Objectives: •
To precipitate nickel from the solution by adding dimethyl glyoxime.
•
Filtration of precipitate using sintered glass crucible.
•
Mass of nickel calculated from the mass of precipitate.
Theory: Gravimetric analysis is one of the most accurate analytical methods available. It is concerned with the determination of a substance by the process of weighing. The element or radical to be determined is conve converte rted d into a stable stable compo compound und of defini definite te comp composi ositio tion n and the mass mass of the compou compound nd is determined accurately. accurately. From this, the mass of element or radical is calculated. The gravimetric analysis involves a precipitation b filtration c washing of the precipitate and d drying, ignition and weighing of the precipitate. Following are the four fundamental types of gravimetric analysis! ".
#hysical gravimetry
$.
Thermogravimetry
%.
#recipitative gr gravimetric an analysis
&.
'lec 'lectr trod odep epos osit itio ion. n. The These se diff differ er in the the pre prepa parratio ation no off the the s sam ampl ple e bef befor ore e wei weigh ghin ing g of of the the anal analyt yte. e. Physical gravimetry: #hysical #hysical gravime gravimetry try involves involves the physical physical separation separation and classificati classification on of matter in environmental samples based on volatility and particle si(e )e.g., total suspended solids. It is the most common type used in environmental engineering. engineering. Thermogravimetry: In Thermogravimetry: In this method the samples are heated and the changes in sample mass are recorded. *olatile *olatile solid analysis is an important example for this type of gravimetric analysis. Precipitative gravimetry: The chemic chemical al preci precipit pitati ation on of an analyt analyte e occur occurs s in the preci precipit pitati ative ve gravimetry. The most important application of this techni+ue in the environmental field is the analysis of sulphite. Electrodeposition: It invo involv lves es the the elec electr troc oche hemi mica call redu reduct ctio ion n of meta metall ions ions at a cath cathod ode e and and simultaneous deposition of the ions on the cathode. The steps commonly followed in gravimetric gravimetric analysis are )" )$ )%
#reparation
of
a
solution
-eparation eighing
containing
of
a
the the
known
weight desired
isolated
of
the
sample.
constituent. constituent.
)& /omputation of the amount of the particular constituent in the sample from the observed weight of the isolated substance.
Pecipitative Gravimetric Analysis:
#recipitative gravimetric analysis re+uires that the substance to be weighed be readily removed by filtration. In order for a non0filterable precipitate to form, it must be supersaturated with respect to its solubility product constant. 1owever, if it is too far above the saturation limit, crystal nucleation may occur at a rate faster than crystal growth )the addition of molecules to a crystal nucleus, eventually forming a non0filterable crystal. hen this occurs, numerous tiny micro0crystals are formed rather than a few large ones. In the extreme case, micro0crystals may behave as colloids and pass through a fibrous filter. To avoid this, precipitating solutions may be heated. 2ecause the solubility of most salts increases with increasing temperature, this treatment will lower the relative degree of super saturation and slow the rate of nucleation. 3lso, one might add the precipitant slowly with rapid mixing to avoid the occurrence of locally high concentrations. #recipitative gravimetry is often practiced at high ionic strengths. This is to reduce the electric double layer thickness )salting0out effect of the slowly forming crystals. hen this occurs, electrostatic repulsion between the crystal and its precipitating molecules is reduced. /rystal growth can then occur more rapidly. It is very important that the precipitate be pure and has the correct stoichiometry. Co-precipitation: For certain gravimetric analysis some other substances besides the desired substances also get precipitated in small amounts, the phenomenon is called co0precipitation. It occurs when an unwanted ion or molecule becomes trapped in the precipitate. This may be due to inclusion or occlusion. Inclusion is the term used for a single substitution in the crystal lattice by an ion of similar si(e. 4cclusion refers to the physical trapping of a large pocket of impurities within the crystal. 4ne techni+ue for minimi(ing these problems is to remove the mother li+uor, re0dissolve the precipitate and then re0precipitate. The second time the mother li+uor will contain fewer unwanted ions capable of co0precipitation.
Conditions of Precipitation: ".
#recipitation
should
be
carried
out
in
dilute
solutions.
$. The reagents should be mixed slowly and with constant stirring. This will assist the growth of large
crystals.
%. #recipitation is effected in hot solutions, provided the solubility and stability of the precipitate permit it. 'ither one or both of the solutions should be heated to 5ust below the boiling point. 1igh
temperature assists
)a coagulation and increase )b
velocity of crystalli(ation.
&. /rystalline precipitates should be digested for as long as possible, preferably overnight. Precipitating Reagents: Ideally a gravimetric precipitating agent should react specifically or at least selectively with the analyte. -pecific reagents which are rare, react only with a single chemical species. -elective reagents which are more common, react with a limited number of species. In addition to specificity and selectivity, the ideal precipitating reagent would react with analyte to give a product that is a
'asily
filtered
and
washed
free
of
contaminants
b 4f sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing c
6nreactive
with
constituents
of
d 4f known chemical composition after it is dried or, if necessary, ignited.
atmosphere
Most of the inorganic ions have yielded to gravimetric analytical techni+ues, but one finds many interfering ions. The table below illustrates both the abundance of reagents available for use as well as the problems which can be encountered by interfering ions! Analyt
Precipitate
Measured form
Interferences
78
72)/91:&
72)/91:&
;1&8, 3g8, 1g$8, Tl8,
Mg$8
Mg);1&.91$4
Mg$#$4=
Many metals )none from ;a 8and
e
78 /a$8
/a/$4&.1$4
/a/4% or /a4
Many metals )none from Mg$8, ;a8 and 78
2a$8
2a-4&
2a-4&
;a8,78,>i8,/a$8,3l%8,/r%8,Fe%8,-r$8, #b$8
Ti&8
Fe%8,@r&8,/u$8,/$4& $0, citrate, 1F
Ti4):,=0dibromo0?0
Ti4):,=0dibromo0?0
hydroxy+uinoline $
hydroxy+uinoline $
*4&%0
1g%*4&
*$4:
/l0,2r0,I0,-4&$0 , /r4&$0,3s4&%0,#4&%0
/r%8
#b/r4&
#b/r4&
;1&8,3g8
Mn$8
Mn);1&.1$4
Mn$#$4=
Interferences from numerous metals
Fe%8
Fe)1/4$%
Fe$4%
Interferences from numerous metals
/o$8
Fe%8,@r&8,#d$8
/o)"0nitroso0$0
/o-4& )by reaction
naphtholate%
with 1$-4&
;i)dimethylglyoxima
;i)dimethylglyoxima
te$
te$
/u$8
/u-/;
/u-/;
;1&8,#b$8,1g$8,3g8
@n$8
@n);1&.1$4
@n$#$4=
Interferences from numerous
;i$8
#d$8,#t$8,2i%8,3u%8
metals. /e&8
/e)I4%&
/e4$
Th&8,Ti&8,@r&8
3l%8
3l)?0
3l)?0
Interferences from numerous
hydroxy+uinolate %
hydroxy+uinolate %
metals.
-n)cupferron&
-n4$
/u$8,#b$8,3s)III
-n&8
#b$8
#b-4&
#b-4&
/a$8,-r$8,2a$8,1g$8, 3g8,1/l, 1;4%
;1&8
;1&2)/91:&
;1&2)/91:&
78,
/l0
3g/l
3g/l
2r0, I0, -/;0, -$0, -$4%$0, /;0
2r0
3g2r
3g2r
/l 0, I0, -/;0, -$0, -$4%$0, /;0
I0
3gI
3gI
2r0, /l0, -/;0, -$0, -$4%$0, /;0
-/;0
/u-/;
/u-/;
;1&8,#b$8,1g$8,3g8
/;0
3g/;
3g/;
/l0, 2r0, I0 , -/;0, -$0, -$4%$0
F0
)/91:%-nF
)/91:%-nF
'xcept alkali metals, many interferences, and -i4&&0 , /4%$0.
/l4&0
7/l4&
7/l4&
-4&$0
2a-4&
2a-4&
;a8,78,>i8,/a$8,3l%8,/r%8,Fe%8,-r$8, #b$8
#4&%0
Mg);1&.91$4
Mg$#$4=
Many interferences except ;a 8,78.
;4%0
;itron nitrate
;itron nitrate
/l4&0, I0, -/;0, /r4&$0,/l4%0, ;4$0, 2r0, /$4&$0
/4%$0
/4$ )by addition of
/4$
acid
/4$ is trapped as ;a $/4% on 3scarite.
There are a number of organic functional groups which precipitate with metal ions by one of two routes! )" chelating agents are organic compounds which Awrap aroundA a metal ion thanks to cationic side chains which form coordinate covalent bonds with the ion, and )$ a straightforward ion0 ion bond which produces a new species that excludes water of solvation and thus precipitates. Good examples of chelating agents include 'thylene Biamine Tetraacetic 3cid )'BT3, oxalic acid, glycine, ?0 hydroxy+uinoline
and
dimethylglyoxime.
-ome common organic precipitating agents! Compound
Ions precipitated
Bimethylglyoxime
;i$8,#d$8,#t$8
' BT3 )'thylenediamine tetraacetic acid
@n$8, /u$8, #b$8, /a$8, ;i$8, Fe%8
/upferron
Fe%8,*4$8,Ti&8, @r&8,/e&8,Ga%8,-n&8
?01ydroxy+uinoline
Fe%8,3l%8,Mg$8,@n$8,/u$8,/d$8,#b$8, 2i%8, Ga%8,Th&8, @r&8, Ti4$8, 64$$8
-alicylaldoxime
2i%8,;i$8,#d$8,@n$8, /u$8,#b$8
"0;itroso0$0naphthol
Fe%8,/o$8,#d$8, @r&8
;itron )/$C1"9;&
;4%0, /l4&0, 2F&0, 4&$0
-odium tetraphenylborate
;1&8, organic ammonium, 3g8, /s8,
Tetraphenylarsonium chloride
/r$4=$0, Mn4&0,
The Gravimetric Estimation of Nickel: The nickel is precipitated as nickel dimethyl glyoxime by adding alcoholic solution of dimethyl glyoxime /&19);41$ and then adding a slight excess of a+ueous ammonia solution.
hen the p1 is buffered in the range of : to D, the formation of the red chelate occurs +uantitatively in a solution. The chelation reaction occurs due to donation of the electron pairs on the four nitrogen atoms, not by electrons on the oxygen atoms. The reaction is performed in a solution buffered by either an ammonia or citrate buffer to prevent the p1 of the solution from falling below :. If the p1 does become too low the e+uilibrium of the above reaction favors the formation of the nickel )II ion, causing the dissolution of ;i )BMG$ back into the mother li+uor. 3 slight excess of the reagent has no action on the precipitate, but a large excess should be avoided because of the possible precipitation of the reagent itself. The precipitate is soluble in the free mineral acids. It is therefore crucial to avoid the addition of too large and excess of the reagent because it may crystalli(e out with the chelate. It is also important to know that the complex itself is slightly soluble to some extent in alcoholic solutions. 2y adding small amount of chelating agents will minimi(e the errors from these sources. The amount of the reagent added is also governed by the presence of other metals such as cobalt, which form soluble complexes with the reagent. If a high +uantity of these ions is present, a greater amount of BMG must be added. The nickel dimethylglyoximate is a very bulky precipitate. Therefore, the sample weight used in the analysis must be carefully controlled to allow more convenient handling of the precipitate during the transfer to the filtering crucible. The compactness of the precipitate is improved by ad5usting the p1 to % or &, followed by the addition of ammonia solution. 3 slow increase in the concentration of ammonia in the solution causes a slight increase in the p1 gradually and results in the precipitation of the complex. The result is the formation of a denser precipitate. 4nce the filtrate has been collected and dried, the nickel content of the solution is calculated stoichiometrically from the weight of the precipitate.
The structure of DMG & the complex with nickel ions is given below;
BMG
;ickel solution
;i0BMG /omplex
/rucible
Besiccator
