2-Hydroxy-1-naphthaldehyde-P-hydroxybenzoichydrazone : A New Chromogenic Reagent for the Determination of Thorium ( IV ) and Uranium ( VI )

Simple, sensitive, selective, direct, derivative, and simultaneous spectrophotometric methods are developed for the determination of uranium and thorium individually and simultaneously.emethods are based on the reaction of 2-hydroxy-1-naphthaldehydep-hydroxybenzoichydrazone (HNAHBH) with thorium(IV) and uranium(VI). HNAHBH reacts with thorium and uranium at pH 6.0 forming stable yellow and reddish brown coloured complexes, respectively. [(IV)-HNAHBH] complex shows maximum absorbance at 415 nm. Beer’s law is obeyed over the concentration range 0.464–6.961 μμgmL with a detection limit of 0.01 μμgmL and molar absorptivity, εε, 3.5 × 10 Lmol cm. Maximum absorbance shown by [U(VI)-HNAHBH] complex is at 410 nm with Beer’s law range 0.476–7.140 μμgmL, detection limit 0.139 μμgmL and molar absorptivity, εε, 1.78 × 10 Lmol cm. Highly sensitive and selective second-order derivative methods are reported for the direct and simultaneous determination of(IV) and U(VI) using HNAHBH. e applicability of the developed methods is tested by analyzing water, ore, fertilizer, and gas mantle samples for thorium and uranium content.


Introduction
Among the various actinides, uranium and thorium due to natural abundance are very important, especially in view of energy sources, but seriously hazardous in view of environmental pollution.Powdered thorium metal is oen pyrophoric and should be handled carefully.Exposure to thorium can lead to increased risk of cancers of the lung, pancreas, and blood.Exposure to thorium internally leads to increased risk of liver diseases.All isotopes and compounds of uranium are toxic, teratogenic, and radioactive carcinogens.When uranium gets inside the body, it can lead to cancer or kidney damage.Inhaled uranium increases the risk of lung cancer.ere is also the possibility of groundwater contamination with the toxic chemicals used in the separation of the uranium ore.On these considerations measurement of these metals under the microgram levels is very important [1].Many advanced methods have already been developed for the determination of uranium and thorium.ese methods include alpha spectrometry [2], inductively coupled plasma atomic emission spectrometry (ICP-AES) [3], inductively coupled plasma mass spectrometry (ICP-MS) [4], and capillary zone electrophoresis (CZE) [5].However, these methods are hampered by less sensitivity or by matrix effect.Furthermore, availability of these techniques is limited due to expensive equipment and higher running cost.
Spectrophotometry is a relatively easy alternative method which provides some advantages, like simplicity and moderately low cost.A number of reagents have been used for the spectrophotometric determination of uranium and thorium.Among the various chromogenic agents reported for the determination of thorium (IV), most of them were found to be nonselective involving in serious interference from the diverse ions [6][7][8][9].Majority of the reported methods require prior removal of closely associated metal ions and hence lack selectivity.Some of the early chromogens employed for the spectrophotometric determination of uranium include Arsenazo-III [10], dibenzoyl methane [11], and 1-(2-pyridylazo)-2-naphthol [12,13].ey were highly sensitive reagents with less selectivity involving prior separation of interference.In the present investigation simple, sensitive and nonextractive direct and derivative spectrophotometric methods for the determination of thorium and uranium using HNAHBH are reported.A selective simultaneous second-order derivative spectrophotometric method is also proposed for the determination of (IV) and U(VI).[26].e monazite sand was collected from the Arabian Sea coast at Mangalore, India.1.0 g of the monazite sand was mixed with 5 mL of concentrated sulphuric acid and heated at 250 ∘ C for about 4 hours to digest the monazite sand.e resultant viscous paste was dissolved in distilled water by heating at 45 ∘ C for half an hour.e resultant solution was �ltered and thorium from the �ltrate is precipitated as hydroxide by adding ammonia solution.e precipitate was �ltered off and dissolved in minimum quantity of dil HCl and then diluted to 25 mL.[27].0.5 g of gas mantle sample was accurately weighed and placed in a 100 mL beaker.10 mL of con HNO 3 was added and gently boiled for 20 minutes.e residue was diluted with 10 mL of distilled water and �ltered.e resultant solution was diluted to 250 mL in a volumetric �ask with distilled water.

Preparation of Phosphate Rock and
Fertilizer Sample [28,29].e phosphate rock which is the raw material for manufacturing of phosphate fertilizers, NPK and DAP fertilizers, was collected from a fertilizer industry, Anantapur, India.e collected samples were �nely grounded.10 g of each sample was transferred separately into Ernermeyers �ask containing 100 mL of 0.1 M critic acid.All these �asks were incubated in the orbital shaker at 30 ∘ C at 100 rev min −1 .ese samples were removed and centrifuged to remove solid suspension.[30].0.25 g of 20283 MALINIM-G (Granite) or 20383 MALINIM-L (Lujavrite) ore samples was weighed and dissolved in a mixture of 15 mL of con H 2 SO 4 , 5 mL of con.HNO 3 , and 45 mL of HF by boiling in a te�on beaker.e solution was evaporated to dryness, and the residue was dissolved in 10 mL of distilled water.

Preparation of Water Samples.
Natural water samples from different sources of Anantapur town were collected and �ltered through Whatman �lter paper no.41 to remove the suspended particulate matter.e �ltered water was then treated with 5 mL of con.HNO 3 to prevent the possible hydrolytic precipitation of some mineral salts.Different known amounts of U(VI) and (IV) were then spiked into 100 mL of these treated water samples.

Apparatus.
A Perkin Elmer (LAMBDA25) spectrophotometer controlled by a computer and equipped with a 1 cm path length quartz cell was used for UV-Vis spectra acquisition.Spectra were acquired between 350-600 nm (1 nm resolution).An ELICO model LI-120 pH-meter furnished with a combined glass electrode was used to measure pH of buffer solutions.

Foreign ion
Tolerance limit (g mL −1 ) Foreign ion Tolerance limit (g mL −1 ) Foreign ion Tolerance limit (g mL Masking agents, a in the presence of 800 ppm of EDTA; b 500 ppm of tartrate.

Direct Spectrophotometric Determination of orium(IV) and Uranium(VI)
. HNAHBH forms yellow and reddish brown coloured complexes with (IV) and U(VI), respectively.e colour of complexes is stable for more than 72 hours.In�uence of pH, reagent, and surfactants on the absorbance of the complexes was investigated as follows.
3.1.1.pH Effect.e study of effect of pH 1.0-10.0 on the absorbance of both the reaction mixtures [U(VI)-HNAHBH] and [(IV)-HNAHBH] showed that maximum absorbance was obtained in the pH region of 5.5-6.5.erefore further studies were carried out at pH 6.0.[(IV)-HNAHBH] complex shows maximum absorbance at 415 nm and that of [U(VI)-HNAHBH] complex is at 410 nm.Reagent blank shows the least absorbance at these wave lengths.

Effect of Reagent (HNAHBH) Concentration
3.1.5.Calibration Curves.e calibration curves were constructed for both the complexes taking variable amounts of metal ions at their respective absorption maxima.e calibration plots were linear over wide concentration ranges which are shown in Table 10 along with the slope, intercept, standard deviation, detection, and determination limits.
3.1.7.Interferences.e effect of diverse ions in the determination of (IV) and U(VI) was studied in detail, and the tolerance limits were calculated and presented in Tables 1 and  2. Most of the anions and good number of cations did not interfere in the present methods even when present in more than 50-fold excess.Some cations, which interfere seriously, could be masked up to 50-fold excess with suitable masking agents as shown in the Tables 1 and 2.
3.1.8.Application of Proposed Direct Methods.e proposed direct spectrophotometric methods were employed for the determination of thorium in monazite sand and environmental water samples and for the determination of uranium in phosphate rock and fertilizer samples.Suitable aliquots of prepared sample solutions are treated with required amount of reagent and suitable buffer media, and the absorbances of resultant solutions were measured at appropriate wave lengths; the amounts of metal ions present T 2: Tolerance limits of foreign ions.Amount of U(VI) taken = 4.760 g mL −1 pH = 6.0.

Foreign ion
Tolerance limit (g mL −1 ) Foreign ion Tolerance limit Foreign ion Tolerance limit (g mL

Derivative Spectrophotometric Determination of (IV) and U(VI).
Second-order derivative spectra at pH 6.0 in the wavelength region of 350-600 nm is recorded for T 5: Analysis of phosphate rock and fertilizers for their uranium content. of [(IV) : HNAHBH] complex show maximum amplitude at 434 nm and 453 nm with zero crossing at 443 nm (Figure 1).e second derivative spectra of yellowish brown coloured [U(VI)-HNAHBH] solution show a small crust at 448 nm, a small trough at 459 nm, and an intensi�ed crust at 477 nm with zero cross at 453 nm and 466 nm (Figure 2).e measured derivative amplitudes at appropriate wavelengths as mentioned above for second derivative spectra were plotted against the amount of (IV) and U(VI).Linear plots were obtained in the range of 0.232-6.961g mL −1 for (IV) and 0.238-2.38g mL −1 for U(VI).Other analytical properties are listed in Table 10.

Effect of Foreign Ions in Derivative Methods.
Interference of various metal and anions were studied on the derivative amplitudes.It was noticed that all metal ions and anions which did not interfere in direct methods also did not interfere in derivative methods.e metal ions which interfere seriously in zero order method are tolerable up to 20-25 fold excess (Tables 6 and 7).e above studies reveal that the derivative methods are more sensitive and selective than proposed direct methods.

Determination of orium in Gas Mantle.
A known aliquot of the prepared sample solution was treated according to the recommended procedure, and the amount of thorium was evaluated from the predetermined calibration plot.e results are shown in Table 8 along with recovery percentages.

Analysis of Environmental
Water Samples for the Uranium Content. 100 mL of each of the sample was �ltered using �hatman �lter paper and spiked with known amounts of uranium.Suitable aliquots were taken and analysed for uranium amount (Table 9).

Simultaneous Second-Order Derivative Spectrophotometric Determination of Uranium and orium.
A good number of spectrophotometric methods have already been developed for the simultaneous determination of uranium and thorium [31][32][33].e present method provides a simple and selective derivative spectrophotometric procedure for the simultaneous determination of uranium and thorium without prior separation and without solving simultaneous equations.

Derivative Spectra. e 2nd-order derivative spectra recorded for [U(VI)-HNAHBH] and [(VI)
-HNAHBH] at pH 6.0 showed sufficiently large derivative amplitude for thorium at 448 nm, while the U(VI) species exhibits zero amplitude, at 477 nm maximum derivative amplitude was noticed for U(VI) where there is no amplitude for (IV) (Figure 3).is facilitates the determination of U(VI) and (IV) simultaneously by measuring the second derivative amplitudes of binary mixtures containing U(VI) and (IV) at 477 nm and 448 nm, respectively.

Determination of U(VI) and (IV).
Aliquots of solutions containing 0.238-2.380g mL −1 of U(VI) or 0.232-4.640g mL −1 of (IV) were transferred into a series of 10 mL calibrated volumetric �asks.HNAHBH (1 × 10 −2 M, 0.6 mL), CPC (1%, 0.5 mL), and buffer solution  coefficients of the prepared calibration plots were calculated and presented in Table 11.e data in table indicate that the presence of (IV)/U(VI) is not in�uencing the calibration plot of U(VI)/(IV) to any signi�cant extent.is allows the determination of U(VI) and (IV) in their mixtures without any signi�cant error and without the need for their prior separation.U(VI) and (IV) were mixed in different proportions and then treated with required amount of HNAHBH in the presence of buffer solution (pH 6.0) and 0.05% of CTAB and diluted to the volume in 10 mL volumetric �asks.e secondorder derivative spectra for these solutions were recorded (350-600 nm), and the derivative amplitudes were measured at 477 nm and 448 nm.e amounts of U(VI) and (IV) taken in the mixtures were calculated from the measured derivative amplitudes using the respective predetermined calibration plots.e results obtained along with the recovery percentage and relative errors are presented in Table 12, which indicate the usefulness of the proposed method for the simultaneous determination of U(VI) and (IV).

Application of
Simultaneous Method.0.5 mL of the prepared ore samples and suitable volumes of water samples was analyzed by the proposed second-order derivative spectrophotometric method using HNAHBH and the results obtained and relative errors are tabulated in Tables 13  and 14.

Conclusions
e analytical results evaluated in the present paper in direct, derivative, and simultaneous methods were compared with those reported in some of the recently reported methods and presented in the Table 15.e comparison of the results indicate that the proposed method for the determination of uranium is more sensitive than number of reported methods [14][15][16][17][18] and more selective than the method reported by Khan et al. [20].Regarding the determination of thorium the present method is more sensitive than those reported by Khan et al. [21], Sharma and Eshwar [22].e results obtained in the simultaneous determination of U(VI) and (IV) are well comparable with the reported methods [23][24][25].Above all most of the reported methods involve extraction in spurious organic solvents, whereas the present reported methods are simple, nonextractive, and reasonably accurate methods.

2 F 3 :
Second order derivative spectra of (a) [(IV)-HNAHBH] and (b) [U(VI)-HNAHBH].(IV) (g mL −1 ): 0.464; 0.928; 1.392: U(VI) (g mL −1 ): 0.952; 1.428; 1.904.(pH6.0, 4 mL) were added to each of these �asks and diluted to the mark with distilled water.e zero crossing points of [U(VI)-HNAHBH] and [(IV)-HNAHBH] species were determined by recording the second-order derivative spectra of both the systems with reference to the reagent blank.Calibration plots for the determination of U(VI) and (IV) were constructed by measuring the second derivative amplitudes at zero crossing points of [(IV)-HNAHBH] (448 nm) and [U(VI)-HNAHBH] (477 nm), respectively and plotting against the respective analyte concentrations.[U(VI)-HNAHBH] and [(IV)-HNAHBH] coloured solutions obeyed Beer's law in the range 0.238-2.380g mL −1 and 0.232-4.640g mL −1 at 477 nm and 448 nm, respectively.To test the mutual independency of the analytical signals of U(VI) and (IV) in the presence of each other, the calibration plots were drawn for each metal ion alone and in the presence of a known amount of the other metal ion.e slopes, intercepts, and correlation Masking agents, a in presence of 600 ppm of Tartrate, b 300 ppm of oxalate, c 500 ppm of thiocyanate.
T 11: Linear regression analysis of the determination of U(VI) and (IV) in mixture by second derivative spectrophotometry.
* Average of four determinations.T 13: Simultaneous determination of uranium and thorium in ore samples.