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The turbidimetric determination of sulfate without the use of additives
SHORT
COMMUNLCATIONS
The turbldimetric
327
determination
of sulfate without
the use of additives
Nearly all of the numerous methods described in the literature for the turbidimetric or nephelometric determination of sulfate involve the addition of various compounds to the turbid dispersion, ostensibly for stabilization of the dispersion. The most recent publication, in which no additives were used, seems to be that by HII~BAWJ in 19241; he used a solution of barium chloride as the precipitating agent, but the accuracy was poor (+ 18.7~/~error). Owrza seems to have been the first to use additives. HOCHSEGANG has reviewed this determination as well as nephelometry ancl turbidirnetry in generals. In the present note, an accurate turbidimctric determination of sulfate without additives is described and compared with the standard method”. The simplicity of the described method and the stability of barium sulfate dispersions formed without the use of additives refute the traditional necessity of additives.
Absorbance measurements using x-cm matched silica cells.
PhparaliorL
were made with a Beckman
DB spectrophotometer
of standard ctcrves
Pipet 25 ml of standard sulfate solution (prepared from A.R. potassium sulfate in deionized water), containing IO-IOO p.p.m. of sulfate, into a 5o-ml mixing cylinder. Add a scoop (ca. 0.25 6) of barium chloride dihydrate (20-30 mesh), stopper the Cylinder and shake for about 30 sec. After a minimum of 20 min, and not longer than z h, mix the contents again by inverting the cylinder several times. Transfer the turbid solution to a r-ctn cell and measure the absorbance at 480 nm. Use deionized water to set the instrument at xooo/o transmittance. The procedure previously used in this laboratory in the past was similar to the standard methoda. To a measured volume (or weight) of sample, add 15 ml of salt-acid solution (73.x 6 of sodium chloride and 20.7 ml of concentrated hydrochloric acid per liter), 25 ml of alcohol-glycerin solution (334 ml of glycerin, 666 ml of isopropanol and I 1 of deionized water) and make up to 75 ml in a roo-ml mixing cylinder with deionized water. Add I scoop of 20-30 mesh barium chloride dihydrate, invert the cylinder 22 times, let stand exactly 2 min and read the absorbance in a x-cm cell at Ggo nm.
RcstcZts and discttssioft Figure 3 shows working curves for the proposed and standard methods. The proposed method yields a straight line for the range IO-xoo p.p,m. whereas the method employing additives exhibits curvature. The line for the present method fits theequation: p.p.m. sulfate = absorbance ‘~02.5 +3. The accuracy of the method was checked with standard solutions of potassium sulfate and also by comparison with gravitnetric analysis of some phosphate rock samples (Table I), For the precision, eight determinations on a so-p.p.m. standard showed an average deviation of 1.0%. The stability of turbidities produced from 3 different concentrations of sulfate wasstudied by both methods. The suspensions were poured into a x-cm cell and the %T recorded VS. time (Fig. 2) ; the 4So-nm wavelength was used for both methods for the Anal.
CLim.
A&a,
42 (rgG8) 327-329
328
SHORT
COMMUNICATIONS
sake of comparison. In each case, a minimum in O/,X’is apparent. However, the time required to reach a minimum for the additive method varies from I0 to 50 min, whereas for the proposed method a minimum appears in about 20 min in each case i.e. the time required to form the maximum number of barium sulfate particles is nearly constant. It should be noted that at no time do all 3 concentrations exhibit
0
IO
20
50
40
80
ppn
Fig. I. Stan&ml TABLE ACCURACY
50:-
60
70
80
90
100
curves.
I DATA
I~OR DtYt%ttMlNATION
OF SULFATIS
Sywlketic slmdar&s +$.P.tn, Preset84
p+7ta.
X5.0 25.0
x5.1 24.6 38.7 49.4 P-7 87.5
40.0 50.0 72.0
00.0
i;btttrd
O/O B 0.7 1.G 3.2 I.2
I.8 2.8
Av.
m&/o
a minimum simultaneously when the standard method is used; this may explain the curvature of the working curve, The nlininlum in the 8o-p.p.m. curve does not last very long, but this posed no problem provided that the suspension was left in the mixing cylinder and reshakcn just before the absorbance was measured. For example, a suspension of IOO p,p.m. of sulfate showed a percentage transmittance Anal. Chim. Ada,
42 (x968)
327-321)
SHORT
COMMUNICATIONS
329
of x1.2 after 20-30 min; the value changed only to 11.4 after 280 rnin, when the suspension was shaken before the measurement. This stability is convenient for routine work. Figure 2 shows that additives serve little purpose, except possibly to decrease the solubility of barium sulfate and thus increase slightly the sensitivity of the method. If more sensitivity is required, longer cells can be used; d-cm cells were used with the proposed method and a straight line was obtained for the range 5-25 p.p.m.
Oar
WltN
ADDITIVES
WITHOUT
ADDITIVES
c
I5
(YIN)
TIME
Fig.
2.
Comparison
PPM
of the stability
of B&04
turbitlitics.
The effect of temperature was not studied, all the tests being made at room temperature (zz-2G”). Obviously, high temperatures should be avoided. The advantages of the proposed method are as follows: fewer manipulations and fewer reagents are required, it is not necessary to follow exactly an elaborate technique, and the timing of measurements is not critical. Research and Development Department, Contine9ttal Oit Company, Ponca City, Okta. (U.S.A.)
J. W.
WIMBERLEY
P. L. HIBBARD, J. I*rd. Eng. Chem.. IG (1924) 804. A.W. OWE, Kolloid-Z.. 32 (x923) 73. 3 I. M. KOLTHOFP AND P. J. ELVING, Trealise on ArtaZyticalCJretnislry, Part I, Vol. 5, Section D-3, 1964, p. 3289-3328. 4 Standard Methods for the Examination of Water and Wastewater, 11th Edn., American Public I
2
Health
(Received
Association
Inc.,
December
New
York,
p.
241442.
zrst, rgG7) Allal. C/rim. Acfa, 32 (x968)
327-329
COMMUNLCATIONS
The turbldimetric
327
determination
of sulfate without
the use of additives
Nearly all of the numerous methods described in the literature for the turbidimetric or nephelometric determination of sulfate involve the addition of various compounds to the turbid dispersion, ostensibly for stabilization of the dispersion. The most recent publication, in which no additives were used, seems to be that by HII~BAWJ in 19241; he used a solution of barium chloride as the precipitating agent, but the accuracy was poor (+ 18.7~/~error). Owrza seems to have been the first to use additives. HOCHSEGANG has reviewed this determination as well as nephelometry ancl turbidirnetry in generals. In the present note, an accurate turbidimctric determination of sulfate without additives is described and compared with the standard method”. The simplicity of the described method and the stability of barium sulfate dispersions formed without the use of additives refute the traditional necessity of additives.
Absorbance measurements using x-cm matched silica cells.
PhparaliorL
were made with a Beckman
DB spectrophotometer
of standard ctcrves
Pipet 25 ml of standard sulfate solution (prepared from A.R. potassium sulfate in deionized water), containing IO-IOO p.p.m. of sulfate, into a 5o-ml mixing cylinder. Add a scoop (ca. 0.25 6) of barium chloride dihydrate (20-30 mesh), stopper the Cylinder and shake for about 30 sec. After a minimum of 20 min, and not longer than z h, mix the contents again by inverting the cylinder several times. Transfer the turbid solution to a r-ctn cell and measure the absorbance at 480 nm. Use deionized water to set the instrument at xooo/o transmittance. The procedure previously used in this laboratory in the past was similar to the standard methoda. To a measured volume (or weight) of sample, add 15 ml of salt-acid solution (73.x 6 of sodium chloride and 20.7 ml of concentrated hydrochloric acid per liter), 25 ml of alcohol-glycerin solution (334 ml of glycerin, 666 ml of isopropanol and I 1 of deionized water) and make up to 75 ml in a roo-ml mixing cylinder with deionized water. Add I scoop of 20-30 mesh barium chloride dihydrate, invert the cylinder 22 times, let stand exactly 2 min and read the absorbance in a x-cm cell at Ggo nm.
RcstcZts and discttssioft Figure 3 shows working curves for the proposed and standard methods. The proposed method yields a straight line for the range IO-xoo p.p,m. whereas the method employing additives exhibits curvature. The line for the present method fits theequation: p.p.m. sulfate = absorbance ‘~02.5 +3. The accuracy of the method was checked with standard solutions of potassium sulfate and also by comparison with gravitnetric analysis of some phosphate rock samples (Table I), For the precision, eight determinations on a so-p.p.m. standard showed an average deviation of 1.0%. The stability of turbidities produced from 3 different concentrations of sulfate wasstudied by both methods. The suspensions were poured into a x-cm cell and the %T recorded VS. time (Fig. 2) ; the 4So-nm wavelength was used for both methods for the Anal.
CLim.
A&a,
42 (rgG8) 327-329
328
SHORT
COMMUNICATIONS
sake of comparison. In each case, a minimum in O/,X’is apparent. However, the time required to reach a minimum for the additive method varies from I0 to 50 min, whereas for the proposed method a minimum appears in about 20 min in each case i.e. the time required to form the maximum number of barium sulfate particles is nearly constant. It should be noted that at no time do all 3 concentrations exhibit
0
IO
20
50
40
80
ppn
Fig. I. Stan&ml TABLE ACCURACY
50:-
60
70
80
90
100
curves.
I DATA
I~OR DtYt%ttMlNATION
OF SULFATIS
Sywlketic slmdar&s +$.P.tn, Preset84
p+7ta.
X5.0 25.0
x5.1 24.6 38.7 49.4 P-7 87.5
40.0 50.0 72.0
00.0
i;btttrd
O/O B 0.7 1.G 3.2 I.2
I.8 2.8
Av.
m&/o
a minimum simultaneously when the standard method is used; this may explain the curvature of the working curve, The nlininlum in the 8o-p.p.m. curve does not last very long, but this posed no problem provided that the suspension was left in the mixing cylinder and reshakcn just before the absorbance was measured. For example, a suspension of IOO p,p.m. of sulfate showed a percentage transmittance Anal. Chim. Ada,
42 (x968)
327-321)
SHORT
COMMUNICATIONS
329
of x1.2 after 20-30 min; the value changed only to 11.4 after 280 rnin, when the suspension was shaken before the measurement. This stability is convenient for routine work. Figure 2 shows that additives serve little purpose, except possibly to decrease the solubility of barium sulfate and thus increase slightly the sensitivity of the method. If more sensitivity is required, longer cells can be used; d-cm cells were used with the proposed method and a straight line was obtained for the range 5-25 p.p.m.
Oar
WltN
ADDITIVES
WITHOUT
ADDITIVES
c
I5
(YIN)
TIME
Fig.
2.
Comparison
PPM
of the stability
of B&04
turbitlitics.
The effect of temperature was not studied, all the tests being made at room temperature (zz-2G”). Obviously, high temperatures should be avoided. The advantages of the proposed method are as follows: fewer manipulations and fewer reagents are required, it is not necessary to follow exactly an elaborate technique, and the timing of measurements is not critical. Research and Development Department, Contine9ttal Oit Company, Ponca City, Okta. (U.S.A.)
J. W.
WIMBERLEY
P. L. HIBBARD, J. I*rd. Eng. Chem.. IG (1924) 804. A.W. OWE, Kolloid-Z.. 32 (x923) 73. 3 I. M. KOLTHOFP AND P. J. ELVING, Trealise on ArtaZyticalCJretnislry, Part I, Vol. 5, Section D-3, 1964, p. 3289-3328. 4 Standard Methods for the Examination of Water and Wastewater, 11th Edn., American Public I
2
Health
(Received
Association
Inc.,
December
New
York,
p.
241442.
zrst, rgG7) Allal. C/rim. Acfa, 32 (x968)
327-329