- Research article
- Open Access
Evaluation of purity with its uncertainty value in high purity lead stick by conventional and electro-gravimetric methods
© Singh et al.; licensee Chemistry Central Ltd. 2013
- Received: 6 February 2013
- Accepted: 10 June 2013
- Published: 26 June 2013
A conventional gravimetry and electro-gravimetry study has been carried out for the precise and accurate purity determination of lead (Pb) in high purity lead stick and for preparation of reference standard. Reference materials are standards containing a known amount of an analyte and provide a reference value to determine unknown concentrations or to calibrate analytical instruments. A stock solution of approximate 2 kg has been prepared after dissolving approximate 2 g of Pb stick in 5% ultra pure nitric acid. From the stock solution five replicates of approximate 50 g have been taken for determination of purity by each method. The Pb has been determined as PbSO4 by conventional gravimetry, as PbO2 by electro gravimetry. The percentage purity of the metallic Pb was calculated accordingly from PbSO4 and PbO2.
On the basis of experimental observations it has been concluded that by conventional gravimetry and electro-gravimetry the purity of Pb was found to be 99.98 ± 0.24 and 99.97 ± 0.27 g/100 g and on the basis of Pb purity the concentration of reference standard solutions were found to be 1000.88 ± 2.44 and 1000.81 ± 2.68 mg kg-1 respectively with 95% confidence level (k = 2). The uncertainty evaluation has also been carried out in Pb determination following EURACHEM/GUM guidelines. The final analytical results quantifying uncertainty fulfills this requirement and gives a measure of the confidence level of the concerned laboratory.
Gravimetry is the most reliable technique in comparison to titremetry and instrumental method and the results of gravimetry are directly traceable to SI unit. Gravimetric analysis, if methods are followed carefully, provides for exceedingly precise analysis. In classical gravimetry the major uncertainties are due to repeatability but in electro-gravimetry several other factors also affect the final results.
- Conventional gravimetric
- Electro gravimetric
- Uncertainty evaluation
There is an increasing interest in Pb in the various matrixes because of its harmful effects on human beings. The Pb transfer through various processes from rocks, sediments, ground water, sludge etc. and causes negative effect on human beings. Lead as a metal has been widely used in industry for many years. Lead is omnipresent in the environment, either from natural resources or from pollutants. WHO/Codex alimentarius has established 0.01 mg L-1 maximum permissible limits of Pb in drinking water and 0.02 to 0.3 mg L-1 in food items . Exposure to high concentrations of Pb can cause serious health problems, including nervous system dysfunction of fetuses and infants, hemotoxic effects, reproductive dysfunction, gastrointestinal tract alterations, nephropathies, Alzheimer’s disease, anemia etc. . Thus accurate and precise determination of Pb is of prime importance because it is directly related to human health. To determine Pb at ppm or sub-ppm levels, some analytical instruments with high-sensitivity detection capabilities are available, including inductively coupled plasma mass spectroscopy (ICP-MS) and graphite furnace atomic absorption spectroscopy (GFAAS) [3–8]. Ion chromatography has also been used for lead determination; however, its sensitivity is not as good as that of ICP-MS and GFAAS [9, 10]. ASTM E1613-04 is the Standard Test Method for determination of lead by ICP-AES, Flame Atomic FAAS, or GFAAS Techniques. These techniques are very sensitive and can be used only for trace analysis and always required reference standard to calibrate the instrument. The higher concentration can not be determined by these techniques due to several dilution factors. Usually the higher concentration of lead can be determined by titremetry using sodium thiosulphate, EDTA, ammonium molybdate as titrant, or electro gravimetry as PbO2 or conventional gravimetry as lead chromate, lead molybdate, lead sulphate, and lead subacetate . Out of these techniques conventional gravimetry method is the primary method of measurement and most reliable technique in comparison to titremetry and instrumental method and results of gravimetry are directly traceable to SI unit.
In chemical metrology the reliability of the results in international trade is required for the acceptance of the analytical result by the users within the country or outside the country. This can be achieved by providing traceability of data characterizing products or for quality control/assurance of methodology used to analyze products. Quantifying uncertainty of analytical results fulfills this requirement and gives a measure of the confidence level of the analytical result. The estimation of uncertainty in the measurements has been given in ISO/EURACHEM guidelines [13–15]. The measurement uncertainty is carried out by quantifying of uncertainty in measurement of various steps and further by combination of potential sources of uncertainty of the entire experiment. The uncertainty has been classified as ‘Type A’, which is determined by statistical analysis of several observations. Second is or ‘Type B’ method which is determined by other ways than statistical analysis of a series of results.
Keeping in view the above facts, an attempt has been made by analyzing Pb by using two different gravimetry approaches; conventional gravimetry and electro-gravimetry. In the proposed process we have determined the purity of the Pb content with uncertainty value in the high purity lead stick procured from Sigma Aldrich. The uncertainty value determined is depends on different sources viz. uncertainty due to precision, repeatability, weighing, atomic mass of Pb and air buoyancy. In literature number of studies has been published on the evaluation of uncertainty in chemical and physical analysis [16–19] in various matrixes but the evaluation of the uncertainty by mass is very scanty. The present study highlights the various potential sources of uncertainty involved in the result obtained by conventional gravimetry and electro-gravimetry.
The gravimetric analysis only provides analysis of a single element, or a limited group of elements, at a time. In gravimetric analysis, the substance to be analyzed is separated from the other constituents in the form of an insoluble precipitate. The precipitates must be stable, non-hygroscopic and should be unaffected by the atmosphere. Besides this the precipitate should be of known chemical composition, sufficiently insoluble, easy to filter and amounts lost in the washings are insignificant. In most cases gravimetric analysis was used to determine the atomic masses of many elements up to six decimal point accuracy. It provides very less probability for instrumental error and does not require a series of standards for calculation of an unknown and also do not require expensive equipment. Due to its high degree of accuracy when performed correctly it can also be used to calibrate other instruments in lieu of reference standards. There are two basic variations of electro-gravimetric method: controlled-potential or controlled current. Electro-gravimetry analysis is more or less similar to conventional gravimetric analysis. Gravimetric analysis is one of the oldest analytical techniques and very accurate method of analysis when a large sample is available. Modern techniques require much less sample but may not be as accurate. Under the proper conditions, lead may be separated quantitatively from other metals as an insoluble sulfate. In addition, PbSO4 is somewhat soluble in nitric acid so there should be no nitric acid in the solution. So fuming should be strong to ensure complete removal of nitric acid from the solution. The solubility of lead sulfate increases with temperature and is dependant upon the weight percent of sulfuric acid in solution. In controlled-potential electro-gravimetry, a known constant potential is applied to the electrode for a sufficiently time to deposited out 100% of the analyte. The current decreases as the metal is deposited out. This provides a measure of selectivity if there are two or more metals that can plate out; the metal ion that is more easily reduced can often be plated out quantitatively without any of the other metal also plating out. In controlled-current electro-gravimetry, voltage is applied to the working electrode. The current is often on the order of milliamps (mA). If the concentration of metal ion in the electrolysis solution is insufficient to consume all the current, then other reactions such as hydrogen ion reduction may occur in order to consume all the current. And, if any interfering species are present, they will also plate out. In conventional gravimetry the major uncertainties are due to repeatability but in electro-gravimetry several other factors also affect the final results. After analysis of data by both the techniques, purity was found to be 99.97 ± 0.27 and 99.98 ± 0.24 g/100 g, while concentration of lead stock solutions were found to be 1000.81 ± 2.68 and 1000.88 ± 2.44 mg kg-1 using electro-gravimetry and conventional gravimetry technique respectively with 95% confidence level (k = 2).
A generalized scheme for planning a measurement and a simple, practical approach to estimating and combining uncertainties has been demonstrated for the determination of purity of Pb stick and the concentration of Pb in stock solution by electro and conventional gravimetric techniques. The results obtained by these two methods are comparable, but standard deviation and combined uncertainty of electro-gravimetry is more in comparisons to conventional gravimetry. So on the basis of results it has been observed that conventional gravimetry is more reliable than electro-gravimetry. But before making any conclusion we should consider the limitation of the process and various sources which contribute uncertainty in final measurement. Gravimetric analysis, if methods are followed carefully, provides for exceedingly precise analysis. The proposed process can be used for the certification of purity of Pb, as traceability statement is primary requirement for the preparation of standard solutions.
The first set of experiment has been carried out by electro-gravimetrically using Toshniwal electrolytic Analyzer system Model E-30, while the other set has been analyzed following classical gravimetry technique. All the weighing work has been carried out using Mettler make; AX 204 model and Sartorius make CC3002 model balances. The glasswares used were of Borosil glass works India Limited. All the acid digestion work was carried out in cleaned laminar flow bench equipped with the proper exhaustive system.
Nitric acid (69%), Hydrochloric acid (35%) of GR grade (Guaranteed Reagent) of Merck-India was used after purified by sub boiling point distillation of quartz glass unit. Merck make sulfuric acid (98%) GR grade was used. The Pb Sticks purity 99.999% as per certificate procured from Fluka AG, Chem (Germany), Febrik CH-9470 Buchs, 15310, Lot No; 244362587; has been used for solution preparation. De-ionized water (18.2 MΩ resistivity) prepared from Millipore milli- Q element water purification system, USA was used throughout the process.
Lead purity determination by conventional gravimetry and electro gravimetry
Stock solution of Pb of approximately 1000 mg kg-1 concentration was prepared by dissolving Pb metal stick in 2 M Sub-boiled nitric acid and final volume was made to 2 kg by de-ionized water. For conventional gravimetric analysis approximately 50 g of stock solution was taken in a beaker and it was diluted to 100 mL. To this solution 4–5 ml of concentrated sulfuric acid was added. It was heated on hot plate for strong fuming for at least one hour. Then the beaker was kept in ice cold water bath after addition of 100 ml de-ionized water in to it and allowed to precipitate PbSO4. The precipitate was filtered through pre-weighted G-4 crucible, washed 3–4 times with 2% H2SO4 followed by de-ionized water, then cooled to the room temperature. The precipitate was dried in an oven at 110°C for 2 h there after it was cooled to the room temperature in a desiccator. The weight of the precipitate along with the G4 crucible has been taken followed by drying and cooling to the room temperature several times till a constant weight is achieved. The concentration/purity of lead was calculated by the weight difference.
Calculation of concentration in stock solution and purity of lead in lead stick
Various evaluated components for the determination of Lead
Precipitate obtained as PbO2or PbSO4,(WPb),(g)
Weight of sample taken for analysis, Wsamp,(g)
Concentration of lead found by electro-gravimetry and gravimetry (mgkg-1), ( C(M Pb ) Exp )
Average concentration of lead found by electro-gravimetry and gravimetry, C Pb (mg kg-1)
Theoretical Concentration of stock solution (mg kg-1) ( C(M Pb ) Theo )
Percent of lead analyzed by electro-gravimetry and gravimetry ( C(M Pb ) Exp )/ C(M Pb ) Theo )
Average (g/100 g) of lead analyzed by electro-gravimetry and gravimetry
The combined standard uncertainty uc(y) is an estimated standard deviation and characterizes the dispersion of the values that could reasonably be attributed to the measurand ‘y’. The sensitivity coefficient = p i y/x i; Where pi is the power of the terms in the equation (1).
Evaluation of associated uncertainty budget
Uncertainty in purity of lead metal
Summary table for Uncertainty Budget for purity determination in (g/100 g) by classical gravimetry
u std (x)
u std (x) / X
U Ex (mg kg-1); k=2
Atomic mass of PbSO4
Balance (Mettler 220 g)
Balance (Sartorius 41 kg)
Error in buoyancy
Weight of PbSO4 precipitate (Mettler 220 g)
Weight of solution for deposition(Mettler 220 g)
Summary table for Uncertainty Budget for concentration determination in mg kg -1 by classical gravimetry
u std (x)
u std (x) / X
U Ex (mg kg-1); k=2
Atomic mass of PbSO4
Balance (Mettler 220 g)
Balance (Saritorious 41 kg)
Error in buoyancy
Weight of PbSO4 precipitate (Mettler 220 g)
Balance (Sartorios 41 kg) for stock solution
Summary table for Uncertainty Budget for purity determination in (g/100g) by electro-gravimetry
u std (x)
u std (x) / X
U Ex (%); k=2
Atomic mass of PbO2; (At. Wt)
Balance (Mettler 220 g); (W metal )
Balance (Sartorious 41kg) (W Obs. stock solution )
Error buoyancy; (Bu w.r.t Wtrue mass stock soln.)
weight of PbO2 (Mettler 220 g) (W ppt )
Weight of soln. for deposition(Mettler 220 g) (W aliquote )
Summary table for Uncertainty Budget for concentration determination in mg kg -1 by electro-gravimetry
ustd(x) / X
Atomic mass of PbO2
Balance (Mettler 220 g)
Balance (Sartorious 41 kg)
Error in buoyancy
weight of PbO2 (Mettler 220 g)
Weight of solution taken for electro-gravimetry experiment (Mettler 220 g)
Uncertainty in atomic mass
Uncertainty in atomic mass of lead Metal
Atomic mass of lead (g)
Uncertainty in weight (g)
Combined standard uncertainty
Uncertainty in weighing
Uncertainty due to balance of Ohaus make (200 g) and Sartorius make (41 kg)
Balance used for weighing
Weight taken (g)
Uncertainty in weight as per certificate at k=2
Standard uncertainty in total weight u(x)/x
Weight of the metal m(g)
Mettler AX 204 220g
Weight of solution taken for PbO2 deposition
Weight of PbO2 deposited
Weight of solution taken for PbSO4 deposition
Weight of PbSO4 deposited
Weight of stock solution
Sartorius; CC3002; 41 kg
Calculation of buoyancy correction
In the proposed process uncertainty due to buoyancy has been consider first time, which has major effects on weighing. The mass of the solution taken in an electronic balance is actually measure its weight force. So in presence of air the weight taken in an electronic balance varies remarkably for the same sample. As there is a deviation in the weight taken in an electronic balance due to air buoyancy, so the total uncertainty budget by mass is calculated considering buoyancy correction. The EURACHEM/CITAC Guide CG 4 ; Jones, Schoonover and Gonz´alez [22–24] have presented the process to calculate correction due to air buoyancy. For the calibration of single-pan electronic control balance, a standard weight of known mass has to be used to builds an apparent mass calibration into the balance. The reference density of the apparent mass scale is the density of the standard mass used for the calibration and the reference air density is the air density at the time of calibration.
Where; W obs is the observed weight of the unknown sample; W T is the True weight of the unknown sample; ρ a is the density of air at the time of balance calibration; ρ a M is the density of air at the time of measurement; ρ x is the density of the standard used to calibrate the balance; ρ s is the density of the sample. It is shown that the relative error in neglecting buoyancy is about 0.1 %. So, the exact value of the error is easily calculated as follows
W obs = W T × F; where,
The absolute error (Abs er ) is given by
Abs er = W T (1 – F)
Putting the value of WT & F in Eq. (6)
Absolute error due to Buoyancy is calculated as 2.0012 g
So the relative error is u(Abs er )/ W T = 0.000993.
The uncertainties associated with each component are combined and expanded Uncertainty with 95% confidence level.
Calculation of combined uncertainty
The authors are grateful to Prof. R. C. Budhani; Director, National Physical Laboratory for encouragement and permission to publish this work.
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