Gold is rare, universally accepted, and can be easily bought and sold around the world. It is used in coinage and as a standard for monetary systems in many countries. Gold is the best form of money the world has ever known. The Earth's crust contains an average of 0.004 grams per ton. Gold occurs in association with ores of copper, lead, iron sulfide, in quartz veins, in the gravel of stream beds etc. Sea water also contains astonishing amount of gold, but the process of recovery is not economical. There are four basic colors of Gold, yellow, white, red and green. The colors of gold are obtained by the use of two or more varied quantities of base metals. The base metals are silver, copper, zinc and nickel. The alloy of gold, silver, and copper, having silver predominates, is called “green gold”. The alloy of the same three elements in which copper predominates is called “red gold”, whereas an alloy of gold and nickel is called “white gold”.
There is a large and rapidly growing demand for gold in industrial processes worldwide. The world consumption of new gold produced is about 50% in jewellery, 40% in investments, and 10% in industry (Soos [2011]). Gold, like other precious metals, is measured in troy weight, when alloy with other metals, the term carat is used to express the amount of gold present, 24 carats means pure gold, while 18 Kt, 14 Kt and 10 Kt Gold called 75%, 58.3% and 41.7% pure respectively. Gold’s superior electrical conductivity, malleability and resistance to corrosion has made it vital in components used in a wide range of electronic products and equipment, including computers, telephones, cellular phones, and home appliances. Gold has high reflective power that protects space crafts and satellites from solar radiation. In medical and industry gold-coated reflectors is used to focus light energy. The small quantity dissolved in glass or plastic sheets prevent the passage of infrared radiation and make an efficient heat shield. Its lack of toxicity and compatibility with living systems make it indispensable in dentistry and medicine. As gold is biologically inactive; it has become a vital tool for medical research and even used in the direct treatment of arthritis and other intractable diseases (Tiekink [2003]).
There are basically three techniques for the determination of gold, gravimetry, titremetry and instrumental techniques. The main instrumental techniques for gold determination are Atomic Absorption Spectrometry (Reddi & Rao [1999]), Neutron Activation Analysis (Heurtebise et al. [1973]), Inductively Coupled Plasma Spectroscopy (Brill & Wiedemann [1992]), X-ray Fluorescence Spectroscopy (Jalas et al. [2002]), Low-energy ED-XRF (Roessiger & Nensel [2003]; Marucco [2004]), Laser Induced Plasma Spectroscopy (Corsi et al. [2001]), Laser Induced Breakdown Spectroscopy (Jurado-Lopez & Luque De Castro [2003]), and Knudsen Effusion Mass Spectrometry (Bardi et al. [2008]). But the costs of the instruments, assayer’s skill, make these techniques out of many small-firms budget. In addition, some of these methods are devoted to the determination of trace elements and hardly applicable to the accurate determination. Elci et al. (Elci et al. [2007]) has determined gold by Atomic Absorption Spectrometer after extraction in Amberlite XAD-200 and also study the effect of acid concentration, sample volume, interfering ions etc. on the recovery of gold. Soylak et al. (Soylak et al. [2000]) has also determined Gold, Silver and Palladium by flame Atomic Absorption Spectrometer after formation of complex with dithiophosphoric acid and DDTP followed by sorbed on activated carbon column. Bulut etal (Bulut et al. [2011]) has determined Gold (III) spectrophotometrically as ODBOCA–Au(III) complex after extraction in chloroform. In gravimetry method Gold is determined as metal or stable salt by using reducing agents like sulphur dioxide, oxalic acid, ferrous sulphate (Vogel [1961]), 2,4,6-triphenylpyrylium chloride (Thomas [1974]) and hydroquinone (Beamish et al. [1937]). Caporali (Caporali et al. [2010]) has determined gold in alloys by using potentiometric titration and claimed process as alternate of fire assay method.
Besides these techniques there are some other techniques like spot test (Sen [1959]) acid testing kit, Electronic gold testers and Touchstone testing. Acid testing is probably the most widely used of all the gold testing methods. This method of testing is done by placing a small drop of acid onto the surface of the test item. Genuine or solid gold will usually leave a brown stain and non gold items will leave a green stain on the metal indicating that the item is either gold plated or contains copper as a base metal. The electronic gold tester is an electronic circuit with a test plate of reference gold. Touchstone testing involves rubbing of test item of gold onto a touchstone. Theses methods still requires the use of acids and also need gold reference standard to compare the test results.
In chemical metrology most of the important decisions are based on the quality and the reliability of analytical results of quantitative analysis. In this context it is also important to have reliability of the results in international trade because analytical results should be acceptable to all users within the country or outside the country. This can be achieved by estimation of uncertainty in the measurements as per ISO/EURACHEM guidelines (ISO [2003]; VIM 3rd edition JCGM 200 [2012]). 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. Thus all the above methods do not fulfill the requirement for precise and accurate determination of gold and due to gold’s high economic value; its quantification must be carried out with high accuracy and precision. The little variation in the results may cause huge losses to traders. Nevertheless, lead fire-assay, owing to its high precision and accuracy, is still the preferred method for determination of gold in jewellery (Ronlad [1980]; Peter [1997]; UNI EN ISO 11426 [2000]). The laboratories that perform gold assay consider the fire assay method the most preferred technique. In the fire assay so many steps are involved from alloying to final determination and there is a chance of loss of gold during analysis. The fire assay method is time consuming and required expertise to perform the test. The measurement uncertainty increases on increasing steps. The quality, reliability and confidence level of the analyzed data depends upon the measurement of uncertainty of the whole process.
In the proposed method an attempt has been made for direct determination of gold gravimetrically in three steps: dissolution, reduction in metallic form and weighing. Several experiments were carried by taking 0.050 to 0.2 g of test sample, but measurement uncertainty has been calculated by analyzing five replicates of approximate 0.2 g following EURACHEM/GUM guideline. In last few years we have also published some papers related to measurement of uncertainty in various matrixes (Kayal et al. [2009]; Nahar et al. [2011]; Kayal & Nahar [2010]).