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EUTECH INSTRUMENTS CADMIUM EPOXY Скачать руководство пользователя страница 14

Cadmium Electrode

Instruction Manual

14

interference from hydroxide is given by a dashed line to the left of the dotted area on Figure 4.
Within this dotted area, cadmium combines with hydroxide to form Cd(OH)

2

. Since only free

cadmium concentration can be measured with the cadmium ion electrodes, a false reading results.

Electrode Life

A cadmium electrode will last six months in normal laboratory use. On-line measurements might
shorten operational lifetime to several months. In time, the response time will increase and the
calibration slope decreases to the point calibration is difficult and electrode replacement is required.

Electrode Storage

The cadmium electrode may be stored for short periods of time in 1.0x10

-2

M cadmium solution. For

longer storage (longer than two weeks), rinse and dry the sensing pellet and cover the membrane tip
with any protective cap shipped with the electrode. The reference portion of the combination
electrode (or the outer chamber of the reference electrode) should be drained of filling solution, if
refillable, and the rubber insert placed over the filling hole.

ELECTRODE THEORY

Electrode Operation

The electrode consists of sulfides of cadmium and other metals bonded into an epoxy or glass body.
When an electrode potential develops across the membrane, the membrane is in contact with a
solution containing cadmium ions. The electrode potential is measured against a constant reference
potential, using a standard pH/mV meter. The level of cadmium ions, corresponding to the
measured potential, is described by the Nernst equation:

E = Eo + S log X

where: E = measured electrode potential

Eo = reference potential (a constant)

S = electrode slope (

∼

27 mV/decade)

X = level of cadmium ions in solution

The activity, X, represents the effective concentration of free cadmium ions in solution. Both
bound, Cb, and free, Cf, cadmium ions are included in the total cadmium ion concentration, Ct. The
cadmium ion electrode will only respond to free cadmium ions, the concentration of which is:

Cf = Ct + Cb

The activity, X, represents the effective concentration of the ions in solution. The activity is related
to the free cadmium ion by the activity coefficient,

γ

, by:

X =

γ

Cf

Activity coefficients vary, depending on total ionic strength, I, defined as:

I = ½

Σ

CxZx

2

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Содержание CADMIUM EPOXY

Страница 1: ...Instruction Manual Cadmium Electrode 1 EUTECH INSTRUMENTS PTE LTD INSTRUCTION MANUAL CADMIUM ION ELECTRODE ...

Страница 2: ...6 Direct Measurement of Cadmium using an ion meter 7 Low Level Cadmium Determination using a pH mV meter 8 Low Level Cadmium Determination using an ion meter 9 Titration 9 Titration of Cadmium 10 ELECTRODE CHARACTERISTICS 11 Reproducibility 11 Interferences 11 Precipitation and Complexation 11 Temperature Influences 12 Electrode Response 12 Limits of Detection 13 pH Effects 13 Electrode Life 14 El...

Страница 3: ...d add 425 grams reagent grade sodium nitrate NaNO3 Swirl the flask gently to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution 3 Cadmium Nitrate Standard Solution 0 1M To prepare this solution from your own laboratory stock half fill a one liter volumetric flask with distilled water and add 30 85 grams of reagent grade Cd NO3 2 4H2O Swirl the ...

Страница 4: ...d to the beaker When the reading has stabilized record the mV reading 3 Using a pipet add 10 ml of the standard used above to the beaker When the reading has stabilized record the mV reading 4 Determine the difference between the two readings The electrode is operating correctly if the mV potential has changed 27 2 mV assuming the temperature is between 20o and 25o C See the TROUBLESHOOTING sectio...

Страница 5: ...stilled water and blot dry Use a clean dry tissue to prevent cross contamination For samples with high ionic strength prepare standards whose composition is similar to the sample Use fresh standards for calibration Use 2 ml of ISA for each 100 ml of sample or standard Always check to see that the membrane is free from air bubbles after immersion into the standard or sample Dilute concentrated samp...

Страница 6: ...pare standards with a composition similar to the samples if the samples have an ionic strength above 0 1M 2 Place 100 ml of the most dilute solution 1 0X10 4 M or 10 ppm in a 150 ml beaker on the magnetic stirrer and begin stirring at a constant rate After assuring that the meter is in the mV mode lower the electrode tips into the solution After the reading has stabilized record the mV reading 3 P...

Страница 7: ... After the reading has stabilized record the mV reading Using the calibration curve determine the sample concentration 7 The calibration should be checked every two hours Assuming no change in ambient temperature immerse the electrode tips in the mid range standard After the reading has stabilized compare it to the original reading recorded in Step 3 above A reading differing by more than 0 5 mV o...

Страница 8: ...change in ambient temperature will necessitate the repetition of Steps 2 6 above The meter should be re calibrated daily Low Level Cadmium Determination using a pH mV meter This procedure is recommended for solutions with cadmium concentrations of less than 1 0X10 5 M 1 1 ppm If the solution is high in ionic strength but low in cadmium ion use the same procedure but prepare a calibration solution ...

Страница 9: ... low level calibration curve daily Check the calibration curve every two hours by repeating Steps 3 7 above Low Level Cadmium Determination using an ion meter 1 Using 20 ml of standard ISA dilute to 100 ml with distilled water This low level ISA 1 0M NaNO3 is added at the rate of 1ml low level ISA to each 100 ml of solution The background ionic strength will be 1 0X10 2 M 2 Follow the steps given ...

Страница 10: ...le into a 150 ml beaker place the beaker on the magnetic stirrer and begin stirring at a constant rate Adjust the sample to pH 10 by adding ammonia 3 Position the burette tip in the beaker slightly above the liquid level in the beaker and slightly off center Position the electrode tips in the solution about halfway between the center of the beaker and the beaker wall 4 Begin adding the EDTA in 0 5...

Страница 11: ...lishing procedure The cadmium ion electrodes do not respond to anions or most cations The electrode membrane is poisoned by solutions containing copper mercury and silver These ions must be absent from the solution If the level of ferric ion or lead ion exceeds the level of cadmium ion in the sample the measurement will be affected Sodium fluoride added to the sample will eliminate ferric ion Prec...

Страница 12: ...lope Temp oC S 0 27 10 10 28 10 20 29 08 25 29 58 30 30 07 40 31 07 50 32 06 If changes in temperature occur the electrodes should be re calibrated The temperature range for the Eutech Cadmium Ion Electrode is 0o 80o C provided that temperature equilibrium has occurred If the temperature varies substantially from room temperature equilibrium times up to one hour are recommended Electrode Response ...

Страница 13: ...ns the upper limit of detection is above 1 0x10 2 M cadmium but the possibility of a liquid junction potential developing at the reference electrode and the salt extraction effect are two limiting factors Some salts may infuse into the electrode membrane at high salt concentrations causing deviation from the theoretical response Either dilute samples between 0 1M and 1 0x10 2 M or calibrate the el...

Страница 14: ...ution if refillable and the rubber insert placed over the filling hole ELECTRODE THEORY Electrode Operation The electrode consists of sulfides of cadmium and other metals bonded into an epoxy or glass body When an electrode potential develops across the membrane the membrane is in contact with a solution containing cadmium ions The electrode potential is measured against a constant reference poten...

Страница 15: ... of ions in the two solutions Electrode charge will be carried unequally across the solution boundary resulting in a potential difference between the two solutions since ions diffuse at different rates When making measurements it is important to remember that this potential be the same when the reference is in the standardizing solution as well as in the sample solution or the change in liquid jun...

Страница 16: ...ed substitute another cadmium ion electrode that is known to be in good working order for the questionable electrode If the problem persists and you are using an electrode pair try the same routine with a working reference electrode 4 If the problem persists the reagent may be of poor quality interferences in the sample may be present or the technique may be faulty See Standards and Reagents Sampl...

Страница 17: ...the samples prior to testing to eliminate a problem before it starts See Measuring Hints Sample Requirements and Interferences Technique Be sure that the electrode s limit of detection has not been exceeded Be sure that the analysis method is clearly understood and is compatible with the sample Refer to the instruction manual again Reread sections GENERAL PREPARATION and ELECTRODE CHARACTERISTICS ...

Страница 18: ...ctrodes in solution in solution Noisy or Unstable defective meter check meter with Readings readings shorting strap continuously or rapidly changing air bubble on remove bubble by membrane re dipping electrode electrode exposed soak electrode in to interferences cadmium standard defective electrode replace electrode ISA not used use recommended ISA meter or stirrer ground meter or not grounded sti...

Страница 19: ... bubble on remove bubble by membrane redipping probe Incorrect Answer incorrect scaling plot millivolts on but calibration of semi log paper the linear axis On curve is good the log axis be sure concentration numbers within each decade are increasing with increasing concentration incorrect sign be sure to note sign of millivolt number correctly incorrect standards prepare fresh standards wrong uni...

Страница 20: ...tance 1 Mohm Reproducibility 2 Samples Aqueous solutions only no organic solvents Size 110 mm length 12 mm diameter 1 m cable length Storage Store in cadmium solution ORDERING INFORMATION CODE NO DESCRIPTION EC CD 03 Cadmium Ion Combination Electrode epoxy body EC ISA CD1 BT Ionic Strength Adjuster ISA 5M NaNO3 EC MIS PP Polishing Paper for the Cadmium Ion Electrodes ...

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