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The homogeneous precipitation of lead molybdate
SHORTCOMMUN1CATIONS
261
The authors thank Professor 13. P. MITRA, Head of the Department of Chemistry for providing facilities, and the Ministry of Education (Government of India) for the award of a scholarship to G.S. Departme& of C/aemistr~~, Uptiversity of Delhi, Delhi 7 (hdia)
I<. 12. KAR GURBIR SINGH
" I I. hf. ICoL-rtloFP AND 12.B. SANDBLL, Texlbooh of Qtra+JCiLalive I?JO~@PJ~C A?Jalysis, 3rd Ed., Macmillan Co,, New York, rgsz , p. 347. 2 v. SHVEDOV, Z~J. AzJrdir. J
(Iicceived October
zotli, x965) Ar~al. ClJiwJ. Acla,
The
homogeneous
precipitation
35 (lgG6) 25g-261
of lead molybdate
The determination of molybdenum as a lead salt was proposccl as early as 1871 by CHATARIII; subscqucnt work has been rcviewcd by 33usov~. Although accurate, the present gravimetric mcthocls arc time-consuming. Taking advantage of the slowness of reaction between chromiun~(III) and EDTAS-6 we have developed a method of homogeneous precipitation based on the reactions H2Y”-+
Pb’t
+ PbY”-+z
PbY 3- + Cr3” + MoO~Q- +
H+ CrY - + PbMoOs
where HzY z- rcprcscnts the ionized form of the disodium salt of ethylenediaminetetraacetic acid. Precipitates obtained in this fashion were more granular and more readily filtered and washed than those obtained by heterogeneous precipitation. In addition, the precipitate showed no tcndcncy to cling to the walls of containers as did precipitates formed in the traditional manner. Procedure The order of addition of reagents was critical; for samples containing ca. 5.6. x0-4 moles of molybdenum the following method of operation gave good results. Place the sample in a I5o-ml beaker, together with a magnetic stirring rod. Add 21 ml of 0.1 M NasHaY, adjust the PH to 4 (acetic acid or sodium hydroxide) and adjust the total volume to about 50 ml. Place the beaker on a combination magnetic stirrer-hot plate. Cover with a special watch glass containing a small hole in the center. Adjust the stirrer to stir gently and heat the solution to near boiling. Add 15 ml of 0.1 M lead nitrate. If the solution becomes cloudy, add more NazHaY dropwise until the cloudiness just disappears. Increase the temperature toobtaingentle boiling. When boiling has been maintained for ca. 60 set, place a buret containing 0.2 M chromium(III) nitrate through the hole in the cover glass so that the tip is about 1.5 cm below the level of the glass. Add the chromium(II1) solution very slowly until sufficient is Anal.
Chitn.
Acla,
35 (IgGG)
26x-262
262
SHORT
COtiMUNICATrONS
present for complete precipitation of the molybdate. For the concentrations of molybdate and other reactants used here, 6.57.0 ml of chromium(III) solution is recommended. Remove the but-et and continue heating at the boiling point for an additional x0 min. Cool to ca. 40”. Decant the purple liquid through a filtering crucible containing a glass fiber filter pad. Wash the precipitate with water to remove all traces of purple material. Transfer the precipitate to the filter, thoroughly cleaning the magnetic Heat the crucible and contents at 200’ to stirring bar with a rubber policeman. constant weight, and weigh as PbMoOd.
Synthetic samples of molybdate were prepared from samples of NaaMoOs 2HzO. The results of the analysis of these samples arc shown in the Table. Qualitative experiments indicated that the best results were obtained when the amount of chromium(II1) added did not surpass the amount of EDTA present. Since a I: I chromium-EDTA complex is formed, judgcmcnt could be made as to how much chromium(II1) was needed. TABLE
I
RESULTS I'OItSYNTIIETIC --_~~-_--_-__-
Na&foO~* aHnO
taken (wag) (96.7’;/, fwuily)
SAMPLES -_--.._.
-
Tlteorelical
13G.G 137-o 138.7 139.0
200.4 20X.X 203.G 204.0
139.1
204,1
FOWZd
20X.X
201.3 204.0
204.0 205.4 203.3
Avcrngc 202.6 --------.------
__
-.-_--_
PAM004 (mg)
The same treatment was applied to sulfate samples as that used for molybdatc. However, there was no noticeable Cr-EDTA complex formed. Instead of the characteristic reddish purple color of the complex, a green colored solution was produced, and no precipitate of lead sulfate appeared. It may bc postulated that the sulfate inhibits Cr-EDTA complex formation, although more investigation is necessary to ascertain the exact role of the sulfate. @epartment of Chemistry, Villasova Univcvsity, Villa?tova, Pa. (U.S.A.) I F. M. CUATARD, Afn.J. Sci.. 2 A. I. Bussv, Andiliclreslrnyn 1962.
I (1871) 416 Klriwaiya Molildem,
GEORGE NBWCOMB, JR.* JAMES J. MARKH.A~~ Izclatcl’stvo Akndcmii Nauk SSSR. 3Ioslcvn.
3 R. C. HAMM, J. Am. Chew. Sot., 75 (1953) 5670. 4 V. I;. RAO, D. S. SUNDAR AND M. N. SASTR~, Chemic?-A 5 C. B, HBDRICIC. J. C/rem. E’dwz., 42 (x965)479.
(Received * Fbscnt Anal.
November
mdysC,
54 (1965)
86.
x7th, 1965)
adclrcss: Pcnnsnlt
Chemicals,
Ckim. Acta, 35 (xg6G) 26x-262
Research
Dept.,
King of Prussia, Pennsylvania.
261
The authors thank Professor 13. P. MITRA, Head of the Department of Chemistry for providing facilities, and the Ministry of Education (Government of India) for the award of a scholarship to G.S. Departme& of C/aemistr~~, Uptiversity of Delhi, Delhi 7 (hdia)
I<. 12. KAR GURBIR SINGH
" I I. hf. ICoL-rtloFP AND 12.B. SANDBLL, Texlbooh of Qtra+JCiLalive I?JO~@PJ~C A?Jalysis, 3rd Ed., Macmillan Co,, New York, rgsz , p. 347. 2 v. SHVEDOV, Z~J. AzJrdir. J
(Iicceived October
zotli, x965) Ar~al. ClJiwJ. Acla,
The
homogeneous
precipitation
35 (lgG6) 25g-261
of lead molybdate
The determination of molybdenum as a lead salt was proposccl as early as 1871 by CHATARIII; subscqucnt work has been rcviewcd by 33usov~. Although accurate, the present gravimetric mcthocls arc time-consuming. Taking advantage of the slowness of reaction between chromiun~(III) and EDTAS-6 we have developed a method of homogeneous precipitation based on the reactions H2Y”-+
Pb’t
+ PbY”-+z
PbY 3- + Cr3” + MoO~Q- +
H+ CrY - + PbMoOs
where HzY z- rcprcscnts the ionized form of the disodium salt of ethylenediaminetetraacetic acid. Precipitates obtained in this fashion were more granular and more readily filtered and washed than those obtained by heterogeneous precipitation. In addition, the precipitate showed no tcndcncy to cling to the walls of containers as did precipitates formed in the traditional manner. Procedure The order of addition of reagents was critical; for samples containing ca. 5.6. x0-4 moles of molybdenum the following method of operation gave good results. Place the sample in a I5o-ml beaker, together with a magnetic stirring rod. Add 21 ml of 0.1 M NasHaY, adjust the PH to 4 (acetic acid or sodium hydroxide) and adjust the total volume to about 50 ml. Place the beaker on a combination magnetic stirrer-hot plate. Cover with a special watch glass containing a small hole in the center. Adjust the stirrer to stir gently and heat the solution to near boiling. Add 15 ml of 0.1 M lead nitrate. If the solution becomes cloudy, add more NazHaY dropwise until the cloudiness just disappears. Increase the temperature toobtaingentle boiling. When boiling has been maintained for ca. 60 set, place a buret containing 0.2 M chromium(III) nitrate through the hole in the cover glass so that the tip is about 1.5 cm below the level of the glass. Add the chromium(II1) solution very slowly until sufficient is Anal.
Chitn.
Acla,
35 (IgGG)
26x-262
262
SHORT
COtiMUNICATrONS
present for complete precipitation of the molybdate. For the concentrations of molybdate and other reactants used here, 6.57.0 ml of chromium(III) solution is recommended. Remove the but-et and continue heating at the boiling point for an additional x0 min. Cool to ca. 40”. Decant the purple liquid through a filtering crucible containing a glass fiber filter pad. Wash the precipitate with water to remove all traces of purple material. Transfer the precipitate to the filter, thoroughly cleaning the magnetic Heat the crucible and contents at 200’ to stirring bar with a rubber policeman. constant weight, and weigh as PbMoOd.
Synthetic samples of molybdate were prepared from samples of NaaMoOs 2HzO. The results of the analysis of these samples arc shown in the Table. Qualitative experiments indicated that the best results were obtained when the amount of chromium(II1) added did not surpass the amount of EDTA present. Since a I: I chromium-EDTA complex is formed, judgcmcnt could be made as to how much chromium(II1) was needed. TABLE
I
RESULTS I'OItSYNTIIETIC --_~~-_--_-__-
Na&foO~* aHnO
taken (wag) (96.7’;/, fwuily)
SAMPLES -_--.._.
-
Tlteorelical
13G.G 137-o 138.7 139.0
200.4 20X.X 203.G 204.0
139.1
204,1
FOWZd
20X.X
201.3 204.0
204.0 205.4 203.3
Avcrngc 202.6 --------.------
__
-.-_--_
PAM004 (mg)
The same treatment was applied to sulfate samples as that used for molybdatc. However, there was no noticeable Cr-EDTA complex formed. Instead of the characteristic reddish purple color of the complex, a green colored solution was produced, and no precipitate of lead sulfate appeared. It may bc postulated that the sulfate inhibits Cr-EDTA complex formation, although more investigation is necessary to ascertain the exact role of the sulfate. @epartment of Chemistry, Villasova Univcvsity, Villa?tova, Pa. (U.S.A.) I F. M. CUATARD, Afn.J. Sci.. 2 A. I. Bussv, Andiliclreslrnyn 1962.
I (1871) 416 Klriwaiya Molildem,
GEORGE NBWCOMB, JR.* JAMES J. MARKH.A~~ Izclatcl’stvo Akndcmii Nauk SSSR. 3Ioslcvn.
3 R. C. HAMM, J. Am. Chew. Sot., 75 (1953) 5670. 4 V. I;. RAO, D. S. SUNDAR AND M. N. SASTR~, Chemic?-A 5 C. B, HBDRICIC. J. C/rem. E’dwz., 42 (x965)479.
(Received * Fbscnt Anal.
November
mdysC,
54 (1965)
86.
x7th, 1965)
adclrcss: Pcnnsnlt
Chemicals,
Ckim. Acta, 35 (xg6G) 26x-262
Research
Dept.,
King of Prussia, Pennsylvania.