The instrument that used in this research are UV\Vis spectrometer (SPECORD S600), Analytical balance (METTLER TOLEDO, AL 104) and Digital pH meter (METTLER TOLEDO, Seven Easy) shaker water bath (GFL 10983), Hot plate stirrer (HP-3000) and Heating Drying oven (Model DHG).

An analytical grade chemical reagents were used, as received without further treatment Cr(NO₃) 39H₂O 98% Panreac, Ninhydrin 98.0% GCC Company, KOH 98.0% NTL (NEN TECH LTD), NaCl 99.0% GCC Company, KNO₃ 99.0% S.D Fine.chem. LTD, CaCl₂ 98.0% s S.D Fine-CHEM LTD INE, CuSO₄ 98.0% LOBA chemie and ZnCl₂ 98.0% GCC Company.

Many experiments were done to find the optimum wavelength which is the maximum one for the product of the reaction between Cr³⁺ ions with ninhydrin. All the experiments were done in alkaline media using 0.0089 M of KOH, at room temperature and with final volume of 50 mL. A stable greenish-violet colored complex (Cr³⁺ - ninhydrin) was obtained after many trials after 15 minutes, and its maximum value of absorption was found at 375 nm in the visible region using 6.24 × 10⁻³ M of Cr³⁺ ions with 2.4 × 10⁻³ M of ninhydrin. At each trial the absorbance was measured in the range of wave length (350 - 700) nm. The spectra of the each blank and 6.24 × 10⁻³ M of Cr³⁺ ions solutions were measured separately also in the range of wave length (350 - 700) nm as shown in Figures 1-3.

Different parameters were used to get the best circumstances for the reaction between chromium ions and ninhydrin. The following variables were examined to develop this analytical method: contact time, temperature, initial ninhydrin concentration, initial chromium ion concentration, initial concentration of KOH. The thermodynamic and kinetic studies were done at the found optimum conditions.

Three prepared solutions which of Cr³⁺, ninhydrin and KOH which have the following concentration respectively 6.24 × 10⁻³ M, 2.4 × 10⁻³ M and of 8.9 × 10⁻³ M are mixed with together at room temperature with final volume of 50 mL to investigate the effect of time of reaction on the absorbance of the colored product. The intervals time were 5, 10, 15, ··· until 155 minutes. The absorbance of the product was measured every 5.0 minutes over the 155 minutes. All of the experiments were repeated and the average result was taken.

Different initial concentrations of Cr³⁺ ions were prepared in the range of (2.00 × 10⁻⁴ - 1.86 × 10⁻²) M, each single solution was mixed separately with constant amount of each KOH with final concentrations of 0.0089 M and ninhydrin with final concentration of 0.0024 M. The solution was left for 15 min to complete the reaction and developing the deep greenish-violet colored complex with constant intensity the solution was diluted to 50 mL using distilled water. The absorbance was measured at 375 nm. All of the experiments were repeated and the average result was taken.

By measuring the absorbance of solutions containing fixed concentrations of both Cr³⁺ (6.24 × 10⁻³ M) and KOH (8.9 × 10⁻³ M) together with varying concentrations of ninhydrin (4.8 × 10⁻⁴ - 9.6 × 10⁻³ M) in a final volume of 50 mL, it was possible to determine the impact of ninhydrin concentration on the intensity of the colored complex at the chosen wavelength which is at 375 nm, the absorbance of each solution was determined. The average outcome was determined after repeating each experiment.

The effects of the initial potassium hydroxide concentration on the outcomes of this combination were investigated through a number of experiments while keeping the other conditions constant, the concentrations of both Cr³⁺ of 6.24 × 10⁻³ M and ninhydrin of 2.4 × 10⁻³ M also kept constant, different volumes of 0.0089 M of KOH were transfers into a series of 50 ml volumetric flask 0, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 mL these solutions were completed to final volume. At 375 nm, the absorbance of each solution was determined. The average outcome was determined after repeating each experiment.

The following temperatures were chosen: R.T (room temperature), 15˚C, 20˚C, 25˚C, 30˚C, 35˚C, 40˚C, 45˚C, 50˚C, 55˚C, and 60˚C. Several tests were conducted to examine the impact of temperature on the stability of the complex (Cr-Ninhydrin). The concentration of Cr³⁺ is 6.24 × 10⁻³ M, ninhydrin is 2.4 × 10⁻³ M, and KOH is 8.9× 10⁻³ M, with other conditions being held constant. At 375 nm, the absorbance of each solution was determined. After conducting each experiment a second time, the average result was established.

The experiment was conducted using 50 mL volumetric flasks containing 3.12 × 10⁻³ M of Cr³⁺, 8.9 × 10⁻³ M of KOH, and 2.4 × 10⁻³ M of ninhydrin under the optimum conditions. Separate amounts of the ionic salts NaCl, NaNO₃, ZnSO₄ and CaCl₂ were added separately to each flask before filling it to the mark. The absorbance of each individual solution was then measured at 375 nm. The average outcome was determined after repeating each experiment.

The calibration curve was created using the spectrophotometric method under the ideal conditions: the concentration of Ninhydrin is 2.4 × 10⁻³ M and 8.9 × 10⁻³ M KOH kept constant while different initial concentration of Cr³⁺ ions were used. The final volume of each solution is 50 mL using volumetric flask, After 15 minutes, and the reaction of each solution results in a greenish-violet color. The solution is then diluted to the appropriate concentration with The absorbance in the visible range was measured in comparison to a reagent blank made in the same way but without Cr³⁺ and kept at room temperature.

The method’s precision was calculated using intermediate precision (intra-day and inter-day). On the same day (with intra-day precision) and four days in a row, three replicates of three distinct standard Cr³⁺ concentrations were examined (inter-day precision). The similarity of the reference value and the obtained value is what determines how accurate an analytical procedure is.

The optimum contact time of the reaction between the chromium ions and ninhydrin was reached by following the development of the maximum absorbance at specific time at room temperature, several experiments were done to investigate the influence of time on the absorbance of the Cr³⁺–ninhydrin reaction, whilst the other conditions remained constant, The following time intervals were recorded: 5, 10, 15, 30, ···, and 155 minutes. Every 5 minutes, the product’s absorbance was measured over the 155 minutes, after 5 minutes of reaction the initial absorbance was 0.67 then it is increasing until reaching 0.73 after 15 minutes, which is the time for the full developed color of the product, at 20 minutes and after the signal of absorbance decreasing to reach 0.1 at 155 minutes as shown in Figure 4. Dissociation of this complex may begin to occur after about 20 minutes. The optimum time for measurement of this complex is after about 15 minutes.

The effect of initial concentration Cr³⁺ was investigated while keeping the other variables constant. As was observed from Figure 5, the absorbance of the

complex increased by increasing the concentration of Cr³⁺ ions in the range (0.0008 - 0.02) M while the concentration of ninhydrin kept constant, then sharp decreasing in the absorbance was observed by raising the concentration of Cr³⁺ ions in the range of (0.024 - 0.04) M, this decreasing may be due to the dissociation of the complex as the concentration of metal ions increased, the equilibrium of reaction will shift to left.

Various ninhydrin concentrations between (4.8 × 10⁻⁴ - 2.1 × 10⁻²) M were added to constant concentration of Cr³⁺ ions, the other variables kept constant, According to the standard procedure, the absorbance was measured. Figure 6 shows how the concentration of ninhydrin affects the intensity of the produced complex. The findings show that the absorbance decreases as ninhydrin solution concentrations increase.

The findings show that the absorbance of the complex decreases as ninhydrin solution concentrations increase because the intensity of the colored complex increased, the absorbance reached the maximum value when 2.4 × 10⁻³ M of ninhydrin is used, the decreasing in absorbance was noticed when higher concentration of ninhydrin is used, dissociation of the complex may occurred by increasing ninhydrin concentration.

According to the accepted practice, the absorbance was measured. Figureshows how the concentration of ninhydrin affects the complex (6). The findings show that the absorbance decreases as ninhydrin solution concentrations increase.

The influence of temperature was investigated at various temperature intervals were chosen in the range of 15˚C until reaching 60˚C as shown in Figure 7, the temperature values from 15˚C to 40˚C showed that the stability of the formed

colored complex has not been impacted, while in the range of 45˚C to 60˚C the stability decreased gradually and the color of the complex became less dense, and decreasing in absorbance readings was observed. This mean that the reaction is stable at low and moderately high temperature range, while at higher temperature instability of the complex is observed which means an exothermic reaction may be occurs.

Investigations were also done into the effect of KOH concentration on the reaction product’s absorbance. From Figure 8, it was observed that the absorbance

of this colored complex decreased when the KOH concentration was raised until it was zero. A competition between ninhydrin and OH^- to interact with the Cr³⁺ ions may occurred and another complex was formed.

The impact of foreign species frequently discovered with Cr³⁺ were examined under optimal conditions to evaluate the efficiency of the suggested technique by adding known concentrations of diverse ions of standard Cr³⁺, the influence of three different cations on the determination of Cr³⁺ was investigated and results are shown in Table 1. The results indicate that almost all salts had no effect on absorbance reading and hence have no influence on complex stability.

Based on Beer’s Lambert law, the linear correlation coefficient between absorbance and Cr³⁺ ion concentration was determined, from the regression analysis the data used to estimate the intercept (a), slope (b), and the correlation coefficient (R²) as can we see in Table 2 and illustrated in Figure 9, the limit of detection and the limit of quantification also contained in Table 2.

With three different Cr³⁺ solution concentrations, the precision and accuracy were studied. The precision was confirmed [16] [17] in terms of intra-day intermediate precision, and these solutions were created and merged with ninhydrin under the optimal conditions. For intra-day precision, the products were examined three times on the same day. For each concentration of Cr³⁺ ions, the standard deviation (SD) and relative standard deviation (%RSD) were estimated, and the range of (%RSD) for the three different concentrations was 0.90% - 3.18% as shown in Table 3. The repeatability in three days (inter-day) for each

The absorbance measured in a glass cuvette of-section are 1 cm².

concentration of Cr³⁺ ions in terms of %RSD were acquired in the range 1.24% - 3.71%. The precision was also determined in terms of intermediate precision inter-day. The products were examined in three repeats over three consecutive

days (inter-day precision) by selecting three different concentrations of Cr³⁺ ions that mixed with ninhydrin under the optimal parameters.

Investigation of the analytical method’s accuracy resulted in the calculation of the absolute error for each analysis (both intra- and inter-day accuracy), as indicated in Table 3. The taken amount and the found amount are likely close to one another based on the minimal absolute error values [16] [17]. The F-test was used in the statistical analysis for the intraday and interlay, and the results demonstrated that there is no bias between the taken values and the observed ones at alpha = 0.05, disproving the null hypothesis.

By using the limiting logarithmic technique under ideal conditions, the stoichiometric ratio between the Cr³⁺ ion and the ninhydrin was assessed [18]. Two sets of tests were made using this method: the first set varied the concentration of Cr³⁺ while maintaining a constant concentration of ninhydrin, and the second set varied the concentration of Cr³⁺ while maintaining a constant concentration of ninhydrin. To identify the order of reaction of the Cr³⁺ with respect to ninhydrin or vice versa, log absorbance versus log [Cr³⁺] and log ninhydrin were plotted to evaluate the slope of the corresponding line, as shown in Figure 10. It was observed that the slope was 1.0049 for various concentrations of Cr³⁺. The slope of Figure 11 is at varying ninhydrin concentrations. At different ninhydrin concentrations, the slope from Figure 11 was equal to about 2. This validated the 1:2 molar ratio of coupling between Cr³⁺ and ninhydrin.

Depending on the data that obtained from Figure 10 and Figure 11, the proposed chemical equation of the reaction between Cr³⁺ and ninhydrin is shown at Scheme 1.