Cat. No.

51935-88

Gel-filled pH Electrode
Models 51935-00, 51935-11, 51935-22

© Hach Company, 1999-2005. All rights reserved. Printed in China te/dk 03/03 2ed
2
Table of Contents
Specifications..................................................................................5
Safety Precautions..........................................................................7

Section 1 Introduction ..............................................................9

1.1 Electrode Description..................................................................9
1.2 Electrolyte Description ............................................................. 10
1.3 Conditioning the Electrode ...................................................... 10
1.4 Measuring Hints ........................................................................ 11
1.5 Checking the Slope ...................................................................12
1.6 General Applications ................................................................12

Section 2 Applications ..........................................................13

pH, Water and Wastewater .............................................................15

Section 3 Electrode Maintenance........................................21

3.1 Storing the Electrode ................................................................ 21
3.2 Cleaning the Electrode ............................................................. 21

Section 4 pH Electrode Characteristics .............................23

4.1 Theory of Operation..................................................................23
4.2 Sodium Error Interferences......................................................24

Section 5 Troubleshooting.....................................................25
Electrode Service Request Questionnaire ..................................29
Glossary .........................................................................................31

General Information ................................................................35

How To Order................................................................................37
Repair Service ...............................................................................38
Warranty........................................................................................39

3
4
Specifications
Specifications are subject to change without notice.
Electrode Type
pH combination with temperature probe
Range
0-14 pH units
Isopotential Point
7.00 ±0.5 pH units (0 ±29 mV)
Electrode Resistance
250 Mohms at 25 °C (new)
Slope
-58 ±3 mV at 25 °C
Temperature Range
Routine Use—0 to 45 °C (32 to 113 °F)
Occasional Use—0 to 100 °C (32 to 212 °F)
Storage
-40 to 50 °C (-40 to 122 °F)
Reference Half Cell
Ag/AgCl
Dimensions
Tip Diameter—12 mm (0.472 inches)
Total Length—152.4 mm (6 inches)
Cable Length—0.91 m (36 inches)
Cable Connector
• sension 5-pin Connector (Cat. No. 51935-00)
• Hach One® Meter Series BNC & 3.5 mm Phone Connector
(Cat. No. 51935-11)
• EC Series BNC & DIN Connector (Cat. No. 51935-22)

5
6
Safety Precautions
Please read this entire manual before unpacking, setting up, or
operating this instrument. Pay particular attention to all danger
and caution statements. Failure to do so could result in serious
injury to the operator or damage to the equipment.
To ensure the protection provided by this equipment is not
impaired, do not use or install this equipment in any manner
other than that which is specified in this manual.

Use of Hazard Information

If multiple hazards exist, this manual will use the signal word
(Danger, Caution, Note) corresponding to the greatest hazard.
DANGER
Indicates a potentially or imminently hazardous situation
which, if not avoided, could result in death or serious injury.
CAUTION
Indicates a potentially hazardous situation that may result in
minor or moderate injury.
NOTE
Information that requires special emphasis.

Precautionary Labels
Read all labels and tags attached to the instrument. Personal
injury or damage to the instrument could occur if not observed.

This symbol, if noted on the instrument, references the

instruction manual for operational and/or safety information.

7
8
Section 1 Introduction

1.1 Electrode Description

The sension™ Gel-filled pH Electrode (Figure 1) is a reliable
electrode designed for general purpose pH measurement.

Figure 1 sension Gel-filled pH Electrode

Gel-filled pH Gel-filled pH Gel-filled pH

Electrode with Electrode with Electrode with
temperature probe, temperature probe, temperature probe,
5-pin connector for BNC & 3.5 mm BNC & DIN
use with sension™ connector for use connector for use
pH meters with Hach One® with EC series
(Cat. No. 51935-00) pH meters pH meters
(Cat. No. 51935-11) (Cat. No. 51935-22)

9
Introduction, continued
The sension Gel-filled pH Electrode uses a silver/silver chloride
reference element with a glass frit junction and a full-range pH
glass that can be used in a wide variety of applications,
especially at extreme levels of pH. If using the sension System,
the meter will automatically account for buffer temperatures
during calibration and factor the information into the reported
pH. The Auto Buffer Recognition/Auto Temperature
Compensation calibration program built into sension meters
contains accurate pH profiles for buffers of pH 4.01, 6.86, 7.00,
and 10.01 at temperatures from 0 to 60 °C.

1.2 Electrolyte Description

The reference electrolyte in this non-refillable pH electrode is a
low viscosity gel that slowly diffuses through the porous
reference junctions (Figure 2). The gel is saturated with
potassium chloride (KCl) and silver chloride (AgCl2). Additional
KCl crystals are added to the electrode during manufacture to
offset losses through diffusion and maintain KCl saturation
during the lifetime of the electrode.

Figure 2 Electrode End View

pH Bulb

Reference

END VIEW

Temperature
Sensor

1.3 Conditioning the Electrode

When a pH bulb is immersed in an aqueous solution, a hydrated
layer slowly forms at the glass/liquid interface. The formation
characteristics of this layer depend upon the type of glass, the
age of the glass, the previous usage history of the glass, the

10
 Introduction, continued
temperature and ionic strength of the aqueous solution, etc.
This hydrated layer affects the sensing properties of the bulb
(charge transfer, and ion transport). A dry bulb will not
function.
To ensure the full development of the hydrated layer, electrodes
should be conditioned for several minutes in a solution
comparable to the sample in terms of pH and ionic strength.
Normal Conditioning
(For routine measurements in samples of moderate to high
conductivity, approximately 150 µS and greater):

Initial use: During shipping, the electrode is kept hydrated by

a cotton/wool ball saturated in a pH 4.00 buffer/KCl solution.
The electrode and saturated ball are encased in a protective
vinyl cap. Before using the electrode, remove the cap and soak
the electrode tip in a pH 4.0 or pH 7.0 buffer. If the cotton ball
is saturated, the electrode will be hydrated after several minutes
in the NIST standard. If the cotton ball and electrode are dry,
the bulb typically requires soaking in the standard for 30
minutes before becoming hydrated.

Between uses: Between sample measurements of medium to

high conductivity, store the electrode in Hach Electrode Storage
Solution, a pH 6.35/1.7 M KCl buffer solution. The solution
keeps the bulb hydrated and prevents the reference gel from
solidifying in the reference junction. Hach electrode storage
solution is available in powder pillow form for dilution with
deionized water or as a prepared solution.

1.4 Measuring Hints

These suggestions will improve the accuracy of your
calibration and sample measurement.
• Allow the potential to stabilize completely
(<1 mV/minute drift) before accepting a calibration point
or sample reading. Meters set to higher resolutions will take
longer to stabilize than meters set to lower resolutions.

11
Introduction, continued
• Use calibration standards and samples that are at the same
temperature to improve accuracy.
• Use fresh calibration standards to achieve greater accuracy.
• Thoroughly rinse and blot the electrode dry between
sample measurements or rinse with small volumes of
sample or deionized water.
• Measure samples within a short period of time after
collecting. Weakly buffered alkaline solutions can absorb
carbon dioxide causing downward drift of pH readings.
Cover samples if necessary.
• A multi-point calibration will ensure more accurate
measurements than a single point calibration.

1.5 Checking the Slope

After calibrating the electrode with fresh buffer, check the slope
using the sension meter’s calibration review option. A slope of
58 ±3 mV indicates a properly functioning electrode.

1.6 General Applications

Section 2 describes a general application for the Gel-filled
pH Electrode. Specific applications include:
• Water and wastewater treatment to monitor influent and
effluent quality.
• Agricultural applications including soil, fertilizer, food, and
feed.
• Chemical and biological research.
• Industrial processes in the manufacture of foods,
beverages, pharmaceuticals, dyes, photographic film,
plating, and chemicals.
• Continuous sampling in aqueous samples up to 45 °C and
intermittently in aqueous samples up to 100 °C.
For information regarding applications not described in this
manual, contact Hach Technical and Customer Service.

12
Section 2 Applications

(sension2, 3, & 4 meters)

13
14
pH, Water and Wastewater (above 150 µS/cm)

pH mV setup

1. Connect the 2. Turn on the meter 3. Press SETUP. Press

electrode to the by pressing I/O. Press the up arrow three
sension2 pH/ISE PH MV until the times. Press ENTER to
meter. display shows pH. move to the desired
Note: Ensure that the number of decimal
electrode has been places, then EXIT to
conditioned according leave setup.
to instructions in
Section 1.3 on page 10.

15
pH, Water and Wastewater, continued

cal
0

4. In two 50-mL 5. Press CAL. The 6. Rinse electrode in

beakers or cups, display will show: deionized water and
prepare buffers of 4.0 Standard 1?. blot dry.
and 7.00 pH or 7.00
and 10.0 pH.
Note: The sample pH
should fall within the
range of the calibration
buffers.

Note: pH 6.86 buffer

may be used instead of
pH 7.0. Autobuffer
recognition for either pH
6.86 or 7.0 is user
selectable in the
sension meter setup
function.

16
 pH, Water and Wastewater, continued

READ READ
enter enter

7. Place the electrode 8. When a stable pH is 9. Place the electrode

in pH 7.0 buffer. Press determined, the in the 4.0 (or 10.0) pH
ENTER. The display display will show buffer. Press ENTER.
shows: Standard 2?. Remove
Stabilizing... the electrode from the
cup. Rinse and blot
Note: Temperature
variation causes
dry.
changes in buffer pH. Note: The meter will
Hach sension meters select a stable reading
correct for these using default meter
changes during parameters and the
calibration when using specified resolution. To
buffers of pH 4,6.86, 7, override the default
and 10. meter parameters, see
the meter manual.

READ
enter

OR

10. After the last 11. The display will 12. Rinse the
calibration point has show Store?. Press electrode with
stabilized and the ENTER to store the deionized water
display reads calibration or EXIT to followed by a small
Standard 3?, press leave the calibration amount of sample and
EXIT. mode without storing blot dry.
Note: For a three-point the values.
calibration, repeat steps
9-10 with an additional
buffer.

17
pH, Water and Wastewater, continued

OR

13. Place the 14. Store or record the

electrode in the pH and temperature
sample. readings when they
stabilize.

Sampling and Storage

Collect samples in clean plastic or glass bottles. Fill completely
and cap tightly. Cool to 4 °C and determine pH within six hours.
If samples cannot be analyzed within six hours, report the
actual holding time with the results.

Accuracy Check
Checking Electrode Response
An electrode is responding properly if its calibration slope is
58±3 mV per pH unit.
Checking Calibration Accuracy
Returning the electrode to a calibration buffer and determining
the pH provides a good test of the system. Rinse and
recondition the electrode before measuring subsequent
samples.
Checking the Precision of the Sample Reading
When practical, measure the pH of the sample several times and
report the average pH. sension2 meters will store and calculate
the average of up to 99 readings.

18
 pH, Water and Wastewater, continued
Method Performance
Precision
In a single lab using an ASTM Substitute Wastewater Standard,
specification number D5905-96, and two electrodes with a
single sension2, a single operator obtained a standard
deviation of 0.005 pH. Each electrode was exposed to seven test
solutions with no rinsing in between and with a default
stability of 0.5 mV/min.

Interferences
Sodium error may occur in samples with pH 12 or higher. See
Section 4.2 on page 24.

Summary of Method
Water with relatively high conductivity typically has a fairly
high buffer capacity. Slight pH changes due to absorption of
carbon dioxide are usually not significant. If the sample
conductivity is not known and high accuracy is desired, use the
Platinum Series pH Electrode and follow either the Low Ionic
Strength or high purity method.
The sension Gel-filled pH Electrode responds to the hydrogen
ion concentration (activity) by developing an electrical
potential. At a constant temperature, this potential varies
linearly with the pH of the solution being measured.

19
pH, Water and Wastewater, continued
REQUIRED REAGENTS AND APPARATUS
Quantity Required
Description Per Test Unit Cat. No.
Gel-filled pH Elect. w/Temp., 5-pin ..1 ..............each ...51935-00
Gel-filled pH Elect. w/Temp., BNC & 3.5mm1 .each ... 51935-11
Gel-filled pH Elect. w/Temp., BNC & DIN1.......each ...51935-22
sension2 Portable pH/ISE Meter/w/elect.1 ......each ...51725-10
sension1 Portable pH/mV Meter/w/elect.1......each ... 51700-11

OPTIONAL REAGENTS
Buffer, Powder Pillow
pH 4.01, color-coded red............................. 15/pkg...... 22269-95
pH 6.86 .......................................................... 15/pkg...... 14098-95
pH 7.00, color-coded yellow....................... 15/pkg.......22270-95
pH 10.00, color-coded blue......................... 15/pkg.......22271-95
Singlet pH Solution, pH 7 and 10 ..... 10 each/pkg...... 27698-20
Singlet pH Solution, pH 4 and 7....... 10 each/pkg...... 27699-20
Singlet pH Solution, pH 4 ........................... 20/pkg.......27700-20
Singlet pH Solution, pH 7 ........................... 20/pkg.......27701-20
Singlet pH Solution, pH 10 ......................... 20/pkg.......27702-20
Singlet pH Solution,Rinse Solution ........... 20/pkg.......27703-20
Buffer Solution, pH 4 (red) ........................ 500 mL...... 22834-49
Buffer Solution, pH 7 (yellow) .................. 500 mL ...... 22835-49
Buffer Solution, pH 10 (blue)..................... 500 mL ...... 22836-49
pH Storage Solution Powder Pillows......... 20/pkg...... 26573-64
Water, deionized..................................................4 L...........272-56

OPTIONAL APPARATUS
Sample Bottles
General Purpose with Screw-cap,
polypropylene, 500-mL................................ each.......27581-10
Cleaned and Certified, HDPE,
for EPA reporting, 500-mL .......................... each.......27582-10

20
Section 3 Electrode Maintenance
The sension™ Gel-filled pH Electrode has been designed to give
trouble-free use, but requires careful handling to extend
longevity. This section presents methods of storing and
cleaning the electrode.

3.1 Storing the Electrode

Proper electrode storage requires different approaches based on
how long the electrode will be stored, how quickly the electrode
needs to be used, and the type of sample being measured.
Storage affects the reference electrolyte gel, the Ag/AgCl
reference element, and the pH sensing glass bulb.

Intermittent storage: Between uses, store the electrode in

solutions of similar ionic strength and pH to the samples of
interest. Carefully rinse the electrode to prevent sample
contamination.

Overnight storage: Store the electrode in Hach Electrode

Storage Solution to keep the electrode hydrated.

Shelf Storage: For very long-term storage, store the electrode

dry. For faster reconditioning in the future, wet the cotton/wool
ball located in the vinyl storage cap with storage solution.
Place the cap over the electrode. This will help keep the bulb
hydrated.

3.2 Cleaning the Electrode

A contaminated glass bulb or fouled electrode may cause slow
response times. Clean the electrode only after following the
steps described in Section 5 Troubleshooting or when the
electrode is known to be dirty. Do not clean the bulb more
frequently than necessary, or bulb life may shorten. Clean the
electrode according to the type of contaminant present:
General contamination—Immerse the electrode tip in 0.1 N
Hydrochloric Acid followed by immersion in 0.1 N Sodium
Hydroxide and again in 0.1 N Hydrochloric Acid, each for a

21
Electrode Maintenance, continued
2-minute period. Rinse with deionized water. Soak in deionized
water for at least 15 minutes.
Oils and fats—Immerse the electrode tip in a detergent solution
such as Alconox™. Use a soft brush or ultrasonic bath if
necessary. Avoid scratching the glass bulb.
Organic films—Use an appropriate solvent, such as methanol or
acetone.
Note: Keep the cable and connector away from dirt, abrasives, and harsh
solvents.

If these steps fail to improve electrode response, complete the

Electrode Service Request Questionnaire on page 29 and
contact Technical Support.

22
Section 4 pH Electrode
Characteristics
4.1 Theory of Operation
pH is a measure of the hydrogen ion activity in a solution and
is defined as: -log10 aH+ where aH+ is the activity of the
hydrogen ion. The 0-14 range of pH measurement is the
measurement of a difference in hydrogen ion concentration of
100,000,000,000,000 (1 x 1014). This means that at pH 0, the
hydrogen ion concentration is 1 x 1014 times greater than at
pH 14. This also means that the hydroxyl ion concentration at
pH 14 is 1 x 1014 times greater than at pH 0.
When the hydrogen and hydroxyl ions are present in equal
numbers (the neutral point), the pH is 7. pH values from 0 to 7
are termed acidic and those from 7 to 14 are termed basic. It is
important to note that a pH change of one unit (for instance
from pH 6 to pH 7) is a factor-of-10 change in hydrogen ion
concentration.
The glass membrane of a pH electrode responds to the
hydrogen ion activity by developing an electrical potential at
the glass/liquid interface. At a constant temperature, this
potential varies linearly with the pH of the solution being
measured. The change in potential per pH unit is termed the
slope of the electrode. The slope of the electrode increases
linearly with temperature.
The potential inside the pH glass bulb is fixed by the filling
solution, and the reference electrode potential is constant.
For these reasons, any change in the potential of the electrode
system at a given temperature will be due to any change in the
pH of the solution being measured.
Effects of temperature on pH measurements depend on the
reference electrode used, pH of the solution within the pH
electrode and pH of the test solution. At a certain pH,
temperature will have no effect on the potential of the electrode
system. This is known as the isopotential point. Also, at some
pH level, the system will exhibit no potential. This is known as
the zero potential point. Both the isopotential point and the
zero potential point are features designed into electrodes. Hach
electrodes are designed so the isopotential and zero potential
points are at pH 7 to minimize temperature effects at this

23
Section 4, continued
calibration point. At 25 °C, an electrode performing at 100%
efficiency will decrease by 59.2 mV for every unit increase
in pH.
4.2 Sodium Error Interferences
Sodium error occurs at elevated pH values when sodium ions in
the sample solution are incorrectly sensed as hydrogen ions by
the glass pH bulb. Below pH 12, sodium error is negligible.
The pH sensing bulbs can be optimized for different
applications by altering glass composition. This electrode has
been designed using a research-grade, full-range glass that is
less susceptible to sodium ion error. Accurate pH values (less
than 0.1 pH error) will be achieved at up to 100 mM Na+
concentrations in the range of 12 to 13 pH units, and up to
50 mM both at 25 °C in samples of pH 14.
In a single lab, using a sample group of three pH sensing
half-cells, in a controlled test versus a Standard Hydrogen
Electrode, the following results were obtained:
Table 1 Sodium Ion Error
pH [Na+] Error
12.1 0.00 M 0.00
12.1 0.01 M 0.01
12.1 0.05 M 0.02
12.1 0.10 0.03
12.5 0.00 M 0.02
12.5 0.01 M 0.02
12.5 0.05 M 0.03
12.5 0.10 0.05
13.7 0.00 M 0.04
13.7 0.01 M 0.06
13.7 0.05 M 0.07

Use the values presented in Table 1 to correct for sodium error

in samples of similar composition. Add the error values listed
below to the pH sensed by the electrode.

24
Section 5 Troubleshooting
Symptom Cause Remedy
No Electrode is improperly Check connection.
response connected to meter.
If a two-channel meter, is Reconnect the electrode to
the electrode connected to the desired channel or
the displayed channel? change the displayed
(A shorting cap on a channel channel on the meter.
input will show ZERO on
the display.)
Connector is dirty or wet. 1. Inspect both male and
female connector.
2. Clean with mild
solvents such as
alcohol or acetone.
3. Clean and dry with a
lint-free cloth or
compressed air.
Reference junction is Immerse in hot water or an
blocked. ultrasonic bath for several
minutes.
pH sensing bulb and Submerge the probe
reference junction are not in deeper into the sample.
contact with the sample.
Cable contains kinks, breaks, Replace or repair
loose connectors, etc. electrode.
The meter is broken. 1. Try another electrode.
2. Try the other channel.
3. Check and replace
batteries, if necessary.
4. Put shorting caps on
and see if meter
‘zeros’.
The meter is not set up to Select the appropriate
read the appropriate connector from the meter
connector. setup menu (5-pin on
sension meters).

25
Troubleshooting, continued
Symptom Cause Remedy
Out-of- See “No response”, above. See “No response”, above.
range
The bulb may be broken. Replace bulb.
response
Erratic Electrode is not plugged-in Plug electrode into meter
response to selected meter channel. channel.
Extreme external electrical Install a shorting cap on
fields are present. (The pH the unused meter terminal.
cable, though shielded, can Do not use stirrers in low
act as an antenna.) ionic strength solutions.
Avoid static electricity,
nearby pumps, power
supplies, etc.
pH sensing bulb and Submerge the probe
reference junction are not in deeper into the sample.
contact with the sample.
The sample chemistry is None.
changing.
The pH of a LIS solution is Use LIS Chamber
changing due to CO2 uptake. Accessory.
The bulb is cracked, If dirty, clean according to
discolored, or contaminated. Section 3.2 on page 21.
If cracked, discard
electrode.
The sample container is Use a clean sample
contaminated. container.

26
 Troubleshooting, continued
Symptom Cause Remedy
Low slope Bulb is old. Replace electrode.
Bulb is dirty. Clean according to Section
3.2 on page 21.
Standards are inaccurate. Make sure standards were
entered into meter
correctly.
Use only NIST traceable
buffers.
Use fresh buffers.
Change buffers frequently.
History of harsh usage. Replace electrode.
Cracked bulb. Replace electrode.
High slope Buffers are incorrect. Use appropriate buffers.
Incorrect measurement or Follow procedures
calibration technique. described in SECTION 2 on
page 13.
Temperature variations in Keep sample and standards
standards. at same temperature.
Standards contaminated. Prepare fresh standards.
High offset Leakage pathway results in Return electrode under
stray voltage via internal warranty.
electrical short.
Reference gel has become Submerge in hot water or
diluted or junction is an ultrasonic bath to
plugged by a clogged frit. attempt to unclog the frit.
Replace electrode if no
improvement occurs.
Cracked bulb. Replace electrode.
Sluggish Sample is cold and of low Wait patiently.
response ionic strength.
Improper conditioning. Condition according to
Section 1.3 on page 10.
Bulb, guard, and electrode Clean according to Section
stem are dirty. 3.2 on page 21.

27
28
Electrode Service Request
Questionnaire
1. What model of meter is the electrode being used with?

2. What is the complete lot code of the electrode (located on

the electrode cable)?

3. What is the date the electrode was purchased?

4. How long has the electrode been in use?

5. What types of samples are being tested?

6. What is the temperature of the samples being tested?

7. How often is the electrode being used?

8. How is the electrode being stored between uses?

9. What is the electrode slope during a typical calibration?

10. If a new electrode, has it been conditioned according

to instructions?

11. If the electrode has been in use for awhile, what

maintenance has been performed?

12. Describe the suspected problem or failure of the electrode.

13. When calling for telephone support, have your meter,

electrode, buffers/standards, and this completed
questionnaire near the phone before calling.

14. Did you use Ionic Strength Adjustor in samples or

standards?

15. Did you use a low range method for Low Ionic Strength
and high purity water samples with conductivity below
150 µ/cm?

16. Is the sample above pH 12 with a high Na+ concentration?

29
30
Glossary
Acid—a chemical compound that contributes hydrogen ions
(H+) to an aqueous solution or causes the pH to be less than 7.0.

Acidic—A solution or system with a pH less than 7.0.

Activity—The measure of work performed by an ion as it moves

through a system. The activity is dependent upon the
concentration of all ions in the system and the charge
associated with the ion.

Alkali—A solution or system with a pH greater than 7.0.

Alkalinity—Capacity of water to accept hydrogen ions (H+).

Indicates presence of carbonate (CO32-), bicarbonate (HCO3),
and hydroxyl (OH-) ions.

Anion—A negatively charged ion (i.e., NO3-, Cl-).

Base—A chemical compound that contributes hydroxide ions

(OH-) to an aqueous solution or causes the pH to be greater
than 7.0.

Buffer—

1. A compound, mixture of compounds, or solution which,

when added to a system, changes pH to a specified,
known value.
2. The ability a solution or system to resist change in pH if
either an acid or a base is added.
Buffering capacity—A measurement of the amount of acid or
base which can be added to a solution before the pH changes.

Cation—A positively charged ion (i.e., NH4+, Ca2+).

Combination electrode—An electrode composed of a reference

half-cell and a sensing half-cell. The reference half-cell is the
half of the electrode pair that maintains a constant potential

31
Glossary, continued
regardless of solution composition. The sensing half-cell
develops a potential proportional to solution composition.

Conditioning—The process of preparing an electrode for use by

soaking the electrode in a solution with a pH and ionic strength
similar to the sample. Conditioning allows the formation of the
hydrated layer, necessary for accurate measurements.

Decade—Any two standards with a concentration ratio of 10.

For example:

Concentration of the higher Standard- = 10

---------------------------------------------------------------------------------------
Concentration of the lower Standard

Dilution factor—A numerical value by which results must be

multiplied in order to obtain the actual value of the sample. For
example, if 1 mL of sample is diluted to 10 mL, then the result
obtained must be multiplied by 10 to obtain the value for the
sample before dilution.

Electrolyte—A solution made of dissolved ions that is used to

facilitate electrical conductance and ionic transport. The
composition is usually potassium chloride (KCl), sodium
chloride (NaCl), ammonium chloride (NH4Cl), or ammonium
sulfate ((NH4)2SO4).

Free-flowing reference junction—The point on the electrode

where reference gel is freely dispensed without obstruction by
frits or other materials.

Hydrated layer—A layer that forms at the glass/liquid

interface. Chemically similar to the sample being measured
(especially in samples of low ionic strength), the formation
characteristics of this layer depend upon the type of glass, the
age of the glass, the previous usage history of the glass, the
temperature and ionic strength of the aqueous solution, etc.

Inert—Does not react with other substances.

32
 Glossary, continued
Ion—An atom, group of atoms or molecule that is electrically
charged as the result of gaining or losing electrons. An ion
resulting from the gain of electrons becomes negative (anion),
while an ion resulting from the loss of electrons becomes
positive (cation).

Ionic strength—A measure of the concentration of any ion in

solution, usually expressed in moles per liter.

Occasional use—Infrequent use where the probe is subjected to

test conditions just long enough to obtain a proper reading.

pH—A relative numerical measurement of the acidic, neutral, or

alkali nature of a solution or system. Mathematically defined as
the negative log of the hydrogen ion concentration.

Potential—The capacity of a system to perform work. When two

ions of opposite charge are separated by a distance, there is a
potential developed from the tendency of the ions to come back
together. Each ion exerts work to come back together.
The amount of work needed by each ion to come back together
is the potential.

Reference element—A silver wire coated with silver chloride,

this element develops a constant potential when immersed in
an electrolyte such as potassium chloride.

Reference half-cell—Provides a constant voltage against which

the pH dependent voltage from the sensing half-cell is
compared.

Reference junction—The point on the electrode where reference

gel is dispensed.

Routine use—Typical use following regular, frequent routines,

but not continuous as with an in-line monitoring process.

Salt—An ionic compound.

33
Glossary, continued
Sensing half-cell—Provides a voltage proportional to the
concentration of a specific ion in solution.

Siemen—The unit of conductivity equivalent to mho (ohm-1).

Conductivity is often expressed as microSiemens per
centimeter (µS/cm) and milliSiemens per centimeter (mS/cm)
and it is a measure of how readily a sample conducts electricity.
More concentrated solutions conduct more electricity and have
higher conductivity.

Sodium error (alkaline error)—Occurs in strongly alkaline

solutions, generally greater than 13 pH, when concentrations
of H+ are low, and small, positively charged ions such as Na+
generate potential differences across the pH glass membrane.
These differences can result in measurement errors between
-0.5 and -1.0 pH.

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General Information

At Hach Company, customer service is an

important part of every product we make.
With that in mind, we have compiled the
following information for your convenience.

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How To Order
By Telephone: By Mail:
6:30 a.m. to 5:00 p.m. MST Hach Company
Monday through Friday P.O. Box 389
(800) 227-HACH Loveland, CO 80539-0389
(800-227-4224) U.S.A.
By FAX: (970) 669-2932

Ordering information by E-mail: orders@hach.com

Information Required
• Hach account number (if available) • Billing address
• Your name and phone number • Shipping address
• Purchase order number • Catalog number
• Brief description or model number • Quantity

Technical and Customer Service (U.S.A. only)

Hach Technical and Customer Service Department personnel
are eager to answer questions about our products and their use.
Specialists in analytical methods, they are happy to put their
talents to work for you.
Call 1-800-227-4224 or E-mail techhelp@hach.com.

International Customers
Hach maintains a worldwide network of dealers and
distributors. To locate the representative nearest you, send
E-mail to intl@hach. com or contact:
In Canada, Latin America, Africa, Asia, Pacific Rim:
Telephone: (970) 669-3050; FAX: (970) 669-2932
In Europe, the Middle East, or Mediterranean Africa:
HACH Company, c/o Dr. Bruno Lange GmbH
Willstätterstr. 11
D-40549 Düsseldorf, Germany
Telephone: +49/[0]211.52.88.0
Fax: +49/[0]211.52.88.231

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Repair Service
Authorization must be obtained from Hach Company before
sending any items for repair. Please contact the HACH Service
Center serving your location.
In the United States:
Hach Company
100 Dayton Avenue
Ames, Iowa 50010
(800) 227-4224 (U.S.A. only)
Telephone: (515) 232-2533
FAX: (515) 232-1276
In Canada:
Hach Sales & Service Canada Ltd.
1313 Border Street, Unit 34
Winnipeg, Manitoba
R3H 0X4
(800) 665-7635 (Canada only)
Telephone: (204) 632-5598
FAX: (204) 694-5134
E-mail: canada@hach.com
In Latin America, the Caribbean, the Far East,
the Indian Subcontinent, Africa, Europe, or the Middle East:
Hach Company World Headquarters
P.O. Box 389
Loveland, Colorado, 80539-0389
U.S.A.
Telephone: (970) 669-3050
FAX: (970) 669-2932
E-mail: intl@hach.com

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Warranty
Hach warrants most products against defective materials or
workmanship for at least one year from the date of shipment;
longer warranties may apply to some items.
HACH WARRANTS TO THE ORIGINAL BUYER THAT HACH
PRODUCTS WILL CONFORM TO ANY EXPRESS WRITTEN
WARRANTY GIVEN BY HACH TO THE BUYER. EXCEPT AS
EXPRESSLY SET FORTH IN THE PRECEDING SENTENCE,
HACH MAKES NO WARRANTY OF ANY KIND WHATSOEVER
WITH RESPECT TO ANY PRODUCTS. HACH EXPRESSLY
DISCLAIMS ANY WARRANTIES IMPLIED BY LAW,
INCLUDING BUT NOT BINDING TO ANY WARRANTY OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR
PURPOSE.
LIMITATION OF REMEDIES: Hach shall, at its option, replace
or repair nonconforming products or refund all amounts paid
by the buyer. THIS IS THE EXCLUSIVE REMEDY FOR ANY
BREACH OF WARRANTY.
LIMITATION OF DAMAGES: IN NO EVENT SHALL HACH BE
LIABLE FOR ANY INCIDENTAL OR CONSEQUENTIAL
DAMAGES OF ANY KIND FOR BREACH OF ANY WARRANTY,
NEGLIGENCE, ON THE BASIS OF STRICT LIABILITY, OR
OTHERWISE.
This warranty applies only to Hach products purchased and
delivered in the United States.
Catalog descriptions, pictures and specification, although
accurate to the best of our knowledge, are not a guarantee or
warranty.
For a complete description of Hach Company’s warranty policy,
request a copy of our Terms and Conditions of Sale for U.S.
Sales from our Customer Service Department.

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