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Coulometric investigation on the use of single crystals of sodium chloride as a primary standard
Adyrlca
Chimlco
Acre
185
Publishing Company. Amsterdam Printed in The Netherbands
Elscvier
COULOMETRIC INVESTIGATION ON THE USE OF SINGLE OF SODIUM CHLORIDE AS A PRIMARY STANDARD TAKAYOSHI
YOSHIMORI
Faculty o/Engineering,
AND TATSUHIKO
Science Uniwrsity
of Tokyo,
CRYSTALS
TANAKA
Kuguruzoku,
Shinjttku-ku.
Tokyo
(Japan)
(Received 10th Decemder 1970)
Sodium chloride is generally used as the primary standard substance for argentimetry or mercurimetry. Several methods for drying the reagent are, however, shown in the literature. Marinenko and Taylor’ dried the reagent at 110” for 3 h in their excellent investigation on the determination of its purity by the precise cbulometric titration. According to the Japanese Industrial Standard (JIS)‘, the reagent should be ignited in a platinum crucible at 500-650” for 40-50 min. Kolthoff and Sandel13 recommend the same method as JIS. These procedures indicate that the drying of the reagent is somewhat troublesome. In the experience of the present authors, each weighing bottle containing the aliquot of the hot salt should be cooled in its own desiccator. Ifa desiccator containing many weighing bottles is opened to remove the first aliquot, moisture is introduced into the desiccator and is adsorbed by the other samples in it. Single crystals ofsodium chloride synthesized froma melt of the salt are readily available as the cell or prism for infrared absorption spectroscopy. It seemed probable that the crystal has sufficient purity for use as a primary standard, and also little water is adsorbed on its surface. The crystal, however, has not been used for this purpose. Taylor and Smith4 showed that drying of the coarse crystal is unnecessary. Bates and Wichers5 prepared single crystals of benzoic acid and compared their purity against primary-standard potassium hydrogen phthalate (NBS Standard Sample 84d) by differential titration. Comparison of their result with that obtained by Eckfeldt and Shaffer6 (the same potassium hydrogen phthalate was analyzed by precise coulometric titration) indicates that the purity of the above crystal was somewhat less than 100°/O. Rokosz’ has prepared single crystals of potassium hydrogenphthalate, but these crystals have not been analyzed and nor have their purities been compared with those of the other standard reagents. These facts indicate that single crystals cannot automatically be regarded as “pure” substances. In this investigation, the purities of single crystals of sodium chloride were determined by precise coulometric titration, and the adsorbed water on their surfaces was also determined coulometrically. EXPERIMENTAL
Apparatus and reagents The instruments for the coulometric generation of the titrants were similar to those shown previouslye. The assembly and the circuit for potentiometric end-point Anal. Chim. Acta,
55 (1971) 185-191
186
T. YOSHIMORI,
T. TANAKA
location are shown in Fig. 1. The IR drop through a standard resistor (10 ohm) was measured with a precise potentiometer. From the certificated values ofthese measuring devices, it could be expected that the standard deviation of the error in the measurement of the generating current was less than 0.005°/o.As the Faraday constant, 96487.2 coulomb was used. A 50-Hz oscillator based on the frequency of a quartz crystal and a cycle cotinter were used to measure the time interval of the electrolysisg. All samples were weighed with a microbalance. A weighing bottle of nearly the same weight as that containing the sample was used as a counterpoise.All weights were corrected for absolute weights, and all weighings were corrected for air buoyancy.
Fig. 1. Apparatus. (mv).
(1) Constant
current
source;
(2) potentiometer;
(3)stabilizer
; (4) timer : (5) potcntiomctcr
The end-point of the titration was located potentiometrically with the circuit shown in Fig. 1. Because the measuring range of the potentiometer was not enough to measure the potential difference between a silver electrode and a saturated calomel electrode, the applied voltage device shown in Fig. 1 was inserted into this circuit. A tall-form beaker (about 650 ml) was used as the electrolytic cell. Its outside was painted red in order to protect silver salts from light. The generator anode was a pure silver plate of large area (10 x 11 cm and about 0.1 mm thick), and the generator cathode was a platinum spiral (0.5 mm diameter and 50 mm length). The diaphragm under the cathode compartment was prepared as follows: the bottom of a polyethylene tube was closed with a polyethylene plate which had many fine holes. An agaragar gel saturated with potassium nitrate was plugged on the bottom, the thickness of the layer being about 15 mm. All reagents were of analytical grade, and were used without further purilication. A 50% (v/v) methanol solution which was 1 A4 in sodium nitrate and 1 M in acetic acid was used as the anolyte. Procedure Preparation Anal. Chim. Acta,
of sample. The sample
55 (1971) 185-191
was a synthetic
optical crystal of sodium
SINGLE-CRYSTAL
SODIUM
CHLORIDE
AS PRIMARY
187
STANDARD
chloride, which was manufactured by the Stockbarger method (Horiba Ltd., Japan). The single crystal was broken into pieces about 0.2 g in weight, and the surfaces of the pieces were polished with an ethanolic chromium(II1) oxide emulsion. After being washed with ethanol, the crystals were wiped with chamois. In order to test the purity, the sample prepared as above was stored, without heating, in a desiccator at room temperature for about 3 h. Magnesium perchlorate was used as a desiccant throughout this investigation. Coulometric titration. .Nitric acid (1 M) saturated with sodium nitrate were poured into the cathode compartment, and then about 0.5 peq of silver nitrate solution was added to it in order to precipitate any traces of halides or other impurities which react with silver ion. About 600 ml of the anolyte specified above was placed into the cell. In order to remove dissolved oxygen from the anolyte, nitrqgen was passed through the solution for about 1 11.Then the gas was allowed to flow over the anolyte throughout the titration. As a pretitration, a solution containing about 3 mg of sodium chloride was added to the cell, and this was electrolyzed at a constant current of about 25 mA. Near the end-point of the titration, the generating current was cut off and the potential of the indicator electrode was measured with the potentiometer. Then the solution was further electrolyzed and the potential was measured again, the small increments of electrolysis being repeated until after the end-point; the titration was then continued forabout 10secor more.Theend-point ofthe titration waslocated from the differential titration curve. Figure 2 shows an example of the curve. After the pretitration had been completed, about 95% of the silver ion required to react with the weighed sodium chloride was generated in the electrolyte at a constant current of about 129 mA.The weighed sample was then introduced into the cell and dissolved by stirring. After the complete dissolution of the sample, the residual chloride ion was titrated with the electrolytically generated silver ion. In 40
1.0
10
0.7
2!
1
2560
lime,
Fig. 2. Potentiometric
2
2570 set
titration
curve
0
and diffcrcntial
titration
curve of single Anal.
crystal.
Chim. Acta, 55 (1971)
185-191
188 TABLE VARIANCE
Method
T. YOSHIMORI,
T. TANAKA
I OF WEIGHTS
OF FOUR
of drying
Room temp., Mg(C10.J2 desiccator llO=‘, 3.5 h 600°; 50 min
SINGLE
CRYSTALS
DRIED
BY DIFFERENT
METHODS
(ill
1
2
3
4
506.154
583.001
142.223
144.315
506.149 506.150
582.996 582.995
142.222 142.222
144.313 144.311
Xllg)
this case, a constant current of 25 mA was used to generate the titrant. The end-point was located by the same method as that used for the pretitration. The time of electrolysis was measured graphically from the distance between the peak of this differential titration curve and that of the pretitration. The amount of electricity used throughout the main titration was calculated from the electrolytic time and the current. RESULTS AND
DISCUSSION
Water
on the surfaces of single crystals The loss in weight of single crystals was lirst measured under various drying conditions. The results obtained (Table I) indicate that the adsorbed water on the surface of the crystal could be removed simply by storage of the sample in a desiccator at room temperature for a few hours. Next, the adsorbed water on the crystal was determined by carrier gas extraction followed by coulometric titration as described previously”. The sample crystal was stored in the desiccator at room temperature for about 3 h, and the heating temperature for generation of water from the sample was 6OOO.The results obtained are shown in Table II. The amounts of water obtained by this method were nearly consistent with those in Table I. These results indicate that satisfactory dehydration of these single crystals can be obtained by storage in a desiccator containing magnesium perchlorate at room temperature for more than few hours. Determination of the purity of the crystal At the start of this investigation, the weighed sample was dissolved with water TABLE
II
DETERMINATION
OF ADSORBED
WATER
Sample weight (w)
Surface area (cm~)
148.216
1.0222 1.0272 0.9108 0.8850
152.627 127.979 122.591
Anal. Chim. Acta, 55 (1971) 185-191
ON SURFACES
OF SINGLE
H =O fowrd (Kl cm-7 0.437 0.547 0.369 0.570 Mean 0.481
CRYSTALS
SINGLE-CRYSTAL TABLE PURITY
SODIUM
CHLORIDE
AS PRIMARY
STANDARD
189
III TESTS OF SINGLE
Somple
I 2 3
No.
of
CRYSTALS
detns.
Meun valire purity (‘4)
6 6 14
99.988 1 99.9932 99.9949
of 0.0143 0.0138 0.0 I 24”
D V- ’ value.
in a small beaker and then transferred to the cell ; with this method the results obtained were always unfavorable. When the solid sample was introduced into the cell, it could be completely dissolved in the electrolyte by stirring for about 20 min. This technique prevented any errors from transference of the solution and from oxygen or other impurities in the washing water. Three single crystals were divided into small portions (150-200 mg) and their surfaces were polished before storage in the desiccator at room temperature. The purities of the samples thus prepared were determined by the above-mentioned procedure. The results obtained are shown in Table III. It may be considered from the results that the original single crystals had somewhat different purities, but each original crystal was homogeneous enough. The deviations of the results are discussed below. The purities of these single crystals were somewhat less than lOO”/,.From the results in Tables I and II, less than 0.002 mg of adsorbed water was present on the surfaces of the crystals, which is not enough to balance the lower results in Table III. Impurities in the crystals were next considered. Firstly, the acidic materials in a crystal were tested by the following method’. A part of the single crystal (2 g) was dissolved in 40 ml of water freed from carbon dioxide and ammonia. The pH value of this solution was 5.3. A solution ofa crystal which had been heated at 600° for 50 min showed a pH value of 5.6. The pH value of a solution prepared by the same method from the primary-standard substance (certificated by the Industrial Inspection Institute, Japan)was 5.9.Theseresults indicate that the crystaland thestandard reagent contain some acidic impurities, of which the contents and the species are unknown, because the pH of the water used for dissolution was 6.3. Secondly, the crystals were analyzed qualitatively by a spectroscopic method. These analyses showed that the crystals contained an appreciable amount of calcium and aluminum and also trace amounts of magnesium, silicon and silver. The amounts of these impurities may be enough to balance the results in Table III. When the accuracy of the ordinary titrimetric procedure is considered, the contents of the impurities are small enough for the crystals to be used as standard materials for argentimetry. The standard deviations of the results were somewhat larger than those reported previously for potassium dichromate 1 l. The detection of the end-point may be the main source of the deviation because of the slight potential break through the end-point of the titration. Comparing these standard deviations with the results obtained by Marinenko and Taylor’, the authors consider that the main source of error is not the inhomogeneity of each crystal but the location of the end-point of the titration. Awl.
C@n.
Acto. 55 (1971)
185-191
190
T. YOSHIMORI,
T. TANAKA
In order to determine the purity, crystals of about 3 meq were used as sample. When the crystal was smaller than 100 mg, the weighing error was increased. With larger crystals (more than 0.3 g), high results were sometimes obtained under these experimental conditions ; silver-rich precipitates of non-stoichiometric composition were perhaps produced first, and then the precipitates gradually changed to stoichiometric composition. This resulted in an unstable potential of the indicator electrode, and an unreasonably long time was needed in order to obtain the equilibrium potential of the electrode. The interference of the condenser current on the generating electrode was also considered and investigated by the following method. The purity of the crystal was determined by the above method, but the generating current was cut 50 or 100 times during the electrolysis. The results obtained were 100.006 or 100.016°! respectively, which indicated that some influence should be considered. If the generating current was cut for too long a time in order to measure the potential of the indicator electrode, some part of it could be used to recharge the electric double layer on the generating electrode. Therefore the current efficiency may be somewhat decreased, when the area of generating electrode is extremely large and the current is cut too often. In ordinary experiments, however, about 10 measurements of the potential of the indicator electrode were enough to locate the end-point of the titration. Accordingly, the interference caused by the condenser current was negligibly small. CONCLUSION
Single crystals of sodium chloride are of suffkient purity (99.997< or more) and homogeneity, for use as the primary standard substance in argentimetry or mercurimetry. The adsorbed water on the surfaces is negligible, when polished crystals are stored in a desiccator over magnesium perchlorate. The authors are grateful to Mr. S. Ishiwari of Hitachi Co. Ltd., for his valuable support of this investigation and to Mr. T. Harada for technical assistance. SUMMARY
The use of single crystals of sodium chloride as a primary standard substance for argentimetry is described. The water on the surface and the purity of the crystal are measured by coulometric titration methods. The adsorbed water amounts to 0.48 ,ug cmB2 when the sample is dried in a desiccator containing magnesium perchlorate at room temperature for about 3 h. The loss in weight of the crystal is negligible when it is heated at about 6OOO.The purities of three crystals, by precise coulometric titrations, are 99.988,99.993 and 99.995%, with standard deviations of 0.0143, 0.0138 and O.O124OA,respectively. It can be concluded that the crystal is useful as a primary standard.
On propose l’utilisation d’un cristal unique de chlorure de sodium comme &talon primaire en argentimetrie. L’eau en surface et la purete du cristal sont deterAnd. China. Acta, 55 (I 971) 185-191
SINGLE-CRYSTAL
SODIUM
CHLORLDE
AS PRIMARY
191
STANDARD
mindes par titrage coulomCtrique. La quantitC d’eau adsorbte est de 0.48 jig crno2 lorsque 1’Cchantillon est sCchC dam un dessicateur contenant du perchlorate de magnbium, B la tempdrature ordinaire, pendant 3 h. La perte en poids du cristal est ndgligeable lorsqu’il est chauffe B 600° environ. Les pure& de trois cristaux, dtterminCes par titrage coulomCtrique, sont respectivement 99,988, 99.993 et 99.995 “/,. avec une dCviation standard de 0.0143, 0.0138 et 0.0124’y0. On peut conclure que le cristal est utilisable comme &alon primaire. ZUSAMMENFASSUNG
Es wird die Verwendungvon Natriumchlorid-Einkristallen als Urtitersubstanz fiir die Argentimetrie beschrieben. Das Wasser an der Oberflsche und die Reinheit des Kristalls werden durch coulometrische Titration bestimmt. Die adsorbierten Wassermengen betragen bis 0.48 c(g cm - 2, wenn die Probe in einem Exsikkator iiber Magnesiumperchlorat bei Raumtemperatur etwa 3 h lang getrocknet wird. Der Gewichtsverlust des Kristalls ist vernachl%ssigbar, wenn er auf etwa 600” erhitzt wird. Die durch genaue coulometrische Titrationen bestimmten Reinheiten von drei Kristallen sind 99.988,99.993 und 99.995 “/, mit Standardabweichungen von 0.0143, 0.0138 und 0.0124”/ Es kann gefolgert werden, dass sich der Kristall als Urtitersubstanz eignet. REFERENCES 1 G. MARIN~NKO ANV J. K. TAYLOH. J. Rcs. Nd. hr. Sari.. 67 A (1963) 31. 2 JIS (Japanese Industrial Standard), K 800.5. 1966. 3 I. M. KOLT~IOFFAND E. B. SANDELL. Textbook of Qlrcrrrtirrrtiw Inorgurtic A~rdysis. 3rd Edn.. Mncmillan, New York, 1952, p. 541. 4 J. K. TAYLOR ANI) S. W. SMITH. Science, 124 (1956) 940. 5 R. G. BAT= AND E. WICFIEKS, J. RCS. Nut/. Bur. Std.. 59 (1957) 9. 6 E. L. ECKFEZLDT AND E. W. SHAFMR, JR.. Anal. C/uwr.. 37 (1965) 1534. 7 A. ROICOSZ. Jagiclloniun Univcrsily. Krakow, private communication. 8 T. YOSHIMORI AND I. MATSUDARA. BUN. Clretrt. Sot. Jupurr. 43 (1970) 2800. 9 T. MIWA. T. YOSHIMORI AND T. TAKEUCIII, J. Chcm. Sm. Jupm, ftrrl. C/rem. Sect., 67 (1964) 2045. 10 T. YOSHIMORI ANI) S. ISHIWAKI. Oull. Ciwrrr. SW. Jupuu. 42 (1969) 1282. I1 T. YOSNMORI. I. MATSUIIARA, K. HIROSAWA AND T. TANAKA. Jupurr Am~/_vsr, 19 (1970) 681.
hul.
Cltitrt. Aclu,
55 (1971)
185-191
Chimlco
Acre
185
Publishing Company. Amsterdam Printed in The Netherbands
Elscvier
COULOMETRIC INVESTIGATION ON THE USE OF SINGLE OF SODIUM CHLORIDE AS A PRIMARY STANDARD TAKAYOSHI
YOSHIMORI
Faculty o/Engineering,
AND TATSUHIKO
Science Uniwrsity
of Tokyo,
CRYSTALS
TANAKA
Kuguruzoku,
Shinjttku-ku.
Tokyo
(Japan)
(Received 10th Decemder 1970)
Sodium chloride is generally used as the primary standard substance for argentimetry or mercurimetry. Several methods for drying the reagent are, however, shown in the literature. Marinenko and Taylor’ dried the reagent at 110” for 3 h in their excellent investigation on the determination of its purity by the precise cbulometric titration. According to the Japanese Industrial Standard (JIS)‘, the reagent should be ignited in a platinum crucible at 500-650” for 40-50 min. Kolthoff and Sandel13 recommend the same method as JIS. These procedures indicate that the drying of the reagent is somewhat troublesome. In the experience of the present authors, each weighing bottle containing the aliquot of the hot salt should be cooled in its own desiccator. Ifa desiccator containing many weighing bottles is opened to remove the first aliquot, moisture is introduced into the desiccator and is adsorbed by the other samples in it. Single crystals ofsodium chloride synthesized froma melt of the salt are readily available as the cell or prism for infrared absorption spectroscopy. It seemed probable that the crystal has sufficient purity for use as a primary standard, and also little water is adsorbed on its surface. The crystal, however, has not been used for this purpose. Taylor and Smith4 showed that drying of the coarse crystal is unnecessary. Bates and Wichers5 prepared single crystals of benzoic acid and compared their purity against primary-standard potassium hydrogen phthalate (NBS Standard Sample 84d) by differential titration. Comparison of their result with that obtained by Eckfeldt and Shaffer6 (the same potassium hydrogen phthalate was analyzed by precise coulometric titration) indicates that the purity of the above crystal was somewhat less than 100°/O. Rokosz’ has prepared single crystals of potassium hydrogenphthalate, but these crystals have not been analyzed and nor have their purities been compared with those of the other standard reagents. These facts indicate that single crystals cannot automatically be regarded as “pure” substances. In this investigation, the purities of single crystals of sodium chloride were determined by precise coulometric titration, and the adsorbed water on their surfaces was also determined coulometrically. EXPERIMENTAL
Apparatus and reagents The instruments for the coulometric generation of the titrants were similar to those shown previouslye. The assembly and the circuit for potentiometric end-point Anal. Chim. Acta,
55 (1971) 185-191
186
T. YOSHIMORI,
T. TANAKA
location are shown in Fig. 1. The IR drop through a standard resistor (10 ohm) was measured with a precise potentiometer. From the certificated values ofthese measuring devices, it could be expected that the standard deviation of the error in the measurement of the generating current was less than 0.005°/o.As the Faraday constant, 96487.2 coulomb was used. A 50-Hz oscillator based on the frequency of a quartz crystal and a cycle cotinter were used to measure the time interval of the electrolysisg. All samples were weighed with a microbalance. A weighing bottle of nearly the same weight as that containing the sample was used as a counterpoise.All weights were corrected for absolute weights, and all weighings were corrected for air buoyancy.
Fig. 1. Apparatus. (mv).
(1) Constant
current
source;
(2) potentiometer;
(3)stabilizer
; (4) timer : (5) potcntiomctcr
The end-point of the titration was located potentiometrically with the circuit shown in Fig. 1. Because the measuring range of the potentiometer was not enough to measure the potential difference between a silver electrode and a saturated calomel electrode, the applied voltage device shown in Fig. 1 was inserted into this circuit. A tall-form beaker (about 650 ml) was used as the electrolytic cell. Its outside was painted red in order to protect silver salts from light. The generator anode was a pure silver plate of large area (10 x 11 cm and about 0.1 mm thick), and the generator cathode was a platinum spiral (0.5 mm diameter and 50 mm length). The diaphragm under the cathode compartment was prepared as follows: the bottom of a polyethylene tube was closed with a polyethylene plate which had many fine holes. An agaragar gel saturated with potassium nitrate was plugged on the bottom, the thickness of the layer being about 15 mm. All reagents were of analytical grade, and were used without further purilication. A 50% (v/v) methanol solution which was 1 A4 in sodium nitrate and 1 M in acetic acid was used as the anolyte. Procedure Preparation Anal. Chim. Acta,
of sample. The sample
55 (1971) 185-191
was a synthetic
optical crystal of sodium
SINGLE-CRYSTAL
SODIUM
CHLORIDE
AS PRIMARY
187
STANDARD
chloride, which was manufactured by the Stockbarger method (Horiba Ltd., Japan). The single crystal was broken into pieces about 0.2 g in weight, and the surfaces of the pieces were polished with an ethanolic chromium(II1) oxide emulsion. After being washed with ethanol, the crystals were wiped with chamois. In order to test the purity, the sample prepared as above was stored, without heating, in a desiccator at room temperature for about 3 h. Magnesium perchlorate was used as a desiccant throughout this investigation. Coulometric titration. .Nitric acid (1 M) saturated with sodium nitrate were poured into the cathode compartment, and then about 0.5 peq of silver nitrate solution was added to it in order to precipitate any traces of halides or other impurities which react with silver ion. About 600 ml of the anolyte specified above was placed into the cell. In order to remove dissolved oxygen from the anolyte, nitrqgen was passed through the solution for about 1 11.Then the gas was allowed to flow over the anolyte throughout the titration. As a pretitration, a solution containing about 3 mg of sodium chloride was added to the cell, and this was electrolyzed at a constant current of about 25 mA. Near the end-point of the titration, the generating current was cut off and the potential of the indicator electrode was measured with the potentiometer. Then the solution was further electrolyzed and the potential was measured again, the small increments of electrolysis being repeated until after the end-point; the titration was then continued forabout 10secor more.Theend-point ofthe titration waslocated from the differential titration curve. Figure 2 shows an example of the curve. After the pretitration had been completed, about 95% of the silver ion required to react with the weighed sodium chloride was generated in the electrolyte at a constant current of about 129 mA.The weighed sample was then introduced into the cell and dissolved by stirring. After the complete dissolution of the sample, the residual chloride ion was titrated with the electrolytically generated silver ion. In 40
1.0
10
0.7
2!
1
2560
lime,
Fig. 2. Potentiometric
2
2570 set
titration
curve
0
and diffcrcntial
titration
curve of single Anal.
crystal.
Chim. Acta, 55 (1971)
185-191
188 TABLE VARIANCE
Method
T. YOSHIMORI,
T. TANAKA
I OF WEIGHTS
OF FOUR
of drying
Room temp., Mg(C10.J2 desiccator llO=‘, 3.5 h 600°; 50 min
SINGLE
CRYSTALS
DRIED
BY DIFFERENT
METHODS
(ill
1
2
3
4
506.154
583.001
142.223
144.315
506.149 506.150
582.996 582.995
142.222 142.222
144.313 144.311
Xllg)
this case, a constant current of 25 mA was used to generate the titrant. The end-point was located by the same method as that used for the pretitration. The time of electrolysis was measured graphically from the distance between the peak of this differential titration curve and that of the pretitration. The amount of electricity used throughout the main titration was calculated from the electrolytic time and the current. RESULTS AND
DISCUSSION
Water
on the surfaces of single crystals The loss in weight of single crystals was lirst measured under various drying conditions. The results obtained (Table I) indicate that the adsorbed water on the surface of the crystal could be removed simply by storage of the sample in a desiccator at room temperature for a few hours. Next, the adsorbed water on the crystal was determined by carrier gas extraction followed by coulometric titration as described previously”. The sample crystal was stored in the desiccator at room temperature for about 3 h, and the heating temperature for generation of water from the sample was 6OOO.The results obtained are shown in Table II. The amounts of water obtained by this method were nearly consistent with those in Table I. These results indicate that satisfactory dehydration of these single crystals can be obtained by storage in a desiccator containing magnesium perchlorate at room temperature for more than few hours. Determination of the purity of the crystal At the start of this investigation, the weighed sample was dissolved with water TABLE
II
DETERMINATION
OF ADSORBED
WATER
Sample weight (w)
Surface area (cm~)
148.216
1.0222 1.0272 0.9108 0.8850
152.627 127.979 122.591
Anal. Chim. Acta, 55 (1971) 185-191
ON SURFACES
OF SINGLE
H =O fowrd (Kl cm-7 0.437 0.547 0.369 0.570 Mean 0.481
CRYSTALS
SINGLE-CRYSTAL TABLE PURITY
SODIUM
CHLORIDE
AS PRIMARY
STANDARD
189
III TESTS OF SINGLE
Somple
I 2 3
No.
of
CRYSTALS
detns.
Meun valire purity (‘4)
6 6 14
99.988 1 99.9932 99.9949
of 0.0143 0.0138 0.0 I 24”
D V- ’ value.
in a small beaker and then transferred to the cell ; with this method the results obtained were always unfavorable. When the solid sample was introduced into the cell, it could be completely dissolved in the electrolyte by stirring for about 20 min. This technique prevented any errors from transference of the solution and from oxygen or other impurities in the washing water. Three single crystals were divided into small portions (150-200 mg) and their surfaces were polished before storage in the desiccator at room temperature. The purities of the samples thus prepared were determined by the above-mentioned procedure. The results obtained are shown in Table III. It may be considered from the results that the original single crystals had somewhat different purities, but each original crystal was homogeneous enough. The deviations of the results are discussed below. The purities of these single crystals were somewhat less than lOO”/,.From the results in Tables I and II, less than 0.002 mg of adsorbed water was present on the surfaces of the crystals, which is not enough to balance the lower results in Table III. Impurities in the crystals were next considered. Firstly, the acidic materials in a crystal were tested by the following method’. A part of the single crystal (2 g) was dissolved in 40 ml of water freed from carbon dioxide and ammonia. The pH value of this solution was 5.3. A solution ofa crystal which had been heated at 600° for 50 min showed a pH value of 5.6. The pH value of a solution prepared by the same method from the primary-standard substance (certificated by the Industrial Inspection Institute, Japan)was 5.9.Theseresults indicate that the crystaland thestandard reagent contain some acidic impurities, of which the contents and the species are unknown, because the pH of the water used for dissolution was 6.3. Secondly, the crystals were analyzed qualitatively by a spectroscopic method. These analyses showed that the crystals contained an appreciable amount of calcium and aluminum and also trace amounts of magnesium, silicon and silver. The amounts of these impurities may be enough to balance the results in Table III. When the accuracy of the ordinary titrimetric procedure is considered, the contents of the impurities are small enough for the crystals to be used as standard materials for argentimetry. The standard deviations of the results were somewhat larger than those reported previously for potassium dichromate 1 l. The detection of the end-point may be the main source of the deviation because of the slight potential break through the end-point of the titration. Comparing these standard deviations with the results obtained by Marinenko and Taylor’, the authors consider that the main source of error is not the inhomogeneity of each crystal but the location of the end-point of the titration. Awl.
C@n.
Acto. 55 (1971)
185-191
190
T. YOSHIMORI,
T. TANAKA
In order to determine the purity, crystals of about 3 meq were used as sample. When the crystal was smaller than 100 mg, the weighing error was increased. With larger crystals (more than 0.3 g), high results were sometimes obtained under these experimental conditions ; silver-rich precipitates of non-stoichiometric composition were perhaps produced first, and then the precipitates gradually changed to stoichiometric composition. This resulted in an unstable potential of the indicator electrode, and an unreasonably long time was needed in order to obtain the equilibrium potential of the electrode. The interference of the condenser current on the generating electrode was also considered and investigated by the following method. The purity of the crystal was determined by the above method, but the generating current was cut 50 or 100 times during the electrolysis. The results obtained were 100.006 or 100.016°! respectively, which indicated that some influence should be considered. If the generating current was cut for too long a time in order to measure the potential of the indicator electrode, some part of it could be used to recharge the electric double layer on the generating electrode. Therefore the current efficiency may be somewhat decreased, when the area of generating electrode is extremely large and the current is cut too often. In ordinary experiments, however, about 10 measurements of the potential of the indicator electrode were enough to locate the end-point of the titration. Accordingly, the interference caused by the condenser current was negligibly small. CONCLUSION
Single crystals of sodium chloride are of suffkient purity (99.997< or more) and homogeneity, for use as the primary standard substance in argentimetry or mercurimetry. The adsorbed water on the surfaces is negligible, when polished crystals are stored in a desiccator over magnesium perchlorate. The authors are grateful to Mr. S. Ishiwari of Hitachi Co. Ltd., for his valuable support of this investigation and to Mr. T. Harada for technical assistance. SUMMARY
The use of single crystals of sodium chloride as a primary standard substance for argentimetry is described. The water on the surface and the purity of the crystal are measured by coulometric titration methods. The adsorbed water amounts to 0.48 ,ug cmB2 when the sample is dried in a desiccator containing magnesium perchlorate at room temperature for about 3 h. The loss in weight of the crystal is negligible when it is heated at about 6OOO.The purities of three crystals, by precise coulometric titrations, are 99.988,99.993 and 99.995%, with standard deviations of 0.0143, 0.0138 and O.O124OA,respectively. It can be concluded that the crystal is useful as a primary standard.
On propose l’utilisation d’un cristal unique de chlorure de sodium comme &talon primaire en argentimetrie. L’eau en surface et la purete du cristal sont deterAnd. China. Acta, 55 (I 971) 185-191
SINGLE-CRYSTAL
SODIUM
CHLORLDE
AS PRIMARY
191
STANDARD
mindes par titrage coulomCtrique. La quantitC d’eau adsorbte est de 0.48 jig crno2 lorsque 1’Cchantillon est sCchC dam un dessicateur contenant du perchlorate de magnbium, B la tempdrature ordinaire, pendant 3 h. La perte en poids du cristal est ndgligeable lorsqu’il est chauffe B 600° environ. Les pure& de trois cristaux, dtterminCes par titrage coulomCtrique, sont respectivement 99,988, 99.993 et 99.995 “/,. avec une dCviation standard de 0.0143, 0.0138 et 0.0124’y0. On peut conclure que le cristal est utilisable comme &alon primaire. ZUSAMMENFASSUNG
Es wird die Verwendungvon Natriumchlorid-Einkristallen als Urtitersubstanz fiir die Argentimetrie beschrieben. Das Wasser an der Oberflsche und die Reinheit des Kristalls werden durch coulometrische Titration bestimmt. Die adsorbierten Wassermengen betragen bis 0.48 c(g cm - 2, wenn die Probe in einem Exsikkator iiber Magnesiumperchlorat bei Raumtemperatur etwa 3 h lang getrocknet wird. Der Gewichtsverlust des Kristalls ist vernachl%ssigbar, wenn er auf etwa 600” erhitzt wird. Die durch genaue coulometrische Titrationen bestimmten Reinheiten von drei Kristallen sind 99.988,99.993 und 99.995 “/, mit Standardabweichungen von 0.0143, 0.0138 und 0.0124”/ Es kann gefolgert werden, dass sich der Kristall als Urtitersubstanz eignet. REFERENCES 1 G. MARIN~NKO ANV J. K. TAYLOH. J. Rcs. Nd. hr. Sari.. 67 A (1963) 31. 2 JIS (Japanese Industrial Standard), K 800.5. 1966. 3 I. M. KOLT~IOFFAND E. B. SANDELL. Textbook of Qlrcrrrtirrrtiw Inorgurtic A~rdysis. 3rd Edn.. Mncmillan, New York, 1952, p. 541. 4 J. K. TAYLOR ANI) S. W. SMITH. Science, 124 (1956) 940. 5 R. G. BAT= AND E. WICFIEKS, J. RCS. Nut/. Bur. Std.. 59 (1957) 9. 6 E. L. ECKFEZLDT AND E. W. SHAFMR, JR.. Anal. C/uwr.. 37 (1965) 1534. 7 A. ROICOSZ. Jagiclloniun Univcrsily. Krakow, private communication. 8 T. YOSHIMORI AND I. MATSUDARA. BUN. Clretrt. Sot. Jupurr. 43 (1970) 2800. 9 T. MIWA. T. YOSHIMORI AND T. TAKEUCIII, J. Chcm. Sm. Jupm, ftrrl. C/rem. Sect., 67 (1964) 2045. 10 T. YOSHIMORI ANI) S. ISHIWAKI. Oull. Ciwrrr. SW. Jupuu. 42 (1969) 1282. I1 T. YOSNMORI. I. MATSUIIARA, K. HIROSAWA AND T. TANAKA. Jupurr Am~/_vsr, 19 (1970) 681.
hul.
Cltitrt. Aclu,
55 (1971)
185-191