Basic Photography Course

reading is taken at the point where the top of the
meniscus intersects the stem of the hydrometer (fig.
9-2).

pH METERS

The acid or alkali state of a solution is measured in

pH values. The pH value of developers and fixers
influences their activity and proper strength. pH is
basically a measure of the degree of acidity or alkalinity

of a solution. It provides an invaluable aid in
determining the degree of accuracy with which the
processing solutions have been prepared. Photographic

developers usually have a pH of 8 to 12, while fixers
range between pH 3.1 and 5.

The following scale indicates the location of acids

and alkalis by their pH value (strength):

pH VALUES

ACIDS

NEUTRAL

ALKALIS

1 2 3 4 5 6

8 9 10 11 12 13 14

A pH of 7 is neutral. Working down from this point,

the figures indicate weak acids with a pH of 6 on to
strong acids with a pH of 1. Working up from a pH of 7,
the figures indicate weak alkalis with a pH of 8 to strong
alkalis with a pH of 14.

The pH values are numbered on a logarithmic scale.

From 0 through 6, each number indicates a degree of
acidity 1/10 as strong as the preceding number, but 10
times stronger than the next succeeding or higher

number. A solution with a pH value of 4, for example,
has a degree of acidity 10 times stronger than a solution
with a pH value of 5, but only 1/100 the strength of a
solution having a pH value of 2. When determining the
degree of alkalinity of a solution, figure it in an opposite
manner. From 8 through 14, each number represents a
degree of alkalinity 10 times as strong as the last
preceding number, but 1/10 the strength of the next
higher number; for example, a solution having a pH
value of 11 indicates that the solution has an alkalinity

1,000 times stronger than a solution having a pH value

of 8, but it would be only 1/100 as alkaline as a solution
having a pH value of 13.

Litmus paper is used to indicate whether a solution

is acid, alkaline, or neutral, but it does not indicate the
actual pH value. For this purpose a pH meter should be
used.

A pH meter is an amplifier meter with a scale that

reads from 0 to 14 and an electrode apparatus (Eg. 9-3).
A pH meter has a reference electrode and a pH
measuring electrode, or these two can be combined into
one combination electrode. The pH electrode actually

measures the pH, while the reference electrode that
contains an electrolyte solution is used only to complete
the electrical circuit. The first step in measuring pH is

to establish a point of reference by a standardization
procedure. To standardize the pH meter, you must place
the electrodes in a calibrated buffer solution.

Buffer solutions are available at exact pH values for

this precise standardization. Always select a buffer with
a pH value as close as possible to the pH of the sample
to be tested; for example, use a buffer at a pH of 4.00 to
test a fixer solution or a pH of 10.00 to test a developer
solution. The instrument should be standardized at
regular intervals during a long series of measurements
or before each use.

9-4

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> Figure 9-2.Hydrometer. reading is taken at the point where the top of the meniscus intersects the stem of the hydrometer (fig. 9-2). pH METERS The acid or alkali state of a solution is measured in pH values. The pH value of developers and fixers influences their activity and proper strength. pH is basically a measure of the degree of acidity or alkalinity of a solution. It provides an invaluable aid in determining the degree of accuracy with which the processing solutions have been prepared. Photographic developers usually have a pH of 8 to 12, while fixers range between pH 3.1 and 5. The following scale indicates the location of acids and alkalis by their pH value (strength): pH VALUES ACIDS NEUTRAL ALKALIS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 A pH of 7 is neutral. Working down from this point, the figures indicate weak acids with a pH of 6 on to strong acids with a pH of 1. Working up from a pH of 7, the figures indicate weak alkalis with a pH of 8 to strong alkalis with a pH of 14. The pH values are numbered on a logarithmic scale. From 0 through 6, each number indicates a degree of acidity 1/10 as strong as the preceding number, but 10 times stronger than the next succeeding or higher number. A solution with a pH value of 4, for example, has a degree of acidity 10 times stronger than a solution with a pH value of 5, but only 1/100 the strength of a solution having a pH value of 2. When determining the degree of alkalinity of a solution, figure it in an opposite manner. From 8 through 14, each number represents a degree of alkalinity 10 times as strong as the last preceding number, but 1/10 the strength of the next higher number; for example, a solution having a pH value of 11 indicates that the solution has an alkalinity 1,000 times stronger than a solution having a pH value of 8, but it would be only 1/100 as alkaline as a solution having a pH value of 13. Litmus paper is used to indicate whether a solution is acid, alkaline, or neutral, but it does not indicate the actual pH value. For this purpose a pH meter should be used. A pH meter is an amplifier meter with a scale that reads from 0 to 14 and an electrode apparatus (Eg. 9-3). A pH meter has a reference electrode and a pH measuring electrode, or these two can be combined into one combination electrode. The pH electrode actually measures the pH, while the reference electrode that contains an electrolyte solution is used only to complete the electrical circuit. The first step in measuring pH is to establish a point of reference by a standardization procedure. To standardize the pH meter, you must place the electrodes in a calibrated buffer solution. Buffer solutions are available at exact pH values for this precise standardization. Always select a buffer with a pH value as close as possible to the pH of the sample to be tested; for example, use a buffer at a pH of 4.00 to test a fixer solution or a pH of 10.00 to test a developer solution. The instrument should be standardized at regular intervals during a long series of measurements or before each use. 9-4 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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