original paper determination of chloride in food grade
TRANSCRIPT
Original Paper
Determination of Chloride in Food Grade Salt
Yasushi NIINO,* Noriko SHIMIZU,* Hitomi NISIMURA** and Noboru OGATA*
Mercurometry (ISO 2481),potentiometry (ISO 6227),and argentometry (JTS) for determination ofchloride in common salt were studied as mutual comparison regarding the operation,error,time requiredand cost.Mercurometry was effected by pH and quantity of diphenylcarbazone.Excessive addition ofnitric acid caused a decrease in repeatability because of an increase in blank value and unstable colorat the end-point.Too little addition of nitric acid gave too little determination value.So addition of0.5 to 1.0ml of 2 N-nitric acid and preparation about pH 2 were recommended.The color of the end-point was changed by the quantity of diphenylcarbazon,but determination value was not affected.Aserious disadvantage in mercurometry was a disagreement of color with the color-matching solution,andthe instability of end-point color gave frequently some anxiety to ,the operator.
Repeatability in potentiometry in the ruled six combinations of electrodes was not different from others,but a combination of silver-ion selective electrode and calomel electrode gave relatively good accurate re-sults.Repeatability of titration was 1:potentiometry,2:argentometry,3:mercurometry in the other ofexcellence.But each standard deviation in the total operation of determination was nearly equal becausethe dilution error was greater than the titration error.The comparison of accuracy of each method wasobtained from the deviation from the standard value of the International Standard Seawater.The argen-tometry was the most accurate,the mercurometry gave lower value,and the potentiometry gave highervalue.
Time required for making a sample was about 25 min in mercurometry,about 30min in potentiometry,and about 20min in argentometry after pipetting the sample solution.The reagent cost,apparatus costand personnel expenses were estimated,and the total cost of argentometry was more reasonable thanmercurometry in spite of expensive reagent cost because the weight of personnel expenses was greaterthan reagents and apparatus cost.
1.INTRODUCTION
Analytical method for determination of chlo-
ride in food grade salt has been discussed inCodex Comittee on Food Additives under Codex
Alimentarius Commission since 1975.ISO 2481,mercurometric method for chloride determinationof sodium chloride for industrial use,was pro-
posed to CCFA in 1982,for food grade salt.Onthe other hand,argentometry (Mohr's method)
and potentiometric method based on ISO 6227are generally used for a standard analytical
procedure for common salt in Japan and someother countries.The authors have believed thatmercuric reagent should preferably be avoided
from the view point of mercuric pollution,al-thotigh a procedure of the waste water treatmentis established for prevention of the mercuric
pollution in Japan specially in food industries.The use of mercuric reagent is not acceptable in
the national sentiments.Moreover,we regardMohr's method the most simple,rapid and ac-
curate method.Therefore,we found the neces-sity of comparison of mercurometry,potentiome-try and argentometry on the accuracy,repeat-
ability,easiness of operation,time required,costof analysis and the others to revise ISO 2481 to
add argentometry as alternative method in chloridedetermination in food grade salt.
2.OUTLINES OF ANALYTICALPROCEDURES
2.1 Mercurometry (ISO 2481)Test sample solution: The sample salt is taken
out to weigh 100 g,disolved,filtrated by glassfilter and diluted in a 1,000 ml volumetric flask.
*Central Research Institute,The Japan Tobacco Inc.,Umegaoka,Midori-ku,Yokohama,Japan
**Odawara Experiment Station,The Japan Tobacco Inc.,Sakawa,Odawara,Japan
―136―
Y. Niino et al.: Determination of Chloride in Food Grade Salt 137
The test solution is prepared by taking 50 ml of
sample solution and dilution in a 500 ml volu-metric flask.
Titration: 25ml of the test solution is placed
in a conical flask,diluted to approximately 200ml,then added 3 drops of the bromophenol bluesolution and the nitric acid solution (2N) drop
by drop until the color changes from blue to
yellow.Further,3 drops of this acid and thesame volume of diphenylcarbazon solution
(0.5%) as the same volume as in the standardend-point matching solution are added.The
titration is carried out with mercury nitratesolution (0.1N) until the color matches the
mauve of the standard end-point matching solu-tion. The consumption of mercury nitrate solu-tion was about 40ml in common salt.
The standard end-point matching solution is
prepared by the same operation as in the abovetitration using 200ml of pure water.
2.2. Potentiometry (ISO 6227)Test sample solution:Chloride in the test
sample solution is contained above 12.5mg andthe volume of solution is adjusted to 50 ml.
Titration:Titration is carried out with silver
nitrate solution (0.1N),and the end-point isdetermined from the point of inflection on the
differential curve of the potential measuredby electrodes.The measuring electrode is asilver-electrode,chloride ion-electrode or silver
ion-electrode,and the reference electrode isa calomel electrode or mercury sulphate
electrode.2.3 Argentometry (Mohr's method by JTS
standard)
Test sample solution:The sample salt is takenand weighed to be 20g,dissolved and diluted in
a 500ml volumetric flask.The test solution is
prepared by taking 50ml of the solution anddilution in a 500ml volumetric flask.
Titration:20ml of the test solution is placedin a conical flask,diluted to approximately 40ml,and added 1ml of potassium chromate
solution (5%) as indicator.The titration iscarried out with the silver nitrate solution
(0.1N) until the color of reddish orange does notvanish any more even though the solution isstirred vigorously.
2.4 The standard solution in this experimentOn common salt,the same test solution was
used in the same series of the test which was
prepared by conformity with 2.3 argentometry asmentioned above.The International StandardSeawater was applied for the tests of accuracy.The test solution was prepared by weighing 200g
of the standard seawater (19.373g/kg) and dilu-tion to 2,000ml.The titrations were carried out
using 25ml of the test solution.The concentration of the mercury (II) nitrate
and silver nitrate solution were prepared ap-
proximately 0.1N.Consequently,the samplingerror should be a little,and the titration values
were about 17ml in common salt and about14ml in the International Standard Seawater,respectively.
3. PROCEDURE OF MERCUROMETRY
3.1 Effect of pHThe effect of pH was already reported by J.V.
Dubsky,1) I.Robert,2) and F.E.Klerke.3) The
pH recommended did not agreed with the onementioned in the above report,but determinationvalues were decreased in acidic solution.In this
Fig.-1 Effect of nitric acid in mercurometry.
(1) Titration values,(2) Difference of titrationvalue and blank.
138 Bull. Soc. Sea Water Sci. Jpn. Vol.39 No.3 (1985)
paper,the relation between the titration valueand the volume of added nitric acid was ex-amined by using 20ml of sodium chloride solu-
tion (0.1N),and the results were shown inFig.-1.The titration and blank value were in-creased with increase of added nitric acid simi-
larly as reported in the previous papers.Accuracy
was relatively good in 1 to 2ml of nitric acidwhose pH was about 2,but the increase of blankvalue was not preferable for repeatability.And
the color was unstable with increase of nitricacid in as Fig.-2.From these results,the
authors would like to recommend that thevolume of nitric acid should not be 3 drops but0.5 to 1ml and about pH 2.
As a supplementary experiment,a test samplesolution containing 25% of sodium glutamatewas titrated by mercurometry for a notice of the
effect of buffer action.The results were asfollows,and the error was not recognized.
non-added 60.54Cl%
added 60.56Cl%3.2 Effect of diphenylcarbazon
Changes of absorbance of the solution at theend-point were examined at 0.5 and 1 ml ofdiphenylcarbazon solution,and the results were
shown in Fig.-3.The absorbances were in-creased in the test solutions and decreased in the
blank solutions with lapse of time.The velocityof change of color was more remarkable in the
Fig.-2 Effect of nitric acid to color stabilityin mercurometry.
(1) 0.3ml,(2) 1.0ml.
case of addition of 1 ml.The color of the test
solution at the end-point was nearly reddish and
gradually changed to mauve,and the color of
blank solution was mauve and relatively stable.
Consequently,the strict agreement of color be-
tween test solution and color matching solution
was impossible.Therefore,the disagreement
and instability of color frequently gave some
anxiety to the operator.From these phe-
nomena,a comparison of determination value
Fig.-3 Effect of diphenylcarbazon to colorstability in mercurometry.
Test solution:(1) 1.0 ml,(2) 0.5 ml.Blank:
(3) 1.0 ml.(4) 0.3 ml.
Fig.-4 Effect of diphenylcarbazon to analyti-
cal results in mercurometry.
○ Member A,△ Member B,● ▲ Average.
Repetition: 8 times.
Y. Niino et al.: Determination of Chloride in Food Grade Salt 139
and repeatability were made by using the Inter-
national Standard Seawater by 2 members.Theresults were shown in Fig.-4.As a result,there
was no significant difference in each determina-tion by added volume of diphenylcarbazon in
spite of the above problems.
4. REPEATABILITY AND ACCURACY
4.1 Choice of electrode
Three kinds of measuring electrodes and twokinds of reference electrodes are ruled in ISO6227.The number of combination is six,the
values obtained were compared on accuracy andrepeatability.The results were shown in Fig.-
5.There was no significant differences on ac-
Fig.-5 Comparison of electrode combination.
Argent: Argentometry, AG: Ag electrode, AGS:
Ag selective ion electrode, CLS: Cl selective ion
electrode, CA: Calomel electrode,HG:Hg sul-
phate electrode, Repetition: 8 times,←→:±1σ
range.
curacy and repeatability according to the statisticestimation.
4.2 Comparison of argentometry and mer-curometry
The test solution of common salt and standardseawater were titrated by argentometry and mer-
curometry. At first,the tests were carried out intwo laboratories and by two operators, and thetitrations were carried out two times a day and
for four days in total.As these tests were madeby using the same solution, the dilution error
could be eliminated. The results were plotted inFig.-6 as the each datum, average,and the
standard value of standard seawater. Argentome-try was more excellent than mercurometry on
repeatability estimated by the standard deviation.At second, the test was carried out in two
laboratories, by six operators and by two times
of titration. As the dilution of sample was in-dependently done from the same sample, thistest included the dilution error. The results
were shown in Fig.-7 as the range of measuredvalues. Repeatabilities and reproducibilities ob-
tained from the data in Fig.-7.Mercurometry Argentometry
Repeatability 0.08% 0.06%
Reproducibility 0.13 0.15
Fig.-6 Comparison of error in argentometry
and mercurometry eliminated the dilu-
tion error.
---: Standard value of standard seawater,
Repitation: 8 times,•©•¨•}1ƒÐ range.
140 Bull. Soc. Sea Water Sci. Jpn. Vol.39 No.3 (1985)
Fig.-7 Comparison of error in argentometry and mercurometry
included the dilution error.---: Standard value of standard seawater, Repetition: 2 times.
4.3 Miscellaneous problemsTime required for making a sample from the
preparation of test solution to the end of analysiswas as follows:
Mercurometry 25min
Potentiometry 30minArgentometry 20min
Mercurometry by ISO 2481 includes the followingextra operations compared with argentometry:1) Neutralization by nitric acid, 2) Comparison
of color, 3) Large titration value, 4) Use,of bigconical flask, 5) Treatment of waste water.In-crease of time required in potentiometry by ISO
6227 was mainly caused from the long stabiliza-tion time of potentiometer.
5. DISCUSSION
5.1 Comparison of repeatabilityThe standard deviations from all of the above
experiments on determination of solar salt were
shown in Table—1. Repeatability was 1: Po-
tentiometry, 2: Argentometry, 3: Mercurometry
in the order of excellence in the case of the data
excluded dilution error, but it was nearly equal
between mercurometry and argentometry in the
case of the data included dilution error. The
variation coefficients in each operation of dilu-
tion were estimated from the previous paper, 4)
and the standard deviations were shown in
Table-2 obtained from assuming the use of a
hand-operated burret at ISO 2481 (mercurome-
try) and an automatic one at JTS (argentometry).
The enlargement of the variation coefficient (ƒÐ/X)
might be caused from use of hand-operated
burret in ISO 2481 method and from decrease of
titration volume in JTS method.As a result,
each of the standard deviations was nearly among
three methods because dilution error was greater
than titration error.
Y. Niino et al.: Determination of Chloride in Food Grade Salt 141
Table-1 Standard deviations on determination of chloride in solar salt.
Table-2 Estimation of standard deviation in operation of dilution.
Table-3 The deviation from the standard value.
Ag: Silver electrode, AgS: Silver-ion selective electrode, ClS: Chloride-ion selective electrode, Ca: Calomel
electrode,Hg:Mercury sulphate electrode,"t" was obtained from t=X/ƒÐ•ãn,and t values showed a prob-
ability on the rightness of the hypothesis which the data from each method will be differed from the
standard value.
5.2 Accuracy
Accuracy of each method was compared with
the deviation of determination values from the
standard value of standard seawater.All data
were compiled in Table-3. The average and
confidence limits,which were calculated from
±3σ/√n,were shown in Fig.-8.
These results showed that argentometry was the
most accurate, mercurometry gave a little lower
value and potentiometry gave a little higher
value. The combination of silver-ion selective
electrode and calomel electrode gave relatively
accurate values in each combination. But we
should not come to an immediate conclusion
because the number of data was short and the
dilution error was relatively large as stated
above.
5.3 Cost of analysis
Argentometry and potentiometry were fre-
quently criticized about the high cost of analysisbecause of expensiveness of silver nitrate and
potentiometer.The cost of reagent was mostlydecided from silver nitrate.A comparison be-
tween mercurometry (ISO 2481) and argentometry (JTS) was shown in Table-4.
The cost of apparatus for a sample were esti-
mated as
mercurometry \ 71.3
potentiometry 113.0argentometry 80.0
which were obtained by assuming the durabilityas follows: glass vessels can be used 300 times,
dispenser, stirrer and burret can be used 3,000times respectively, electric apparatus can be used
142 Bull. Soc. Sea Water Sci. Jpn. Vol.39 No.3 (1985)
Fig.-8 Deviation of determination value from standard seawater.
○: Average, ← →: Confidence limit, ARG: Argentometry, MER: Mer-
curometry, AG: Ag electrode, AGS: Ag selective ion electrode, CLS: Cl
selective ion electrode, CA: Calomel electrode, HG: Hg Sulphate electrode.
Table-4 Comparison of reagent cost.
Table-5 Characteristics of each method.
* The costs regarding reagent and apparatus.
30,000 times.
But personnel expenses are usually much
greater than the expenses of reagents and appa-
ratus in these kinds of hand-operation analysis.
Consequently,the cost of analysis should be
estimated from time required, and we estimated
as an example that a sample of the personnel
expenses was ••1,125 in mercurimetry,••900 in
argentometry, and ••1,350 in potentiometry.
As a result, the total cost of argentometry was
more reasonable than mercurometry in spite of
expensive reagent cost because the weight of
personnel expenses was greater than reagent andapparatus costs.
6.CONCLUSION
Mercurometry (ISO 2481),potentiometry (ISO
6227),and argentometry (JTS) for determinationof chloride in common salt were studied as
mutual comparison regarding the operation,error, time required and cost. Mercurometrywas effected by pH and quantity of diphenyl-
carbazon, whose effects were smaller than the
Y. Niino et al.: Determination of Chloride in Food Grade Salt 143
error originated from preparation of the test
solution.But the disagreement of color with
the color-matching solution and the instability of
end-point color gave frequently some anxiety to
the operator.The peculiarities of these methods
may be characterized as Table-5. But the
accuracies and repeatabilities in the total opera-
tion of determination were nearly equal because
the dilution error was greater than the titration
error.The total cost of argentometry was more
reasonable than mercurometry in spite of use of
expensive reagent because the personnel expenses
were much larger than the expenses of reagent
and apparatus.
Acknowledgment The examples of analysis wereobtained in cooperation with the members of CentralResearch Institute and Odawara Experiment Station,Japan Tobacco Inc.This manuscript has benefitted
from discussion with and comments by H.Yamanakaand T.Hashimoto,Japan Tobacco Inc.Theauthors thank these members.
REFERENCES
1) J. V. Dubsky and J. Trtilek, J. Mikrochem., 12,315 (1933)
2) I. Robert, Ind. Eng. Chem. Anal. Ed., 22, 553
(1936)3) F. E. Klerke, Anal. Chem., 22, 553 (1950)
4) N. Ogata, Kaisui Gakkaishi (Bull. Soc. Sea WaterSci. Jpn.), 24, 259 (1971)
和 文 要 旨
食塩中の塩化物の定量
新野 靖,清 水典子,西 村ひとみ,尾 方 昇
硝酸水銀滴定法(ISO2481),電 位差滴定法(ISO
6227),お よび硝酸銀滴定法(日 本専売公社法)に よ
る食塩中の塩化物イオンの定量について,操 作,精 度,
所要時間,コ ス トなどの面か ら比較検討 した.硝 酸水
銀滴定法は検液のpHと ジフェニルカルバ ゾンの添加
量によって影響された.硝 酸添加量は過剰になるとブ
ランク値を増加させ,ま た色調の不安定を生ずるし,
不足すると定量値が低 くなるか ら,2N硝 酸 添 加 量
0.5~1.0mlと し検液を約pH2に すると適当であっ
た.ジ フェニルカルバ ゾンの添加量によって終点の色
調 は変化す るが,定 量の精度に影響 しなか った.な お
硝酸水銀滴定法の欠点は終点色調 と終点判定用基準液
の色調が一致せず,ま た終点の色調が不安定で分析者
に しばしば不安感を与える点にある.
電位差滴定法は規定 されている6種 の電極の組合せ
では相互に繰返 し精度の差は認 められなか ったが,正
確 さについては銀イオン選択電極 とカロメル電極の組
合せで比較的よい結果をえた.滴 定の繰返 し精度は,
1:電 位差滴定法,2:硝 酸銀滴定法,3:硝 酸水銀滴
定法の順に良好な結果をえた.し かし定量の全操作を
通 じての標準偏差はいずれもほぼ同一になった.こ れ
は希釈誤差が滴定誤差よ り大 きいため と考え られ る.
各方法の正確 さは国際標準海水の基準値か らの片寄 り
によって比較 した.そ の結果硝酸銀滴定法が最 も正確
であ り,硝 酸水銀滴定法はやや低い値を示 し,電 位差
滴定法はやや高い値を示 した.
概略の分析所要時間は検液採取後で,硝 酸水銀滴定
法25分,電 位差滴定法30分,硝 酸銀滴定法20分 となっ
た.試 薬 コス ト,装 置 コス ト,人 件費を試算 したが人
件費が試薬 コス トおよび装置コス トより高いために試
薬が高価な硝酸銀滴定法のほ うが試薬の安 い硝酸水銀
滴定法 よりむ しろ全 コス トとしては安 くな った.
(Received Feb. 9, 1985)