Micro-gravimetric determination of lead with mercaptobenzothiazole

ANALYTICA

2x8

CHIMICA

VOL.

ACTA

MICRO-GRAVIMETRIC DETERMINATION MERCAPTOBENZOTHIA%OLE

OF

LEAD

12

(1955)

WITH

by CH. Laboratory

of Analytical

CIMERMAN

Chemistry

AND

D.

BOGIN

and Microchemistry. Naif a (Israel)

Israel Instatute of Technology,

INTRODUCTION

Mercaptobenzothiazolel a. A normal,

yellow

reacts salt

with

lead cation,

of the following

forming

two

kinds of salts:

composition: N-

2.

A white,

basic salt having

the following

-

S --

structure:

1’bOH

The normal salt is obtained by precipitating lead, in the cold, with an ammoTlic precipitate, however, is always niacal solution of mercaptobcnzot!liazole. contaminated with the basic salt. On boiling the normal salt with cont. amit is quantitatively convcrtccl into the basic form2. The monium hydrosidc, basic salt is also obtained on precipitating lead from a hot neutral solution with an ammoniacal solution of mcrcaptobcnzothiazole. For the purpose of the quantitative application it is important to obtain the salt in its basic form. SPACU AND I
VOL.

12 (1955)

MICRO-GRAVIMETRIC

DETERMINATION

OF LEAD

2x9

amount of the reagent and also the method of its introduction, (4) the digestion of the filtered precipitate with cont. ammonium hydroxide and the volume of ammonium hydroxide used, (5) the filtration, (6) the wash liquid and method used for washing the precipitate, (7) the temperature and time required to dry the precipitate, the authors have elaborated an exact micro-gravimetric method, applying EMICH’S~ micro-technique and using a porcelain filterstick. EXPERIMENTAL

Reagents and standard solution employed were: I. Mercaptobenzothiazole solution, IO/~, in ammonium hydroxide, 240j0. The solution was prepared by dissolving I g of the reagent in IOO ml of ammonium hydroxide, 23%, heating on the water bath to facilitate solution. It is essential to use a fresh solution, i.e. one prepared just before the determination is carried out. 2. Standard Pb(NO& p.a. solution (the titre of this solution was determined gravimetrically, using the PbS04 method; it was 3 mg Pb+2 per ml), 3. Ammonium hydroxide, 24%. 4. Ammonium hydroxide, cont. Remark: The ammonium hydroxide used must be pure and must not give any silica residue on evaporations. 5. Cadmium salt solution, 10%. 6. Bi-distilled water. Procea~rrc

7h50ntlnedea

for

the

micro-gvnviwtclric

detevmi9tati09t

of

lead

The neutral solution (1-3ml), containing from 3 to 8 mg lead cation, is introduced into a Pyrex glass micro-beaker (dimensions of the beaker: height 55 mm, outer diameter 18 mm) from a micro-burette graduated to 0.01 ml*. The volume is made up to 3 ml with bi-distilled water, the solution is heated to boiling on an electric hot plate and the beaker immediately transferred to a nearly boiling water bath (a 250 ml beaker full of water and heated on a hot plate). A 76%-rSoO~ excess of an ammoniacal solution of mercaptobenzothiazole, I%, (Reagent No. I) is added dropwise from a micro-burette graduated to 0.01 ml (i.e. 0.142-o.226 ml are added for each mg of lead present). After the addition of each drop of the reagent, the precipitate formed is stirred with a short, bent glass rod. This rod is attached, by means of a short piece of rubber tubing, to another glass rod serving as a handle during the stirring (Fig. I c,d,e). burette (U.S.A.) 5 ml * The micro-burette employed was a Blue Line “Exax” alloy delivery tip, capacity, with a graduation interval of 0.01 ml and a platinum giving approx. IOO drops per ml. Xefevences

p.

226.

CH.

ZZO

CIMERMAN,

D.

BOGIN

VOL.

12

(1955)

The additional glass rod (e) may be removed during the filtering and drying operations, leaving.only the bent stirring rod (c) in the micro-beaker. After pre-

Fig. I. Ap aratus used for precipitation a. micro- Eealcer b. water bath

c. bent glass stirring rod d. rubber tubing e. additional glass rod, serving handle during the stirring f. micro-burette g. electric hot plate

as a

cipitation, the sides of the beaker and the stirring rod are washed with 2 ml of K-distilled water, using a wash bottle with a capillary delivery tip. The beaker containing the precipitate is allowed to stand for IO-IS minutes outside the water bath, the additional stirring rod having been removed (Fig. I e). The precipitate settles on the bottom of the beaker, while the liquid above it is clarified, becoming transparent and colourless. After the a.m. interval of 10-15 minutes, the precipitatc is filtered, using a porcelain filtcrstick* (Fig. 2) and very low suction. It is most important to keep to the following manner of filtration: the filterplatc of the filtcrstick must merely touch the surface of the clear liquid; as the level of the liquid falls during filtration, the beaker is gradually raised to keep the surface of the liquid just in touch with the filterplate. This prevents the precipitate from being sucked close to the filterplate and, as a result, most of the precipitate is left on the bottom of the beaker, and only a small fraction of it adheres to the filterstick. This manner of filtration is of the utmost importance in order to ensure the complete conversion of the precipitate into the basic salt during its subsequent digestion with boiling ammonium hydroxide, cont. After all the liquid has been filtered off, the filterstick together with the stirring rod and beaker are detached from the suction system, 0.4 ml of cont. ammonium hydroxide (Reagent No. 4) are introduced from a micro-pipette, washing down the sides of the beaker at the I I + The dimensions of the filterstick used were: overall diameter I I mm; stem diameter 3-4 mm; overall height 55 mm. All filtersticks used were U.S.A. Produce, made by Coors Porcelain Co., Golden. RcfcYc?lces

9.

2a6.

VOL.

12 (1955)

MICRO-GRAVIMETRIC

DETERMINATION

OF

LEAD

22.x

same time; the beaker is now put into a boiling water bath for 2-3 minutes, while its contents are stirred with the glass stirring rod held with a special forceps (Fig. 3). The colour of the precipitate changes to white. The beaker is next removed from the water bath, allowed to stand for 5 minutes, the precipitate filtered with low suction and washed with a warm ammonium hydroxide solution, 2-$% (Reagent No. 3). The wash liquid is introduced from a micro wash bottle with a capillary delivery tip; sides of beaker, filterstick and stirring rod are washed

lo pump

Fig.

2.

Apparatus

for filtration

Fig. 3. Forceps

with small amounts of this liquid. Altogether, 4-8 ml of ammonium hydroxide solution, 2$%, are necessary for adequate washing. The filtrate is tested with a cadmium salt solution (Reagent No. 5) for the absence of the precipitating reagent (mercaptobcnzothiazole). The filtrate must give no precipitate whatever with containing the the cadmium salt solution. After washing, the micro-beaker precipitate together with the filterstick and the stirring rod, are dried in a drying block of BENEDETTI-PICHLERO, at I xo” C7 (temperature inside the drying tube; to that end the temperature of the drying block must be 120~ C) , using low suction. The time required for drying is 25 minutes; after that period, the drying tube containing the beaker, the filterstick and the stirring rod is taken out of the drying block and cooled outside the block for 15 minutes, without interrupting the suction. After cooling, the beaker together with the filterstick and stirring rod are removed from the tube, the beaker wiped, as usual, first with a damp flannel cloth and next with two sets of chamois skins, left for 15 minutes on a nickel block near the microbalance, 5 minutes on a nickel block inside the balance, and finally 5 minutes on the pan of the balance. The beaker, together with the filterstick and the stirring rod, is weighed in the twenty-fifth minute. The weight of the precipitate, multiplied by the factor o.53068*, gives the amount of lead in the sample. , + The factor given in WBLCHER,

Zoc. cit., p. I 13, or in PRODINGEH. Zoc. cif.. p. I I’ given bv the authors, 0.53068, is calculated from the Intern,, *d@ Wei@ts of 195za.

is 0.5309. The factor tional

Atomic

Rcfcrc~rccs p. 226.

CH.

222

CIMERMAN,

D.

BOGIN

VOL.

12

(x955)

The time required. for I determination is 3 hours approx. (including preparation of the beaker and filterstick) ; 2 parallel determinations may be carried out in :5 hours approx*.

Cleaning

the beaker,

/ilterstick

The beaker, containing inixture of HNO,-Hz0 (I

stirring

and

the filterstick and stirring allowed to stand for s-10

: I),

TABLE MICRO-GRAVIMETHIC

DETERMINATION

Excess of mercnptobenzothrazole solution I o/o ?4, 2

2 7 8 9

3.000 5.010 6.990 8.010 3.000

3.000 5.010

1Oj

‘8.::: 3:ooo 3.000 5.010 5.010

‘07 150 1.50 150 1.50 150 150

II 12

13 =4 =5 x6 =7 I8 19

180

:*:(r: 81010 3.000 3.000 5.010

180

5.010

180

6.wO

150

180 180

20 21 22

180 180

23 24

‘07 107 150 150

,227 28

180 180

29

* 2 mg of lead may also recipitating reagent must, The upper hmit K o. q-29). less accurate results under

p.

226.

rod, is filled with a warm minutes while the mixture

I LEAD

WIT11 --._~

l’b found

MERCAPTOBENZOTHIAZOLE .--_-__.__

Pb diffcrcnce m6

107

IO

References

Pb

76 76 107 ‘07

3

4

OF

present

5:

1

rod after the detem&tation

,

6490 8.oro 2.010 2.010 2.010 2.010 2.010 2.010

3*cog 5.000 6,973 7.989 2.995 3.006 5.01

I

7.017 7.994 3.009 2.994 2.;;: 61977 8.024 7.987 2.992 3.004 5.018 5.000 6.972 7.005 8.028 2.005

2.015 2.007 2.014 2.015 2.008

_

+o.aog

‘j/0 _

._

.

-t-O._%

-0.010

-0.2

-0.017 -0.02 -0.005

Relative error

-4J.

I

2

-0.3 -0.2

+0.006 +o.oor

d.003 -0.016 -j-o.oog -0.006 +o.oog -0.0 I4

--0.0x3 +0.014 -0.023 -0.008 3-0.004 +0.008 -0.010

-0.018 f0.0rg +0.01s -O.CO~ +o .005 -0.003 fo .004 to.005 -0.002

;:::2

-0.04 -0.2

-1-o-3 -0-L +0.2

-0.3 -0.2 +0.2

--0.3 -0.3 +0.1 +0.2 -0.2

-0.3 +0.2 4-0.2 -0.2 +0.2 -0.1 ::.a -0.1

.

be determined with this mcthocl; the excess of the however, be from 107% to ISO”/, (see Table I, exp. is 8 mg, as the determination carried out on g mg gives the same conditions.

VOL.

12 (1955)

MICRO-GRAVI~fETRIC

DETERMINATION

OF

223

LEAD

is stirred carefully with the filterstick, avoiding any possible injury to the filterplate. The acid is next discarded and the beaker filled with distilled water. The sides of the beaker, the stirring rod and the stem (but not the plate) of the filterstick are cleaned with a rubber policeman. The beaker is now washed, first with 2 x 5 ml of warm HNO,-HBO (mixture I : I) and next with 4 x IO ml H,O, each time sucking the wash liquid through the filterstick. Drying and weighing of the utensils is done as described under the procedure recommended for the micro-gravimetric determination of lead. Table I shows some characteristic results obtained by the above method. Conchsion As indicated GENERAL

by Table

DISCUSSION

OF

I, results are exact FACTORS BI-

TILE

AFFECTING PROPOSED

(maximum THE

relative

error:

DETERMINATION

fo.3’59. OF

LE.41)

METHOD

I. In order to ensure exact results, it is essential to obtain the lead precipitate quantitatively in the basic form. Precipitating from the hot solution. and later boiling the prccipitatc with cont. ammonium hydroxide, as described in the recommended procedure, makes this entirely feasible. 2. During filtration, the filtcrplate may barely touch the surface of the liquid, as explained in detail under the procedure recommended. 3. Boiling the precipitate, after filtration, with cont. ammonium hydroxide, must be done in a boiling water bath. Boiling over an open flame or on an electric hot plate, causes the ammonia to volatilise too quickly, before it has a chance of reacting with the precipitate. 4. The efficient stirring of the solution, after each drop of reagent, during precipitation, is essential to the proper agglomeration of the precipitate; where this is neglected, the precipitate tends to stick to the sides of the beaker and its subsequent conversion to the basic form is incomplete. 5. The ammonium hydroxide must be absolutely pure and must, on no account, leave a silica residue after evaporation. Reliance on the analysis of the reagent, as given on the label, is inadequate; the purity of the ammonium hydroxide must be tested experimentally, as its silica content depends on the quality of the glass in which it has been kept and the duration of its storageb. 6. The mercaptobenzothiazole solution must be freshly prepared just before the determination is carried out, as it remains clear for a few hours only. PRELIMINARY

TBST

The excess of mercaptobenzothiazole solution, I%, recommended under the procedure elaborated, was 76% -1800/~. In order to determine the quantity References

p. 226.

CH.

224

CIMERMAN,

D.

VOL. 12

BOGIN

(1955)

of reagent to be added to a particular sample, the following rapid preliminary test may be carried out: The neutral lead cation solution (2-3 ml, containing s-10 mg 6f lead cation) is introduced into a Pyrex glass test tube (dimensions: length 1x0 mm, outer diameter 16 mm), from a micro-burette graduated to 0.01 ml. The volume is made up to 8 ml with bi-distilled water, the test tube heated in a boiling water bath and an ammoniacal solution of mercaptobenzothiazole, I%, (Reagent No. I) added dropwise from a similar micro-burette. The test tube is shaken and centrifuged for 5 minutes. The precipitate settles to the bottom of the tube and the liquid above it is clarified. The test tube and its contents are hcatcd again, and the reagent again added dropwise to determine whether precipitation has been complete. The shaking, centrifugation, etc. are rcpcated until no precipitate is formed on the addition of the reagent. After 2-3 such experiments, the theoretic amount of reagent, required for this particular sample, may be determined easily, with an average relative error of minus 16%. One preliminary test takes approx. 20 minutes. Table II shows some characteristic results obtained by this preliminary test. TABLE PRELIMINARY BENZOTIIIAZOLI3

Exp. No.

TEST FOR SOLUTION

Standard Pb solution used

ml

II

THE CALCULATION NECESSARY FOR

Pb present

mg

OF THE

TIIE VOLUME OF MICRO-DETERMINATION

Mcrcaptobcn2~~hixzolc theoretic

ml

d #ference

ml

ml

I .GG

4.g8

0.402

0.402

0.340 0.335

-0.oG7

:

I .GG 3.33 3.33 3.33

4.08 4.98 9.09 9.99 9.99

0.402 0.806

o-330 0.325 o.G95

-0.077 -0.072 -0.11 -o.~xG

0.800

O.Oc)O 0.GYg

-O.I”I

I

2

7

I .GG

4.95

^.OG2

I

Avcrngc: -

-

MERCAPTOOF LEAD

solution

USed

2

THE

Relative error cliffercnce calculated with regard to Per “ml used” “theoretic” ml % --0.182 -0.200

-x5.4 -167

-0.2lY -0.237

-17.9 ---lg.1 -13.8 -14.4

-O.IGO -O.IGY

--0.177 -0.192

----_

-1g.o

--tG.o

Conclusion

As indicated by Table II, the average diffcrencc, between the theoretic volume of the mercaptobenzothiazole solution, I%, and the volume used, is 0.192 ml for each ml participating in the reaction in this preliminary test. This difference amounts to an average relative error of minus lficyo, calculated with regard to the theoretic volume of the mercaptobenzothiazole solution. Rcfcrcmxs

p.

226.

VOL,

MICRO-GRAVIMETRIC

12 (x955)

DETERMINATION

OF

LEAD

225

DISCUSSION

I. The average relative error the calculation of the quantity in the final micro-determination thiazole necessary for the final calculated with the aid of the

of minus IG%, nevertheless, definitely permits of mercaptobenzothiazole which must be added of lead. The theoretic, volume of mercaptobenzodetermination (x ml) may be approximately equation: n +

0.1cpn

L-

x

solution, r*/*, used in the preliminary where n = ml of mercaptobenzothiazole test. (The relative error, calculated according to the a.m. equation, for exp. No. 1-7 in Table II, ranges from &0.7~h to &3.G”h.) The effective volume of mercaptobcnzothiazole solution, IO/(, which must be employed in the final determination of lead (y ml), may be calculated from the following equation : x+ax=y

where: a = a coefficient, ranging fron x.3 to r.5# solution, x = the approx. theoretic volume (ml) of mercaptobenzothiazole I */*, required, Y = the effective volume (ml) of mercaptobenzothiaxole solution, ro/d, required for the determination of lead in a lead solution, equal in volume and lead content to that used in the preliminary test, under the condition that the volume of lead solution used in the preliminary test contained 5--x0 mg of lead cation. The coefficient a (r-3-1.5) rclntcs to 130*/&- r5oyG excess of mercaptobenzothiazole, which values lie within the 7GO/;---~SO~/~ optimum range given in the recommended procedure, 2. On the other hand, the preliminary test allows the approx. estimation of the amount of lead cation present in each ml of the lead solution examined (within the same range of relative error, i.e., o.7%--3.G%), using the equation given below : m(9t + o.xgzw) __I”_. .-- -.___ _-

0.807

w

I=- fi mg lead per ml.

(Tbc condition mentioned under section r of this discussion, i.e,, that the lead solution employed in the preliminary test contain 5-10 mg of lead cation, also holds here.) In the above equation: n = ml of mercaptobcnzothiazole solution, I*/~, entering the reaction in the preliminary test, IO == mg mercaptobenzothiazole in I ml of a II% solution, Rejeuences

p.

226.

CH. CIMERMAN, D. BOGIN

22fi

0.807 =- mg of mercaptobcnzothiazole is obtained from the ratio: C,H,NS, .----Pb

=

reacting 167.25 -207.2 1

VOL.

12 (1955)

with I mg of lead; this number

o.8071

v - ml of lead solution employed in the preliminary test. jf) ==.mg of lead per. ml of the lead solution examined. I~~nzrw/~:l’hc above preliminary test, may, thcrcfore, be used as a semi-quantitative mcthocl for the determination of lead. 3. Based on the calculation given in section 2. the volume of lead solution to be talccn for the final determination of lead may bc easily deduced, so that it mg of lead cation (this being the limits of the micro-gravimetric contains .,--8 ’ method recommended). SUMMARY ‘I’l1c authors have claborntccl an esnct micro-grnvrmetric nilnntion ol Icatl with mcrcaptobenzotl11azolc.

method

for the deter-

cxactc

pour la dbter-

I
Mlkro-grawmetrische ausgerrrbeitet. IIEFEKENCES

1

3

3 4

zur Bestimmung

.

1;. J. WELCIII~I<, Ovganzc Analytical Reagents, Vol. IV, D. V;LII Nostrand Co., New York (1c+~H) p. II I. W. PHODINGE~<, Organac lbagents used i3t Quadtfative Iqrovganic Analysis, Blscvier lQbllshin& Co., New Yo1lc (1940) 18, 113. 104 (1936) 88. G. SPACU UND M. ~CUI~AS, %. ana 1p’ . Chem., 17. EMICII, Lafr~bzrch clew Miltvocitemie, 2 Aufl. Verlng J. F. Bergmann, Mtinchen (lOuA) 1’. 1:. ii-Ixcr~*r

b

Methode

84. UNU

J. DONAU, Anovgakxhe Miltvogewichtsanalysc, Verlng J. Springer, Wien (1+10) p. 74. Ausfiihung qunntrtativev Analysoa, 5 Aufl. Vcrlag S. HirI-1. HILT% UN'D W. BILTZ, xl, Zur1cli (1947) p. 29. A. B~~N~~vI~‘PTI-~‘IcI~L~~~,Makvochemie, Pvegl- FestschvLft (rgzc)) p. G. F. Hxcr~r KJNU J. D~NAU, lot. cit., p. 103. CL. DUVAL, lnovgantc l’lrev?nogvavimctvzc A+dyszs, Elsevicr Publishing Co., Amsterdam (1953) p. 471. CtI. 1). ~-IOX)GAIAN, Editor, Handbook of CIlemistvy ami Physzcs, 35th Edition, Chemicnl l
August

I xth,

1954