Note: Descriptions are shown in the official language in which they were submitted.
~o~
The invention relates to ion exchange method
apparatus and for very rapidly approximating the total
dissolved solids content of an aqueous sample solution.
The invention more particularly relates to ion exchange
s method apparatus and for determining the total number of
equivalents of dissolved ionic materials in aqueous
sample solution, such number correlating rather closely
to the percent by weight of all dissolved salts in an
aqueous sample such as a surface water sample or a typical
industrial plant effluent.
There is a constant and ever increasing demand
for analysis of surface waters, boiler feed water and
manufacturing plant effluents for total dissolved solids
content as a measure of water purity for various intended
purposes and reasons. The standard methods long utilized
generally involve drying a measured quantity such as a
100 milliliter quantity of a filtered portion of solution
in a tared dish and weighing the residue. Usually drying
is carried out at 105 to 110C. or 180-2C., and the
method suffers from the difficulty that water of hydration
is frequently not driven off at these temperatures and m-st
be determined as by a Karl Fischer titration and correc-
tion made. The method generally requires about 4-16 hours
of elapsed time for its completion so that answers are
not available in a hurry and furthermore may require
about 45 to 60 minutes of analyst's time, net, per sample ~-~
run in duplicate so that the method is relatively expensive
when run on a large scale. So far as is known, no method
has been available heretofore for the rapid readily auto-
mated relatively inexpensive reliable determination of total
.~,774-F -1-
L~ ~
dissolved solids. Conductivity tests have been used but are not consist-
ently reliable because of widely varying specific conductance among com-
ponent ions. Therefore, a new approach appears to be needed.
The invention resides in the method of determinlng total
ionic content in an aqueous sample solution by quantitative analysis
of a plurality of ionically dissociated compounds in the aqueous sample
solution utilizing ion exchange procedures, which comprises: adding a
predetermined amount of said sample solution to a cation exchange resin
bed means, said resin bed means being charged with a cation exchange
resin in easily elutable cation form capable of exchanging with the
cations of the sample and delivering only one species of cation on elution;
eluting from said resin bed means with water a congregated quantity of
said easily elutable cations corresponding equivalent per equivalent to
the number of equivalents of ion material dissolved in the said predeter-
mined amount of sample solution; passing the effluent from the cation
exchange resin bed means through an anion exchange resin bed means, said
latter resin bed means being in easily elutable anion form capable of
exchanging with the anions of the sample and delivering only one species
of anion on elution thereby exchanging all anionic species in said effluent
for such easily elutable ions; the total exchange capacity of t:he cation
and anion exchange resins being sufficient to at least free, by ion
exchange, a number of equivalents of each of the easily elutable cations
and anions corresponding to the said number of equivalents of ionically
dissociated compounds in the predetermined
-- 2 --
10~8~
amount of sample solution; and passing the effluent from the anion exchange
resin bed means through a conductivity cell having associated readout means
and thereby quantitatively measuring the congregated quantity of easily
elutable cations and anions.
The invention also resides in an apparatus for the determin-
ation of total ionic content of an aqueous sample solution containing a
plurality of ionically dissociated compounds, utilizing ion exchange tech-
niques which comprises: a first and a second ion exchange column and de-
tector means connected in series by liquid conduit means, the first column
being charged with a cation exchange resin in the sodium ion or the lithium
ion form and the second column being charged with an anion exchange resin
in the hydroxide ion or the acetate ion form, and said detector means being
a conductivity cell with associated readout means.
The invention further resides in an apparatus for the determin-
ation of total ionic content of a~ aqueous sample solution containing a
plurality of ionically dissociated compounds, utilizing ion exchange tech-
niques which comprises: a first ion exchange column charged with a cation
exchange resin in easily elutable cation form, a second ion exchange column
charged with an anion exchange resin in easily elutable anion form, said ion
exchange columns each have an inside diameter not exceeding about 10 milli-
meters, means for introducing water and a predetermined amount of sample
solution into the first ion exchange column, means for conveying effluent
from the first ion exchange column to the second ion exchange column, a
conductivity cell and associated
-- 3 --
1~:)4~
readout means, and means for conveying effluent from the second ion
exchange column to the conductivity cell.
The single figure of the drawing is a schematic represent-
ation of an embodiment of the apparatus of the invention showing a
reservoir for water which serves as eluant, a pump, an injection
valve such as a special injection valve regularly used for ion exchange
and chromatographic separations, a first ion exchange column, a second
ion exchange column and a detector such as a conductivity cell, all
connected in series by liquid conduit means.
The present method and apparatus are well adapted for the
rapid and easily automated analysis of the total ionic content of an
aqueous sample solution and is, for example, readily carried out at re-
mote locations or by relatively untrained personnel. The method is
effective for the ion exchange transformation and determination of
any ionic material in solution or any material which is less than
completely dissociated but readily dissociates and reacts with an ion
exchange resin. The method is readily carried out on most any aqueous
sample such as a sample of surface waters, boiler blow down water or
aqueous
- 3a -
plant effluent from a manufacturing plant. More concen-
trated samples such as brines and sea water samples are
also analyzable according to the present method, although
a much smaller sample size is generally utilized for more
concentrated samples, e.g., 2 to 10 microliters of sample
solution. A sample size of the order of 0.1 milliliter
is indicated for the analysis of water of the ionic material
content level of good drinking water. For most natural
fresh surface waters suitable sample sizes are in the range
of about 10 to about 50 microliters. In each case, it is
preferred to use as small sample size as is reasonably
convenient to handle consistent with good instrument practice
on signal to noise ratios. Smaller samples do not ex-
haust the resin beds as rapidly as larger samples.
Referring now to the single figure of the drawings,
the apparatus of the invention is seen basically t:o con-
sist of a first ion exchange column 10 arranged in series
with a second ion exchange column 11 followed by a con-
. ductivity cell or other suitable detector 12 all connected
by liquid conduit means. Sample may be placed on or added
to the column 10 in most any suitable manner as by the
use of a pipet or a buret and eluted with eluant added
from, for example, a buret, and the process carried on by
a gravity flow system. Preferably, by means of a syringe
(not shown), the sample is added to the system at sample
injection valve 13.
The sample injection valve 13 is of a type
commonly used for adding sample to the column in carrying
out chromatographic separations or ion exchange operations
and typically is provided with a bore in the valve plug or
~;,774-F -4-
a loop of tubing connected to two of the valve body ports
either of which, i.e., the bore or the loop of tubing,
determines sample size which is subsequently swept out by
the eluant as well ~nderstood in the art when the valve
plug bore or the loop of tubing, on manipulation of the
valve, is placed in series with a stream of the eluant
leading to the first ion exchange column and the selected
sample portion is thereby swept on into the ion exchange
column.
In the present apparatus, the sample injected
at injection valve 13 is swept through the apparatus by
water 14 which serves as eluant and is drawn from a reservoir
15 or other suitable source of supply by pump 16 and passed
through the sample injection valve 13 to the first ion
exchange column 10. As indicated above, the eluant water
may be added to the first column manually as by pouring
the water from a vessel into an open column but :is prefer-
ably added in a continuous stream to obtain better uni-
formity and reproducibility of results. The effluent i
leaving the ion exchange column 10 with all of t-he cationic
species exchanged for a single easily elutable cation i~
conveyed by liquid conduit means to the ion exchange column
11 wherein the anions present in the solution sample are
exchanged for a single easily elutable anion. The effluent
from the anion exchange column 11 is led by liquid conduit
means to a suitable detector such as a conductivity cell 12
wherein the total amount of the single predetermined or
preselected ion pair species issuing from the preceding
two columns is readily determined as a single concentration
peak as shown by the associated readout means. The readout
16,774-F -5-
means represented in the drawing, schematically, is a con-
ductivity meter 17 and a recorder 18 associated with the
conductivity cell. The recorder 18 is preferably a
recorder-integrator.
The cation exchange resin 22 employed in the
first chromatographic column 10 will ordinarily be placed
in either the lithium or sodium ion form since these
cations are amongst the most easily elutable cations, although
other cations such as potassium ion may be usable with
some samples containing substantially only cations that
will displace potassium ions. The anion exchange resin 23
in the second chromatographic column 11 is ordinarily
placed in either the hydroxide form or the acetate form since
these anions are amongst the most easily elutable anions
and will be readily displaced by any other anion. Like
ions of course in the original sample will produce an effect
as if they had not exchanged and will make their proper net
contribution to the determination of total ionic content.
While it is sufficient for a single determina-
tion that each ion exchange column contain ion exchange
resin with sufficient capacity to exchange all ions in
the sample differing from those in the form of the resin,
as a practical matter it is much preferred that the quantity
of ion exchange resin employed in each bed have a sufficient
total capacity to permit the analysis of a very large number
of samples before regeneration of the resin is required
to put it in its original ion form. As a practical manner
each ion exchange column should be charged with sufficient
resin to permit, without regeneration, the analysis of at
least five samples, much more preferably at least 1000
16, 774--F -6-
10~
samples, and even more preferably at least 5000 samples,
and even more is desirable and attainable, as experience
shows, the columns are readily charged with sufficient
~ high capacity ion exchange resin to accommodate l0,000
samples when small sample sizes such as l0 to 50 microliters
of solution are used.
Periodically as either or both of the ion
exchange resins being used approach exhaustion, it is
necessary to either replace or regenerate each bed to
place it back into the desired ion form. If time demands
upon the use of the instrument are quite great, it may
be desirable to keep on hand an inventory of columns
containing the resin beds in the fully regenerated form.
On the other hand, if desired, the resin beds can be
lS regenerated in place without dismantling the apparatus.
Regeneration is carried out by reversing the ion exchange
process that takes place during analytical determinations.
This is accomplished by bringing into contact with the
ion exchange resin a relatively.concentrated solution,
for example, about l to S molar, of the ion to be estab-
lished at the ion exchange sites. The ion of interest may
be one that is paired with most any suitable counter valent
ion so long as the resulting ion pair is quite soluble in
water. Thus, the cationic exchange resin may be washed,
or soaked if necessary to achieve better replacement,
with a solution of lithium chloride while the anion ex-
change resin may, for example, be washed with a solution
of sodium hydroxide. On the other hand, it is entirely
possible to wash both columns at once with the same solu-
tion upon using an ion pair made up of ions each suitable
16,774-F -7-
for regenerating one of the beds. Examples of such ion
pairs are lithium hydroxide, lithium acetate, sodium
hydroxide or sodium acetate. Rowever, it is best to first
acid wash the cation exchange resin, in this ~ase, before
placing in the sodium or lithium form. If desired, the
acid wash can simply be passed through both columns before
adding the fluid regenerant solution.
~ Referring now to the single figure of the
drawing, regeneration of the cation exchange resin in
column 10 is carried out upon drawing in regenerate solu-
tion such as aqueous solution of lithium chloride through
valve 21 to the pump 16 thence through the bypass of the
portion of the sample injection valve 13 and into the
column 10, the effluent leaving by way of valve 19 and
being directed to discard. Similarly, regenerate solution
for the anion exchange resin in column 11 can, by a suitable
pump, or gravity flow, be directed in through valve 19
to column 11, the effluent leaving by way of valve 20 and
being directed to discard. On the other hand, if the ion
pair is appropriate for regeneration of both columns with a
single regenerant solution, the regenerant solution drawr
into the line through valve 21 is simply directed on
through column 10 to column 11 and out at valve 20 to dis-
card after first acid washing both resin beds with, e.g.,
dilute hydrochloric acid. If desired, either both of the
columns may be backwashed initially to loosen and fluff
up the resin beds in order to avoid channeling phenomenon.
It must be understood that the columns shown
in the drawing are ordinarily small diameter tubing,
usually of glass or stainless steel, the small diameter,
1;,774-F -8-
io'~
together with fast flow rates of water serving as eluant,
facilitating analysis times of generally no greater than
about 5 minu~es, though longer times may be required
- depending upon instrument design. For the purposes of the
present discussion and the appended claims, small diameter
columns are those having an internal diameter (I.D.) of
not more than about 3 millimeters. Larger diameter columns
may be used if desired, such as columns having an I.D.
of 25 or 50 millimeters but, generally one does not
use such large columns and it is preferrèd to use a column
with an I.D. not greater than about 10 millimeters.
In carrying out the present method, the sample
size selected is preferably rather small in order to
facilitate rapid determinations, to avoid unnecessary
exhaustion of the ion exchange beds, and further, to get
sharp peak concentrations of eluted ion pairs to be
detected at the detector. Further, since the detector's
available, especially the conductivity cell, are very
sensitive, sample size can indeed be quite small. A
syringe is conveniently used to inject the portion of the
sample solution into the sample injection valve which
measures out, e.g., from about 0.002 to about 1 milliliter
of the solution to be analyzed. More generally, the sample
size is in the range of about 10 to 50 microliters for
fresh surface waters and in the range of 2 to 10 micro-
liters in the case of sea water and brines. Typically,
conveniently and preferably the number of milliequivalents
of ionic material in the sample portion does not exceed
about 1/500, more preferably 1/2500, and even more pre-
ferably not more than 1/5000 of the total exchange capacity
of each of the ion exchange resin beds employed.
16,774-F -9-
While it is possible to add the sample portion
selected to the first ion exchange column manually and to
add water to the column from a pipet or buret or even a
beaker and to depend on gravity flow ope^rations, it is
much preferred for the sake of reproducibility, con-
venience and speed to use a pump and to supply with a
closed system a substantially continuous stream of water
as eluant and to carry out sample injection and elution
according to good current chromatographic or ion exchange
practice in which the eluant is used to sweep the sample
out of the sampling valve and on to the column. Typical
flow rates for the eluant water fall generally in the
range of about 20 to 500 milliliters per hour when the
columns used are sized in the range of about 2.8 to 9
millimeter I.D:
The ion exchange resins to be used in each of
the columns according to the present invention may each
be any of the generally used, preferably high capacity,
ion exchange resins commercially available such as Dowex
50W and Dowex 1 ion exchange resin. These ion ex-hange
resins are typically polystyrene or modified polystyrene
copolymers cross-linked, for example, with divinylbenzene,
and carrying nuclear groups, the latter providing the
active exchange sites. The strong acid cation exchange
resins carry nuclear sulfonic acid or sulfonate groups along
the polymer chain. The strong base anion exchange resins
carry nuclear chloromethyl groups which have been quater-
nized.
It must be understood that either or both of
the ion exchange beds used herein may, if desired, be
16,774-F -10-
lv~4~'~4
disposed in two columns per bed connected in series and providing the
requisite amount of ion exchange resin as would normally be provided
in one column per bed, and such two column beds are considered to
be merely an obvious equivalent of single column beds and within the
scope of the appended claims, no advantage accruing from such arrange-
ment. On the other hand, both resins may be disposed in separate
layers in a single column and such an arrangement is considered to
be within the scope of the appended claims.
The following examples serve to illustrate the use of the
method and apparatus of the invention and the scope of the invention
is not intended to be limited thereto.
Example 1
A 9 x 250 millimeter tmm) glass column was filled with
a commercial cation exchange, Dowex* 50WX8, 200-400 mesh in the sodium
ion form. A second column of the same dimensions was filled with
Dowex* lX8, 200-400 mesh in the hydroxide ion form and connected to
the first column. The first column was provided with a sample injection
valve, pump and eluant reservoir while the effluent from the second
column was directed to a conductivity cell and readout means all sub-
stantially following the scheme of the drawing. Samples of aqueous
solutions of sodium chloride of various concentrations were injected
to the first column by means of the sample injection valve and eluted
through both columns using deionized water as eluant at a flow rate of
460 ml/hr. The height of the eluter conductivity peaks were measured
*Trade Mark
104~
and found to be approximately proportional to the concen-
trations of the samples injected.
Example 2
Using the same columns and apparat-us of Example 1,
several samples of a waste treatment plant effluent obtained
on different dates were eluted using deionized water as
eluant and the heights of the eluted conductivity peaks
measured. By using injections of sodium chloride solutions
of accurately known concentration`, the heights of the
conductivity peaks of the waste control plant samples were
converted into equivalent ionic content (total ionic content).
These total ionic content values were then correlated with
the values for the total dissolved solids content of the
samples, obtained by evaporating filtered samples to dry-
ness and correcting for water of hydration in the dried
samples. These results are provided in the accompanying
table which illustrates the strong correlation between the
total ionic content as determined by the method described
herein and the total dissolved solids content determined
by evaporation and drying. The relative constancy of the
correlation factor attests to the strength of the correla-
tions.
16,774-F -12-
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16, 774-F -13-
