PASCO CI-6727 User Manual

Instruction Manual and

012-06601A

9/97

Experiment Guide for the
PASCO scientific
Model CI-6727

CALCIUM SELECTIVE ELECTRODE

© 1997 PASCO scientific $7.50

®

10101 Foothills Blvd. • P.O. Box 619011 • Roseville, CA 95678-9011 USA

Phone [phone] • FAX [phone] • web: www.pasco.com

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Calcium Ion Selective Electrode 012–06601A

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012–06601A Calcium Ion Selective Electrode

Table of Contents

Introduction......................................................................................................................................... 1

Theory .................................................................................................................................................. 1

Equipment ........................................................................................................................................... 2

Included ........................................................................................................................................ 2

Additional Required...................................................................................................................... 2

Required Solutions.............................................................................................................................. 2

General Preparation ........................................................................................................................... 3

Electrode Preparation.................................................................................................................... 3

Electrode Slope Check Using Science Workshop .......................................................................... 3

Measurement....................................................................................................................................... 4

Measuring Hints............................................................................................................................ 4

Sample Requirements ................................................................................................................... 4

Units of Measurement................................................................................................................ 4-5

Measurement Procedure .................................................................................................................... 5

Direct Measurement...................................................................................................................... 5

Low Level Calcium Determination .............................................................................................. 6

Titration ............................................................................................................................................... 7

Titration of Calcium................................................................................................................... 7-8

Electrode Characteristics ................................................................................................................... 9

Reproducibility ............................................................................................................................. 9

Interferences.................................................................................................................................. 9

Complexation .............................................................................................................................. 10

Temperature Influences............................................................................................................... 10

Electrode Response ..................................................................................................................... 11

Limits of Detection ..................................................................................................................... 11

pH Effects ................................................................................................................................... 11

Electrode Life ............................................................................................................................. 11

Electrode Storage ........................................................................................................................ 11

Troubleshooting Guide ..................................................................................................................... 12

Glassware/Plastic-ware ............................................................................................................... 12

Electrode ..................................................................................................................................... 12

Standards & Reagents ................................................................................................................. 12

Sample ........................................................................................................................................ 12

Technique .................................................................................................................................... 13

Troubleshooting Hints ...................................................................................................................... 13

Technical Support ............................................................................................................................. 15

i

Copyright, Warranty , and Equipment Return

PleaseFeel free to duplicate this manual
subject to the copyright restrictions below.

Copyright Notice

The PASCO scientific 012-06601A manual is
copyrighted and all rights reserved. However,
permission is granted to non-profit educational
institutions for reproduction of any part of the
Calcium Ion Selective Electrode manual providing
the reproductions are used only for their laboratories
and are not sold for profit. Reproduction under any
other circumstances, without the written consent of
PASCO scientific, is prohibited.

Limited Warranty

PASCO scientific warrants the product to be free from
defects in materials and workmanship for a period of
one year from the date of shipment to the customer.
PASCO will repair or replace at its option any part of
the product which is deemed to be defective in
material or workmanship. The warranty does not
cover damage to the product caused by abuse or
improper use. Determination of whether a product
failure is the result of a manufacturing defect or
improper use by the customer shall be made solely by
PASCO scientific. Responsibility for the return of
equipment for warranty repair belongs to the
customer. Equipment must be properly packed to
prevent damage and shipped postage or freight
prepaid. (Damage caused by improper packing of the
equipment for return shipment will not be covered by
the warranty.) Shipping costs for returning the
equipment after repair will be paid by PASCO
scientific.

Credits

Author: Peter Boyle
Editor: Steve Miller

Equipment Return

Should the product have to be returned to PASCO
scientific for any reason, notify PASCO scientific by
letter, phone, or fax BEFORE returning the product.
Upon notification, the return authorization and
shipping instructions will be promptly issued.

ä

NOTE: NO EQUIPMENT WILL BE

ACCEPTED FOR RETURN WITHOUT AN
AUTHORIZATION FROM PASCO.

When returning equipment for repair, the units must
be packed properly. Carriers will not accept
responsibility for damage caused by improper
packing. To be certain the unit will not be damaged in
shipment, observe the following rules:

➀ The packing carton must be strong enough for the

item shipped.

➁ Make certain there are at least two inches of packing

material between any point on the apparatus and the
inside walls of the carton.

➂ Make certain that the packing material cannot shift in

the box or become compressed, allowing the
instrument come in contact with the packing carton.

Address: PASCO scientific

10101 Foothills Blvd.
P.O. Box 619011
Roseville, CA 95678-9011

Phone: [phone]

FAX: [phone]

email: [e-mail]

web: www.pasco.com

ii

012–06601A Calcium Ion Selective Electrode

E=E

X

C

f

C

t

C

X

γ

C

f

x

C

x

Z

x

γ

Introduction

The PASCO Calcium Ion Selective Electrode is used to quickly, simply, accurately, and economically measure
calcium concentration in aqueous solutions.

Theory

The Calcium Ion Selective Electrode consists of an electrode body containing an ion exchanger in a sensing
module. This sensing module contains a liquid internal filling solution in contact with a gelled organophilic
membrane containing a calcium ion selective exchanger .

An electrode potential develops across the membrane. When the membrane is in contact with a calcium
solution. Measurement of this potential against a constant reference potential, using an ISE Amplifier and
Science Workshop interface, depends on the level of free calcium ion in solution. The level of calcium ions,
corresponding to the measured potential, is described by the Nernst equation:

+ S log

0

where:

E = measured electrode potential
E

= reference potential (a constant)

0

S = electrode slope (~26 mv/decade)
X = level of calcium ions in solution

The activity, X, represents the effective concentration of the calcium ions in the solution. Total calcium
concentration, C
electrodes only respond to free ion, the free ion concentration is:

The activity is related to the free ion concentration, C

includes free calcium ions, Cf plus bound or complexed calcium ions, Cb, since the calcium

t

=

–

b

, by the activity coefficient, γ, by:

f

=

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

121

I =

where:

= concentration of ion X

= charge on ion X

= sum of all of the types of ions in the solution

Σ

In the case of high and constant ionic strength, relative to the sensed ion concentration, the activity coefficient,
is constant and the activity, X, is directly proportional to the concentration.

To adjust the background ionic strength to a high and constant value, ionic strength adjuster (ISA) is added to
samples and standards. The recommended ISA solution for calcium is potassium chloride, KCl. Solutions other
than this may be used as ionic strength adjusters as long as ions that they contain do not interfere with the electrodes
response to calcium ions.

Σ CxZ

2

2

,

1

Calcium Ion Selective Electrode 012–06601A

The reference electrode must also be considered. When two solutions of different composition are brought into contact
with one another, liquid junction potentials arise. Millivolt potentials occur from the inter-diffusion of ions into 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 junction potential will appear as an error in the measured electrode potential.

The composition of the liquid junction, filling solution in the reference electrode is most important. The speed with
which the positive and negative ions in the filling solution diffuse into the sample should be equiltransferent. No
junction potential can result if the rate at which positive and negative charge carried into the sample is equal.

Equipment

Included:

 Calcium Ion Selective

Electrode

 Calcium Ion Selective

Electrode fill solution

 pipette for fill solution

Additional Required:

Required Equipment

 PASCO CI-6738 ISE (Ion

Selective Electrode) Amplifier

 Science Workshop 2.25 or

higher

to ISE
Amplifier

filling pipette

Figure 1

Included Equipment

Calcium Ion Selective Electrode

filling
solution

 PASCO Science Workshop Computer Interface
 Semi-logarithmic 4-cycle graph paper for preparing calibration

curves (Linear graph paper is recommended for low level

measurements)
 magnetic stir plate
 Lab-ware made of plastic, not glass, for all low level

measurements.

Required Solutions

The stock solutions listed in this section may be created as described
in the text or ordered directly form PASCO. The solutions available
for order and their respective prices are listed on the ISE Working
Solution Price List.

 Deionized or distilled water for solution and standard

preparation.
 Calcium Chloride solution 0.1M CaCl

To prepare this solution, half fill a one liter volumetric flask

with distilled water and add 14.7 grams of reagent-grade

calcium chloride (CaCl

.2H20). Swirl the flask gently to

2

dissolve the solid. Fill to the mark with distilled water, cap,

and upend several times to mix the solution.
 Calcium Chloride solution 1000 ppm Ca

To prepare this solution, half fill a one liter volumetric flask

with distilled water and add 3.67 grams of reagent-grade

R

O

E

O

L

1

P

0

1

M

3

A

S

R

n

e

f

e

tio

r

e

u

n

l

c

o

e

S

Fi

l

l

4

I

M

C

K

calcium chloride (CaCl

.2H20). Swirl the flask gently to

2

dissolve the solid. Fill to the mark with distilled water, cap,

and upend several times to mix the solution.
 Calcium Chloride solution 100 ppm Ca

To prepare this solution, half fill a one liter volumetric flask

with distilled water and add 0.15 grams of reagent-grade

calcium chloride (CaCl

.2H20). Swirl the flask gently to

2

dissolve the solid. Fill to the mark with distilled water, cap,

and upend several times to mix the solution.

2

+2

+2

as CaCO

3

2