TURNING THE BALANCE ON

 With no load on the platform, switch the balance ON by pressing the “ON/OFF” button located on the right side of the balance.  When
the balance is first switched ON, all segments of the display should be on as shown:
 
 
 
 
 
 
 
 

 The “display check” will be displayed briefly, the display will momentarily blank and then indicate zero.
 

SELECTING A WEIGHING UNIT

Weight measurements can be displayed in any of the available weighing units.  To select one of these weighing units for displaying weight
measurements, repeatedly press UNITS until the desired unit indicator appears.
 

WEIGHING MATERIAL OR OBJECTS WITH TARE
 

When weighing material or objects that must be held in a container, taring enables the
balance to automatically subtract the weight of the container.  Only the net weight of the material in the container will be displayed.

1.  If necessary, press TARE with no weight on the platform to obtain a zero reading.

2.  Place the empty container on the platform.  Its weight will be displayed.

3.  Press TARE.  Zero will be displayed and the container’s weight will be stored in the balance’s memory.  The weight of the container
will be automatically subtracted from the next weighing.

4.  Add the material to be weighed to the container.  As material is added, its net weight will be displayed.

5.  When the container and its contents are removed from the platform, the tared weight (of the container) will be displayed as a negative
number.

NOTE:  The combined weight of the container and the material must not exceed the capacity of the balance.  The container’s weight will
remain in memory until TARE is pressed again.
 
 

OPERATION OF THE pH METER
 
 

Turn on the instrument by pressing the ON/OFF key.  The instrument will enter into the state or mode when the instrument was last turned
off.  It is not necessary to press the “CLEAR” key.
 
 

CALIBRATION OF THE pH METER (in the AUTOLOCK mode)

 1.  Press the “MODE” key for the LCD display to indicate pH and for the  AUTOLOCK indication LED to be on.

 2.  Rinse the pH electrode in distilled water and immerse in pH buffer 7.00.  The instrument will display the buffer temperature.

 3.  Press the “STAND” key.  The STAND indication LED will be on and the WAIT indication LED will flash, indicating that the
instrument is hunting for a stable reading.  The display will be locked to the buffer value corresponding to the temperature of buffer 7.00 as
measured in the previous instruction #2.  When a stable reading is reached, the WAIT indication LED will stop flashing and stay off.  The
SLOPE indication LED will start to flash, indicating that standardization at buffer 7.00 is complete and the instrument is ready to be sloped
at a second buffer.

 4.  Remove the pH electrode from buffer 7.00, rinse with distilled water and immerse in buffer 4.01.

 5.  Press the “SLOPE” key.  The SLOPE indication LED will stop flashing and stay on.  The WAIT indication LED will start to flash,
indicating that the instrument is hunting for a stable reading.  The display will be locked to the second buffer value corresponding to the
temperature of the second buffer as measured in #4.   When a stable reading is reached the WAIT indication LED will stop flashing and
stay off.

  NOTE:The micro computer inside the instrument will calculate and compensate for the pH electrode slope deviation corresponding to the
values of the two calibration buffers.  The instrument is dual point calibrated and is ready for pH measurements.
 
 

pH MEASUREMENTS
 
 

The STAND and SLOPE indication LED’s must be on, indicating that the
instrument is dual point calibrated and ready for measurements.

 1.  Press the “MODE” key for the LCD display to indicate pH and for the AUTOLOCK indication LED to be on.

 2. Rinse the pH electrode with distilled water and immerse it in the sample to be measured.
 
 3.  Press the “MEASURE” key.  The WAIT indication LED will start to flash, indicating that the instrument is hunting for stable
reading.  The display will track the pH value as sensed by the pH electrode.  When the display changes less than 0.01 pH within
approximately 10 seconds, the AUTOLOCK function will be activated.  The WAIT indication LED will stop flashing and stay off.  The
reading is then “locked” and will not respond to further changes from the pH electrode.  The “locked” display is the pH value of the
sample at the displayed sample temperature.
 
 

OPERATION OF THE
“SPECTRONIC 20” SPECTROPHOTOMETER
 
 

The Spectronic 20 Spectrophotometer is a very commonly used instrument found in many laboratories.  It is therefore described in some
detail.

Description of the Optical System
 
 The operation of the optical system may be understood by referring to Fig. 2.  White light emanating from the tungsten lamp passes
through the entrance slit and is focused by the field lens onto the objective lens.  The objective lens focuses an image of the entrance slit at
the exit slit after it has been reflected and dispersed by the diffraction grating.  To obtain various wavelengths the grating is rotated by
means of an arm which rides on the wavelength cam.  In setting the wavelengths, the cam rotates the grating so that the desired wavelength
passes through the exit slit.  The monochromatic light which passes through the exit slit continues on through whatever sample may be
contained in a test tube or cuvette placed in the light path (the light also goes through the red filter in the infrared wavelength range) and
finally terminates at the measuring phototube where the light energy is converted to an electric signal.  Whenever the sample is removed
from the instrument, an occluder automatically falls into the light beam so that the zero may be set without further manipulation.  A light
control is provided to set 100 percent transmittance or zero absorbance with a reference or standard solution in the sample compartment.
The optics of the lens tube provide an extended range which goes to 340 mm.

 
Operating Procedure - Colorimetry
 
1.  For colorimetric use the wavelength control (Fig. 3  - 1) is rotated until the desired wavelength in millimicrons is indicated by the
wavelength scale(Fig. 3 - 2).  This wavelength for a given substance is found by referring to the literature or may be determined
experimentally.

2.  The instrument is turned on by rotating the zero control
(Fig. 3 - 3)clockwise.  When the instrument is turned on the jewel (Fig. 3 - 4) will glow.

3.  After a few minutes warm-up time, the zero control may be adjusted to bring the meter needle to “0” on the percent transmittance scale
(Fig. 3 - 5).

4.  The test tube which is also called cuvette containing water or other solvent (also referred to as the blank) is then inserted in the sample
holder (Fig. 3 - 6).   Be sure the etched marking on the cuvette lines up on raised mark on the front of the sample holder (Fig. 3 - 6).

NOTE:For best results, matched sets of cuvettes should be employed or the use the same cuvette in which the blank was read.

5.  The light control (Fig. 3 - 7) should now be rotated until the meter reads “100” on the percent transmittance scale.  This knob regulates
the amount of light passing through the second slit to the phototube (Fig. 2).  This adjustment tells the machine to disregard the solvent
material.   because the solvent itself may absorb certain wavelengths.
 
NOTE:  Following this any changes observed when the test solution is placed on the light path would be due to that solution alone.
Whenever a change in wavelength is made the 0 and 100 percent must be reset.  The needle should always return to zero when a tube is
removed.
 
6.  The unknown sample may then be inserted in place of the standard and the percent transmittance or absorbance value read directly from
the meter.  The mirror scale of the meter is to avoid parallax when reading the values indicated on the meter scale by the meter needle.
 

Nomenclature
 
 Absorbance, A.  Logarithm to the base 10 of the reciprocal of the transmittance A  = log 10 (1/T).
 
 Transmittance, T.    The ratio of the radiant power transmitted by a sample to the radiant power incident on the sample.

 Visible.  Pertaining to radiant energy in the electromagnetic spectral range visible to the human eye ( approximately 380 to 780 mm.

 Wavelength.    The distance, measured along the line of propagation, between two points that are in phase on adjacent waves- units A., mm,
and m.