Construction of Novel PVC-Based Membrane Electrodes for the Selective Determination of Fe3+ Ion
Authors: Bandi Koteswara RAO , Singh Ashok KUMAR , Upadhyay Anjali 
Published: February 2020

Abstract

Iron is fourth most abundant elements of the earth crust and is widely and uniformly distributed. It generally present in surface waters as salts containing Fe3+ion. Most of those salts are insoluble and settle out , thus, the concentration of iron in well-aerated waters is seldom high. Siderite, haematite, limonite ad magnetite are the most important commercial iron ores. It finds many applications both in support of animal life and industrial processes. At trace concentration, iron promotes many important biological processes of human life. However, at higher concentration, it is toxic and causes liver and kidney damages. Tissue damage has also been reported from prolonged consumption of acidic foodstuffs cooked in iron kitchenware. Thus it is important to monitor the concentration of iron in various environmental samples.
There are number instrumental technique such as X-ray ﬂuorescence, neutron activation analysis, isotope dilution multiple collector inductively coupled plasma mass spectrometry, flame atomic absorption spectrometry, cathodic stripping voltammetry and spectrophotometry for the determination of Fe3+ ion. Although these methods provide accurate determination of Fe3+ion content but require large infrastructure backup, highly expensive and support of expertise. These requirements make difficult for such a technique to be used in routine manner for the analysis of large number of samples. Thus in the search of analytical technique which is portable, fast and low cost is the obvious choice. Such requirements are generally met by ion selective electrodes. But, a good ion-selective electrode need a good ionophore.
Schiff bases N-(4-(dimethylamino)benzylidene)-thiazol-2-amine [L1], 5-((3-methylthiophene-2yl) methyleneamino)-1,3,4-thiadiazole-2-thiol [L2] and N-((3-methylthiophene-2yl)methylene)thiazol-2-amine [L3] were synthesized and explored as Fe3+ ion-selective electrodes. The effect of various plasticizers (o-nitrophenyloctyl ether (o-NPOE), dibutylphthalate (DBP), 1-Chloronapthalene (1-CN), Dioctylphthalate (DOP) and acetophenone (AP)) and anionic excluders (sodium tetraphenylborate (NaTPB)/ Potassium tetrakis p-(chloro phenyl)borate (KTpClPB)) were also examined and observed that the membrane electrode having a composition of L2: PVC: o-NPOE: NaTPB as 3: 38.5: 56: 2.5 (w/w; mg) was showing the best performance characteristics. A coated graphite electrode (CGE) was also fabricated with the same composition and their potentiometric studies were compared. CGE is found to perform better as it shows wider working concentration range 8.3 × 10-8 ‒ 1.0 × 10-1 mol L-1, lower detection limit 2.3 × 10-8 mol L-1, Nernstian slope 19.5± 0.4 mV decade-1 of activity with response time 10 s. The CGE shows a shelf life of 6 weeks and in view of high selectivity, it can be used to quantify Fe3+ ion in water, soil, vegetable and medicinal plants. It can also be used as an indicator electrode in potentiometric titration of EDTA with Fe3+ion.
Fig. 1. Calibration characteristics of the electrodes (PME and CGE) based on L2.
Table 1. Response characteristics of Fe (III) ion selective PME and CGE.

Properties
Electrode Response

PME
CGE

Working concentration range (mol L-1)
6.7 × 10-7 - 1.0 × 10-1

8.3 × 10-8 - 1.0 × 10-1

Detection limit (mol L-1)
1.4 × 10-7

2.3 × 10-8

Slope (mV decade-1
of activity)
19.6± 0.5
19.5± 0.4

Response time (s)
13
10

Life span
5 weeks
6 weeks

pH range
2.0-5.0
1.5-6.5