Colorimetric Estimation of Ni ( II ) Ions in Aqueous Solution

A rapid and accurate colorimetric method has been proposed for the estimation of nickel(II) in aqueous solution. It is found that nickel(II) ions have maximum absorbance at 393 nm in distilled water and in aqueous sucrose solution (0.3 mol dm). In both case, the Beer’s law was obeyed over the range from 0.04 to 0.08 mol dm of nickel(II).The value of molar absorpitivity was constant 5.13±0.03 mol dm. This method is more rapid than the existing spectrophotometeric methods for the estimation of nickel(II). The variation in the results obtained by the method is ±2.1%.


Introduction
Nickel is extensively used in electroplating, the manufacturing of steel, electronic devices, ceramics and colored glasses.It plays a vital role in many processes of applied sciences and fundamental sciences.It necessitates development of rapid methods for estimation of nickel.
In colorimetric methods colored solutions can absorb light more or less depending upon wavelength of radiation and concentration of colored species present in solution.If the wavelength at which the maximum absorbance of other ions present in solution is different from the maximum wavelength of nickel(II) ions, we can estimate the nickel(II) ions with the colorimetric method in presence of other ions.
Although these methods are more sensitive having less interference of foreign ions but these methods requires considerable time for colorimetric estimation with certain tolerance limits.
The results reported in this study clearly indicate that the direct colorimetric method can be recommended for the rapid spectrophotometeric determination of nickel(II) ions in aqueous solution in absence of foreign ions or foreign ions having different wavelength absorbance (λ max ) from nickel(II) ions.This method may be useful for many studies like kinetic, adsorption etc.

Experimental
The nickel sulphate used for the work was of Qualigens grade L R. The solution of known concentration of nickel was prepared by dissolving an accurate weight of sample in deionized water distilled twice with a small quantity of alkaline potassium permanganate.The specific conductance of water used for the study was of the order 2x10 -6 Ω cm -1 .The solution of different concentrations were prepared by diluting a stoke solution of appropriate concentration.All absorbance measurements were made on an Elico mini spectrophotometer S L 177 with 10 mm matched quartz cells.

Results and Discussion
The colorimetric method gives more accurate results at low concentrations range where the absorbance and concentration satisfies the Beer-Lambert law.In this study we have taken the concentration range of solution 0.04 to 0.08 mol dm -3 .The absorbance of each solution was measured at different wave length in the wave length range 340 to 440 nm.The values of absorbance for each solution against distilled water blank are given in Table 1.The same processor was applied for the measurement of absorbance in aqueous sucrose solution using aqueous sucrose solution as blank.
The absorbance data obtained for each concentration showed maximum absorbance at 393 nm.The values of molar absorpitivity at each concentration was obtained which are listed in the last row of the Table 1.In the all cases the constant value of molar absorpitivity clearly indicate the validity of fundamental laws which governs the spectrophotometeric analysis to test the validity of Beer-Lambert law.The absorbance of different solutions were measured at 393 nm, the result obtained were utilized to prepare a calibration curve by plotting the absorbance versus the concentration of nickel sulphate.The calibration curve is given in Figure 1.
The calibration curve was used to estimate the concentration of nickel solution as sample model (0.045 mol dm -3 ) which gives the value of optical density equal to 0.225.The concentration of model sample was also estimated from the reported method 11 .The estimated values of concentration from standard method and proposed method was 0.046 mol dm -3 and 0.044 mol dm -3 respectively with the percentage error of ±2.2%.The absorbance of nickel(II) ions was also observed in aqueous sucrose solution 0.3 mol dm -3 in this case the maximum absorbance was same at 393 nm.It confirms that method is accurate in absence of foreign colored ions and non ionic colorless foreign solute.

Conclusions
The proposed method for estimation of nickel(II) in aqueous solution is with in the limit of experimental accuracy and is more useful due to the rapidity in comparison to the methods in use.This method can be employed for the routine analysis of the nickel in the sample of similar solvent conditions.

Table 1 .
447 Optical densities of Ni(II) ions at different concentrations with change in wavelength.Figure 1. Concentration of nickel sulphate, mol dm -3 .