3. Selectivity properties

Ni2<sup>þ</sup>

Ni<sup>2</sup><sup>þ</sup>

tions (a) 0.01 M, (b) 0.001 M, and (c) 0.0001 M Ni2+ solutions.

tions (a) 0.01 M, (b) 0.001 M, and (c) 0.0001 M Ni2+ solutions.

296 Cyclodextrin - A Versatile Ingredient

ΔE ¼ Em � Es ¼ ð Þ RT=F ln ki aNi

<sup>s</sup> ⇌ Ni2<sup>þ</sup>

Figure 6. Dynamic response for Ni-selective electrodes with membrane type II containing NPOE, for different concentra-

Figure 5. Dynamic response for Ni-selective electrodes with membrane type I containing DEP, for different concentra-

<sup>m</sup> <sup>þ</sup> <sup>β</sup> � CDXm ⇌ Ni2<sup>þ</sup> � <sup>β</sup>-CDX <sup>2</sup><sup>þ</sup>

m

<sup>2</sup><sup>þ</sup> ½ � <sup>1</sup> <sup>þ</sup> Kf ð Þ carrier <sup>=</sup>ð Þ sites

m

The selectivity of Ni electrode with different membrane types I and II was calculated according to the SSM [16]. Table 3 shows the obtained values of the selectivity coefficient (KPot Ni2+, j z+). From the results, it can be reported that most of the tested interferents for Ni-ISE type I showed perfect selectivity. When the electrode type I was used, the values of the selectivity coefficient toward divalent cations were so perfect to consider this electrode selective for Ni++ cation. The recorded values for most of the tested divalent cations were of the order of 10�<sup>3</sup> . In case of trivalent cations, this type of electrode showed better selectivity coefficient values (of order 10�<sup>5</sup> ). When electrode type II was used, the selectivity coefficient values were higher than that for electrode type I. It showed values of order 10�<sup>2</sup> for trivalent cations and 10�<sup>1</sup> for most of the divalent cations. This can be attributed to that in type II electrode the NPOE has active sites to interact with cationic species which lowers the selectivity toward Ni++ [17]. The tested monovalent cations (Na<sup>+</sup> , K<sup>+</sup> , and NH4 + ) showed interference with the measurements with either type I or type II. Accordingly, it is recommended that measurements with the proposed electrodes should be conducted in the absence of these cations.


4. Determination of nickel in its samples

Table 4. Determination of nickel in its samples using the proposed Ni-ISE.

with membrane type II for 0.0001 M solutions.

\*(4-determinations).

In this chapter, two types of samples containing nickel were used. They were representative for food samples and stainless steel samples. Five steel samples (A–D) and one food (E) sample were chosen. On the one hand, 0.1 g of each stainless steel sample was dissolved into aqua-Regia, heated at 105�C, and diluted to 250 ml using bi-distilled water. This solution was measured directly. On the other hand, 0.5 g chocolate sample (E) was dissolved in 100 ml after digestion with HNO3, HClO4, and H2O2. In this case, 1 ml was diluted to 50 ml, and the result solution was subjected to potential measurements using the proposed Ni-selective electrode. The obtained Ni values into the stainless steel samples (A–D) were between 1.467 and 7.354 ppm. The chocolate sample E showed Ni content 14.707 ppm. All the obtained values agreed with the values given by AAS analysis of the same samples [19]. The obtained values of

No. Sample Ni2+, ppm RSD\*, %

A Test tube holder 1.437 1.467 1.95 B Shaving blade 6.005 7.354 2.51 C Screwdriver 4.181 4.640 2.13 D Coin (1/4 pound) 4.61 3.686 1.81 E Chocolate (Cadbury Dairy Milk) 12.158 14.707 2.34

AAS method Ni-ISE method

z+) for Ni-ISE

299

β-Cyclodextrin as an Ionophore for Membrane Electrode http://dx.doi.org/10.5772/intechopen.73597

Figure 9. Correlation between ionic radius (pm) of the tested cations and selectivity coefficient (KPot Ni2+, j

Figure 8. Correlation between ionic radius (pm) of the tested cations and selectivity coefficient (KPot Ni2+, j z+) for Ni-ISE with membrane type I for 0.0001 M solutions.

The relation between ionic radius [18] of interferent cations and the values of selectivity coefficient for both electrode types is shown in Figures 8 and 9. It was found that there was an increase in the selectivity coefficient values with increasing the ionic radius of the tested cations. This was true for both electrode types I and II. The increment values in case of type I were less than those in case of type II. This was attributed to that the increase in ionic volume was suitable for the β-CDX cavity.

Figure 9. Correlation between ionic radius (pm) of the tested cations and selectivity coefficient (KPot Ni2+, j z+) for Ni-ISE with membrane type II for 0.0001 M solutions.
