LaMotte DHA 3000 User Manual

DHA-3000

L

Mott

DIGITAL pH METER

CODE 1706-02

TABLE OF CONTENTS

WhatispH?................................................ 4

GeneralInformation......................................... 5

MeterSpecifications......................................... 6

MeterAccessories........................................... 7

MeterPreparation........................................... 7

Standardizing............................................... 8

MeasuringpH .............................................. 9

MeasuringthepHofSoil ..................................... 9

MeasuringTemperature...................................... 10

Measuring Millivolts ........................................ 10

Meter Care............................................... 11

WarrantyInformation....................................... 12

LaMOTTE COMPANY

Helping People Solve Analytical Challenges

PO Box 329 • Chestertown • Maryland • 21620 • USA

800-344-3100 • 410-778-3100 (Outside USA) • Fax [phone]

Visit us on the web at www.lamotte.com

3

SM

WHAT IS pH?

pH is one of the most common analyses in soil and water testing. An
indication of the sample’s acidity, pH is actually a measurement of the
activity of hydrogen ions in the sample.

pH measurements run on a scale from 0 to 14, with 7.0 considered neutral.
Those solutions with a pH below 7.0 are considered acids, and those
between 7.0 and 14.0 are designated bases. The pH scale is logarithmic, so
a one unit change in pH actually reflects a ten fold change in the acidity.
For instance, orange juice, pH 4, is ten times more acidic than cottage
cheese, which has a pH of 5.

Many industries rely heavily on pH for their process to work properly, or
to maintain expensive equipment. Breweries maintain the pH between

4.2 and 4.6 to keep infectious bacteria from breeding during the
fermentation process. In many industrial applications if the pH is too low
the water may corrode metal equipment, but if it is too high scaling may
result.

pH can be measured visually or electronically. Visual comparisons use pH
indicators where color changes reflect the pH, which are then matched to
color standards. pH meters, such as the DHA-3000, simplify the pH test.
An electrode is placed in the sample, and the pH read directly from the
meter.

While the meter is very easy to use, the electronics within the meter are
more complex. After the pH electrode measures the millivolts of potential
between the reference electrode and the pH electrode, the meter converts
this reading to pH units using the Nernst Equation:

E=Ex+2.3RT

log(ai)

k

nF

where Ex= constant depending upon reference electrode

R = constant

= absolute temperature

T

k

n = charge of the ion (including sign)
F = constant

= activity of the ion

a

i

For actual use in converting pH readings to millivolts, this equation can
be simplified to:

(pH - 7.00) x (-0.198) = mV

4

GENERAL INFORMATION

The DHA-3000 pH meter consists of three major components: the pH
electrode, the temperature probe and the meter.

The pH electrode consists of a glass, hydrogen-ion selective electrode, and
a reference electrode, combined into a single unit. The glass electrode is
specially treated to measure only hydrogen ions, while the reference
electrode is silver surrounded by silver chloride, and it provides a “zero” or
reference point for the measurement. This “zero” point means any change
in potential measured at the glass electrode is attributed to hydrogen ions,
and therefore expressed as pH.

The temperature probe serves a dual purpose, and may be connected to
the pH electrode. When the temperature probe is in the sample at the
same time as the pH electrode, it measures the temperature and allows the
meter to automatically compensate the temperature to 25°C. Secondly, it
allows the user to measure the actual temperature of the sample in °C.

When the temperature probe and pH electrode are immersed in the
sample, the meter measures the difference in potential between the glass
electrode and the reference electrode. This electronic measurement is
converted from millivolts to pH units, and the result appears on the
display.

If, for some reason, the temperature probe can not be used during the pH
measurement, the meter must be manually compensated for temperature.
To do this standardize the meter with buffers which are the same
temperature as the sample, and use the “Slope” knob to standardize meter
to the proper pH.

5

METER SPECIFICATIONS

Range 0.00-14.00 pH units

±1999 mV

0.0 to 50.0°C

Readout 3

1

/2digit LCD

Readout Resolution 0.01 pH units

1mV

0.1°C

Controls pH

millivolts
°C Temperature
On/Off
Standardize
Slope

Temperature Compensation Automatic by separate probe
Electrode pH: Combination gel-filled epoxy body,

Ag/AgCl with BNC connector, 3’ cable

Probe Temperature: 2.5 mm cable jack

connector, 3’ cable

Power Alkaline (9V) DC battery;

3.5 mm jack for optional adapter

Size 5

7

/8”Lx31/4”Wx13/4”H

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