations apply for the concentrations of the alkali metal and alkaline earth hydroxides in the workplace air: r ˆ F a 10 b V air 1000 Z f base 273 t g 273 t a where: r is the concentration by weight in the ambient air as a function of t a and p a F is the peak area a is the intercept of the calibration curve with the ordinate b is the gradient of the calibration curve in ml/µg f base is the stoichiometric conversion factor: cation? base 10 is the conversion factor for the elution volume of the measured sample in ml 1000 is the conversion factor: µg? mg V air is the sampled air volume in m 3 t g is the temperature in the gasmeter in 8C t a is the temperature of the ambient air in 8C Z is the recovery (to be taken into account if necessary)

10 Analytical Methods 24 8 Discussion of the method The characteristics of the method were determined according to the standard DIN EN 482 [2]. 8.1 Precision To determine the precision, ten quartz fibre filters were spiked with concentrations at the level of twice the limit of quantification, 0.1 times the MAK value, 0.2 times the MAK value and twice the MAK value for each of sodium and calcium hydroxide, and three corresponding concentrations of lithium and potassium hydroxide. These filters were dried in the air, processed and analysed. Table 3. Standard deviation s (relative) and mean variation u, n = 10 determinations. Substance Concentration Standard deviation Mean variation s (rel.) u mg/m 3 % % Lithium hydroxide Sodium hydroxide Potassium hydroxide Calcium hydroxide Recovery The recovery was determined using the results of the determination of the precision. The results were compared with the standard solutions used for calibration. The mean recoveries Z were: Lithium hydroxide: 1.01 Sodium hydroxide: 1.02 Potassium hydroxide: 1.00 Calcium hydroxide: 1.01

11 Limit of quantification Based on a 10-point calibration, the limits of quantification were calculated according to DIN [3]. For a sampled air volume of 420 L the limits of quantification are: Lithium hydroxide: mg/m 3 Sodium hydroxide: mg/m 3 Potassium hydroxide: mg/m 3 Calcium hydroxide: mg/m Shelf-life To determine the shelf-life, two series of quartz fibre filters were spiked with the hydroxides (low and high concentration). The samples were stored first of all for 7 days at room temperature, then in the refrigerator at + 4 8C. On days 1, 3, 6, 9, 13, 17, 21 and 27 two filters at each concentration were analysed. No significant changes in the hydroxide concentrations could be ascertained. 8.5 Effect of higher humidity The influence of the ambient conditions on sampling was investigated at a humidity of 70 %, established with a dynamic test gas apparatus. First of all, ten quartz fibre filters were spiked with a concentration in the middle of the determination range and dried at room temperature. Then, over a period of two hours, air with a humidity of 70 % was drawn through the loaded filters at a flow rate of 3.5 L/min. Humidity was not found to influence the results. 8.6 Sources of error Ion chromatographic determination is highly specific for lithium, sodium, potassium and calcium ions. At very high ammonium concentrations, the ammonium signal can overlap with the sodium signal. 8.7 Discussion The blank values of the quartz fibre filters must be checked regularly. The ubiquitous presence of sodium and calcium compounds leads to blank values being produced by numerous materials and equipment (chemicals, vessels). The blank values of all chemicals and equipment must therefore be carefully checked. As glassware yields high blank values, it may not be used for this analytical procedure.

12 Analytical Methods 26 When sampling hydroxides, other ubiquitous particulate lithium, sodium, potassium and calcium compounds are always collected too. Analytical differentiation is not possible. If electrochemical background compensation is used instead of chemical suppression, similar results are obtained. 9 References [1] Bundesministerium fçr Arbeit und Sozialverordnung (Eds) (2000) TRGS 900, Grenzwerte in der Luft am Arbeitsplatz ± Luftgrenzwerte. In: Technische Regeln und Richtlinien des Bundesministeriums fçr Arbeit zur Verordnung çber gefåhrliche Stoffe. BArbBl. 10/2000: 34±63 [2] Deutsches Institut fçr Normung e.v. (DIN) (1994) EN 482 ±Arbeitsplatzatmosphåre ± Allgemeine Anforderungen an Verfahren zur Messung von chemischen Arbeitsstoffen, Beuth Verlag, Berlin [3] Deutsches Institut fçr Normung e.v. (DIN) (1994) DIN ± Chemische Analytik ± Nachweis-, Erfassungs- und Bestimmungsgrenze. Beuth Verlag, Berlin Authors: Examiners: D. Breuer, B. Heinrich R. Hebisch, A. Johnen Fig. 1. Example of a chromatogram for the separation of the alkali metal and alkaline earth hydroxides analysed as cations (for the chromatography conditions see Section 4). The concentrations were 1.6 µg/ml for lithium and 3.2 µg/ml for ammonium, sodium, potassium, magnesium and calcium.

13 27 Fig. 2. Calibration curves for the alkali metal and alkaline earth hydroxides, analysed as cations.