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Analysis methods by pH and Ion Selective Electrodes


A comparative view of analysis by pH and Ion Selective Electrodes. Types of ISE electrodes. Measurement techniques. Standard addition methods. Molarity to ppm for electrode species. Electrode filling solutions and Internal Stabilization Adjusters (ISA). Preparation and use of TISAB.

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pH Combination Electrodes

Analytical aspects of the pH and ise

Analysis by Electrodes

Ion selective electrodes are a technology that gives direct measurement of cations and anions such as sodium and fluoride, and can also measure dissolved gases such as ammonia and sulphur dioxide. Time consuming steps such as filtrations, weighings and distillation are not required in most cases. Concentrations can be read out directly on a pH/MV/ION meter, or can be read from a constructed calibration curve.
Electrode methods save time, and since electrodes are portable, measurements can be made on a laboratory bench, the bank of a river or pond, the floor of a manufacturing plant, or the tray of a truck.

About pH and ISE

The pH electrode measures only the hydronium ion [H3O+] concentration in a solution, whereas the ion selective electrode (ISE) measures the ionic concentration of an electrode species.
A pH electrode will measure the alkalinity or acidity of a solution in the range of pH 0 - 14. Ph 1 - 7 is acidic and pH 7 - 14 is basic. However, pH 7 is neutral, since it is neither acidic nor basic.
A Fluoride ISE will measure the concentration of F- ions in solution and a sodium ISE will measure the Na+ ions in solution.
pH Glass membrane
Composition : 22% Na2O, 6% CaO and 72% SiO2 - highly selective to hydrogen ions, the pHelectrode.
In ISE, the silicon dioxide glass matrix is doped with a specific chemical e.g. sodium to make the sodium ion specific electrode.
Reference electrode
Silver/siver chloride or calomel pH reference electrodes are used only for pH measurements. For ion selective electrodes a special single or double junction reference electrode must be used. A pH reference electrode cannot be used as a reference electrode for an ISE determination.
Combination electrode
The C30F lab now use the combination type electrodes instead of the half cell types for both pH and ISE determinations. It is convenient, can be used with small volumes, and can be easilly moved in and out of measuring solutions.
In a pH combination electrode the reference is housed in the same cylindrical body as the sensor head. This produces a simple compact unit for making measurements.
In a specific ion combination electrode, a sleeve type reference electrode and a solid-state sensing element is incorporated into a single 13 mm diameter electrode body.

Types of ion selective electrodes

An ion selective electrode measures the potential of a specifc ion in solution against a stable reference electrode of constant potential. The potential difference depends upon the activity of the specific ion in solution. The activity is related to the concentration of that specific ion, hence an alytical measurement can be made.
The most important feature of an ion selective electrode is the selective membrane that it is made off. There are two main types of membrane material :
Solid crystal matrix - a single crystal or a polycrystalline compressed pelet.
Ion carrier - a complex organic molecule impregnated into a plastic, PVC. or rubber film which acts as an ion carrier.
  • Glass membrane : the silicon glass matrix is doped with a specific chemical e.g. sodium to make the sodium ion specific electrode.
  • Liquid membrane : consist of various ion exchange materials on an inert, hydrophobic PVC, polyethelene, or silicone rubber. Used in aqueous solutions only. It comes with a ready to use pre-tested sensing module screwed into a lower replaceable body. e.g. the selective ion electrodes of K, Ca, and Nitrate.
  • Solid-state : sensing element is a crystal sealed flush into one end of the electrode body; a La crystal for the Fluoride ISE and a compressed Ag 2S for silver / sulphide electrodes. Made of unbreakable epoxy bodies, resistant to most inorganic reagents, and can be used intermittantly in most common organic solvents such as methanol, acetone and dioxane.
    Must not be used with strong polar solvents such as DMF, or chloroform. Examples of this type of electrodes are silver / sulphide, chloride, fluoride, and copper.
  • Gas sensing : has a gas permeable membrane, and an internal buffer solution whose pH changes as the gas reacts with it, and this change is detected by a combination pH sensor within the electrode housing There is therefore no need for a reference electrode.
    There are special ISE construction designs for ammonia, carbon dioxide, nitrogen dioxide, and oxygen.
  • Reference : mono electrode or half-cells designed specially for specific ion electrode systems. Has sleeve type construction that minimizes liquid junction potential and avoids clogging problems encountered with frit and fiber type electrodes. Made from virtually unbreakable, chemically resistant plastics.
  • Combination : A sleeve type reference electrode and a solid-state sensing element is incorporated into a single electrode body. Flush clean design - reduce time in cleaning and maintenance.

Laboratory practice

Problems
1. If electrodes function properly in buffers and standards but not in sample, it may be adversely affected by substances in the sample (interferences). Consult electrode instruction manual for more info.
2. If you get an "off scale" or "over range" reading on meter, it could be that the electrode is not properly connected to the meter, an abrazed cable, or the electrode is not immersed into the measuring solution.
Molarity to ppm
The molecular weight of a species is equal to the concentration in ppm at 10-3M. For example, Fluoride has a molecular weight of 19. Hence, a 10-3M concentration is equal to 19 ppm. See
Electrode Conversion Factors (Table III).

Storage

Put pH electrodes in pH buffer 7 storage solution when not in use, and put ion selective electrodes in a 0.1M solution of the respective ion of the electrode. Store a fluoride electrode in 0.1M sodium fluoride solution when not in use. Try to avoid storing electrodes in distilled water because the filling solution will be diluted and the electrode response will be slow.
pH sensing electrode
Fill reference chamber with filling solution, cover fill hole, and put protective cap containing a few drops of storage solution over sensing element.
Ion selective electrode
Drain electrode, rinse internal chamber with distilled water and store dry with protective cap covering sensing element.
Sodium electrode
Do not store sodiun ISE in water or air. Keep electrode in 10-2M sodium standard. For long term storage, rinse electrode with sodium standard and store dry with protective cap covering sensing element.

Technique

Rinse electrodes in deionised water between measurements to prevent contamination by carry over on the electrode, but do not wipe the pH glass electrode bulb because transfer of static charge onto the glass bulb will result in slow or drirty response. Also, do not wipe the liquid membrane tip or surface of an ion selective electrode which may cause damage or contamination to the sensing element.
Remove fill hole cover in pH electrodes during calibration and measurement to ensure a uniform flow of electrode filling solution. Fill electrode filling chamber with electrode filling solution. The filling solution level should be about 1 inch above sample heihght in beakar. Cover fill hole after use and during storage.
Use a magnetic stirrer at uniform speed to stirr standards and samples in order to get representative measurement and improved electrode response time.
Temperature
To account for pH slope, buffer and sample changes due to temperature, an ATC probe must be used.
For ion selective electrodes, calibration and measurement should be done at the same temperature, and ISA buffers added to both standards and samples.
Calibration
Calibration verifies electrode slope and proper function of the meter. For pH, calibrate with 2 or 3 buffers or standards that bracket the expected sample range. Use pH buffers 4 and 7 or 9.
ISE standards should be made up in decade series of concentration e.g. 10-2M, 10-3M, 10-4M standards of the electrode species of interest. You should have an estimate of the values for the samples you are analysing in order to prepare a relevant calibration range, which will bracket the sample values.
A reading of about 56 MV per decade concentration is optimal for monovalent ions, and about 28 mv for divalent ions. Calibrate once a day with 2 to 4 standards.
Always use fresh standards and buffers.
pH buffers may absorb CO2 from the air causing shift in pH value.
ISE standards may become contaminated or prepared wrongly.
ISE Matrix effects
Matrix effects are eliminated by adding a few mls. of a recommended ionic strength adjuster (ISA) to both standards and samples before measurement. The ISA "swamps out" differences in sample ionic strength and makes the ionic strength of samples and standards about the same.

ISE Methods

ISE can measure the ion concentration of samples at very high values ( > 1000 ppm) to very low levels ( < 1 ppm)
Direct analysis
Sample electrode potentials (Mv) are compared to that of standards, having added ionic strength adjusters (ISA) to both standards and samples. However the MV/ION Meter must first be calibrated with at least three standards and concentration read from a calibration curve of concentration vs MV. Concentrations can also be read out directly from the meter in any concentration unit such as molarity, ppm, or percentage.
Incremental methods
These are spiking techniques, adding stadard to sample, or sample to standards. These are standard addition or known addition (KA), known subtraction or standard subtraction (KS), sample addition or analate addition (AA), sample subtraction or analate subtraction (AS).
A 1 : 100 ratio of standard to sample ratio is optimal. For a monovalent electrode, the standard addition should result in a 15 - 30 mV change. For a divalent electrode, a 7 - 10 mV change is acceptable.
However, for incremental methods, certain inputs must be given in order to get a direct readout from the meter.
For example, the required inputs for a 50 ml volume sample in know addition is:
Slope =58 mV
Sample volume = 50 ml
Standard volume = 10 ml
ISA volume = 50 ml
Standard concentration = 100 ppm
The meter (Accumet AR-50) will prompt you through the method to enter the appropriate parameters at the appropriate time.
These techniques are summarized in Table I below.

Table I

Incremental Methods

Parameter Known addition Known subtraction Analate addition Analate subtraction
Does electrode directly sense species being analysed Yes Yes Yes No. Sample species pecipitates or complexes standard species
Does electrode sense standard species Yes No. Standard precipitates or complexes sample species Yes Yes
Solution in which initial electrode potential is measured Known volume of sample solution (typically 100 ml) Known volume of standard solution (typically 100 ml) 0.01 x to 0.1 x expected sample concentration
Increment used Known volume of standard solution (typically 1 to 10 ml) 100 x to 10 x expected sample concentration Known volume of sample solution (typically 1 to 10 ml)
Solution in which final electrode potential is measured Sample solution plus standard solution Standard solution plus sample solution


Table II

ISE Filling solutions and Ionic strength adjusters

Electrode Concentration range pH range Filling soln Buffer/ISA
Ammonia 1.0 to 5 x 10-7M
17000 to 0.01 ppm
11 - 13 0.1M NH4Cl
0.5349 g/100 ml
10M NaOH
400 g/l
Calcium 1.0 to 5 x 10-7M
40,100 to 0.02 ppm
6 - 8 0.1M KCl
0.7455 g/100 ml
1M KCl
74.55 g/l
Chloride 1.0 to 5 x 10-5M
35,500 to 1.8 ppm
2 - 11 10% KNO3
10 g/100 ml
5M KNO3
424.97 g/l
Fluoride saturated to 10-6M
saturated to 0.02 ppm
5 - 8 10% KNO3
10 g/100 ml
TISAB
Nitrate 1.0 to 7 x 10-6M
14,000 to 0.01 ppm
3 - 10 0.4M NH4SO4
0.5286 g/100 ml
2M (NH4SO4)2
264.28 g/l
Potassium 1.0 to 10-6M
39,000 to 0.01 ppm
3 - 10 0.1M NaCl
0.5844 g/100 ml
1M NaCl
58.44 g/l
Silver 1.0 to 10-7M
107,900 to 0.01 ppm as Ag+
2 - 9 10% KNO3
10 g/100 ml
5M KNO3
424.97 g/l
Sulphide 1.0 to 10-7M
32,100 to 0.003 ppm as S--
13 - 14 10% KNO3
10 g/100 ml
SAOB
Sodium saturated to 106M
saturated to 0.02 ppm
9 - 10 10% KNO3
10 g/100 ml
1M NH4OH
35 mls/l



SPECIAL ISE BUFFERS

ISA and Filling solutions
Always use the recommended ISA and Filling solutions for a given ion selective electrode. See Table II below.
Volumes of ISA to be used
For Fluoride and silver electrodes: Add 50 ml ISA to 50 ml volume sample and standard.
For Nitrite and Carbon dioxide electrodes, add 10 ml ISA to 100 ml sample and standard.
For all other electrodes: Add 2 ml ISA to 100 ml sample and standard.

TISAB II (Total ionic strength adjuster)
Used for low level F (-), < 0.4 ppm (2 x 10-5M)
Salt required is CDTA:trans 1,2, diaminocyclohexane N,N,N1,N1-tetra acetic acid, also known as cyclohexylene dinitro tetra acetic acid, or cyclohexylenediamine tetra acetic acid):
C6H10{N(CH2CO2H)2}2.xH2O: FW = 364.34
Dissolve 4 g CDTA, together with 57 mls glacial acetic acid and 58 g NaCl in about 500 mls distilled water, and using a pH meter, adjust to between 5 and 5.5 pH by adding 5M NaOH (200 g/l) and dilute to 1 liter with distilled water (about 130 mls NaOH).
TISAB III
Dissolve 300 g sodium citrate.2H2O (FW=294.10), 22 g CDTA, and 60 g NaCl in 1 liter water.
Use 10% buffer per volume standard and sample. (add 5 ml TISAB III to 50 ml volumes of standard and sample).
TISAB IV
Complexes more than 100 ppm iron or aluminium in presence of 1 ppm F(-)
Dissolve 84 ml conc. HCl, 242 g TRIS (hydroxymethyl aminomethane, and 239 g sodium tartrate (FW=230.08), in about 500 mls water, cool, and transfer to 1 liter volumetric flask and make up to mark.
Use a 1 :1 volume to standard and sample.

An alternative preparation for TISAB.

Dissolve 58.44g NaCl, 61.50g sodium acetate, 0.29g sodium citrate and 15 ml acetic acid in 1 liter distilled water.

SAOB (Sulphide Anti-oxidant Buffer.)
(Salt required is ascorbic acid FW = 176.12; 0.2M in 2M NaOH). Dissolve 36 g ascorbic acid and 80 g NaOH in one about 600 mls distilled water, add 67g disodium EDTA, stir, and make up to 1 liter. Discard and make fresh solution when color changes to brown.
The ascorbic acid retards air oxidation of species being measured, makes the solution basic for measurement, and adjusts the ionic strength. Use a 1 : 1 ratio of sample and standard to SAOB.

Table III
Conversion Factors: Molarity and ppm

See also :
ppm conversions

Electrode Species 10-3M
equals
1 ppm
equals
Electrode Species 10-3M
equals
1 ppm
equals
Aluminium 27.0 ppm 3.7 x 10-5 M Fluoride 19.0 ppm 5.2 x 10-5 M
Ammonia 17.0 ppm 5.9 x 10-5 M Hardness, as CaCO3 100 ppm 1.0 x 10-5 M
Ammonia, as N 14.0 ppm 7.1 x 10-5 M Iodide 127 ppm 0.79 x 10-5 M
Ammonium 18.0 ppm 5.6 x 10-5 M Lead 207 ppm 0.48 x 10-5 M
Boron 10.8 ppm 9.3 x 10-5 M Magnesium 24.3 ppm 4.1 x 10-5 M
Bromide 79.9 ppm 1.3 x 10-5 M Mercury 200 ppm 0.50 x 10-5 M
Cadmium 112.0 ppm 0.89 x 10-5 M Nickel 58.7 ppm 1.7 x 10-5 M
Calcium 40.1 ppm 2.5 x 10-5 M Nitrate 62.0 ppm 1.6 x 10-5 M
Calcium, as CaCO3 M 100 ppm 1.0 x 10-5 M Nitrate, as N 14.0 ppm 7.1 x 10-5 M
Carbon dioxide 44 ppm 2.3 x 10-5 M Perchlorate 99.5 ppm 1.0 x 10-5 M
Carbonate, as CaCO3 100 ppm 2.8 x 10-5 M Potassium 39.1 ppm 2.6 x 10-5 M
Chloride 35.5 ppm 2.8 x 10-5 M Phosphorus, as P2O5 70.9 ppm 1.4 x 10-5 M
Chlorine 70.9 ppm 1.4 x 10-5 M Silver 107.9 ppm 0.93 x 10-5 M
Chromium 52.0 ppm 1.9 x 10-5 M Sodium 23.0 ppm 4.4 x 10-5 M
Chromium, as CrO4= 116 ppm 0.86 x 10-5 M Sulphate 96.1 ppm 1.0 x 10-5
Cobalt 58.9 ppm 1.7 x 10-5 M Sulphide 32.1 ppm 3.1 x 10-5 M
Copper 63.5 ppm 1.6 x 10-5 M Sulphur dioxide 64 ppm 1.6 x 10-5 M
Cyanide 26.0 ppm 3.8 x 10-5 M Zinc 65.4 ppm 1.5 x 10-5 M

References:
1. Orion Research "Analytical methods guide" (1977)
2. Denver Instruments Co. "ISE Filling solutions and ISA"
3. Moody GJ and Thomas JDR (1971) Selective Ion Sensitive Electrodes, Merrow (Bath, UK)

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Signature: Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
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