US2583890A - Quantitative determination of metals - Google Patents

Description

Patented Jan. 29, 1952 Gerold Schwarzenbach, Zurich, Switzerland, as-

- signor to Chemische Fabrik Uetikon (Uetikon Chemical Company), Uetikon, Switzerland, a

firm of Switzerland No Drawing. Application April a, 1946, Serial No. 660,408. In Switzerland May 2, 1945 iz'ciaims.

For the quantitative determination of numerous metals and, especially, of the alkaline-earth metals, at the present time only gravimetric and indirect titrimetric methods are available. Thus, for example, for the determination of the total hardness of a natural water by the standard method, a sample thereof is treated with an excess of a standard solution of sodium carbonate and sodium hydroxide, in order to precipitate the calcium as carbonate. This is effected either by evaporating themixture to dryness or letting it stand for a day. In the former case it is taken up in distilled water. The whole is then filtered and the filtrate is titrated back with hydro chloric acid of known strength. The total hardness can be calculated from the volumes of the sample of water employed, the added solution of sodium carbonate and sodium hydroxide and the hydrochloric acid needed for the back titration. This method is very complicated andtedious. In addition, the result is obtained as a difference between two comparatively large figures and therefore lacks accuracy. methods for determining the total hardness are either similarly troublesome or are suitable only for approximate determination of hardness as, for example, in the case of Blachers method in which a soap solution is employed.

It has now been found that the total hardness of any water can be determined in a particularly simple and reliable manner by adjusting to the same pH value a sample of the Water and a solution of an amino polycarboxylic acid compound containing in relation to one basic nitrogen, atom more than one carboxymethyl, group attached directly thereto, said compound being capable of sequestering calcium in complex form, mixing with the sample of water an amount of thesolution. of amino polycarboxylic acid compound at least sufficient to sequester in soluble complex form the metal cations such as calcium and magnesium responsible for Water hardness, and then adding progressively a standard alkaline solution until the pH value of the mixture is restored to the value to which the water sample and the solution of amino polycarboxylic acid I compound were initially adjusted.

The other known ltshould be here pointed out that the hydrogen which is associated with the amino carboxylic compound is fairly tightly held and contributes virtually no acidity to the aqueous solution in which the amino carboxylic acid compound is employed; When the solution is added to the watersample, and the metal ions react with the amino carboxylic acid compound, thishydrogen is displaced and becomes an active hydrogen ion and thereby produces a proportionate drop in the pH value of the final solution. Consequently, by titrating the solution with a standard alkaline solution such as dilute sodium hydroxide solution, until the pH value of the sample is restored to its original adjusted value which existed prior to the addition of the complex-forming amino polycarboxylic acid compound, the volume of alkali solution required to neutralize the liberated acid represents the equivalent of hardness present in the water sample.

. The pH adjustment of the original water sample and that of the amino polycarboxylic acid compound may be readily effected either electrometrically or more simply with the aid of one of the Well-known acid-base colorimetric indicators, for example methyl orange, methyl red, phenolphthalein, etc.

In addition to calcium, the other alkalineearth metals, and also lithium, aluminium the rare-earth metals, zinc, cadmium, mercury, lead, copper, iron, cobalt, nickel and manganese can be quantitatively determined by the same method. Especially suitable as complex-forming compounds are the aminopolycarboxylic acids of the general formula CHrC 0 0H CHz-C 0 0H wherein R stands for a member selected. from the group consisting of hydrogen and the radicals Iminodiacetic acid, nitrilotriacetic acid, anthraacid-diacetic acid and uramildiacetic acid have, recently been prepared by me (Helvetica chimica acta 28, 1141 (1945); 29, 368 (1946)).

The following are suitable as complex-forming compounds: the salts of iminodiacetic acid,

and of its derivatives, for example, nitrilotriaceimportance of the new method lies in the fact that it enables a considerable number of metals to be titrated with the same standard alkaline solution as. that used for the usual allialimetric titratiohsl Nospecial chemicals or apparatus are necessary for these determinations in addi tion to the complex-forming substances mentioned. Also, some of the complex-forming compounds are easily obtainable in an excellently crystalline form andin a condition. of absolute purity and are therefore pre-emi'nently suitable as substances for titrimetric standards with the aid ofwhich the alkaline and all other stand: ard solutions arerea'dily. adjustable. The accuracyof the metal determinations is increased by the employment of such adjusted standard alkaline solutions. Asia modification of the method outlined above, instead of adding, a standard alkali solution and titrating. with the aid of one of the acid-base. colorimetric indicators mentioned, any one. of a. number of colorimetric indicators particularly suitable for determination of pH may be addedto a. solution, for example, containing water sample and complex-forming compound. The choice of indicator will be de pendent upon the pH range in whichthe analysis complex-former, the next increment of alkali complex-former solution will cause a readily detectable increase in pH value which can be rendered visible by means of a suitable indicator. Thus, cobalt, nickel, zinc, copper and lead can be titrated very accurately with the tripotassium salt of nitrilotriacetic acid, whilst,'for cadmium and manganese, it is better to employ the tetrabe effected petentiometrically by means of an indicator electrode.

It has furthermore been found that in the titration of inetals by means of a standard solution of the complex-forming compound, there can also be; employed an indicator which responds directly by color change to the ions of the metal to be determined, some of these metal ions combining directly with the indicator itself. Thus, for example, iron and cobalt can be ti trated with the complex-former using thiooya nate as the indicator and taking as the end po'int the vanishing red or blue coloration depending upon the metal ion present. For determining zinc, for example, the diphenylthiocarbazone known by the name dithizone'm'ay be employed as an indicator. Especially impressive is the ti tration of calcium with murexide as the indi cator. Murexide (ammonium purpurat'e) gives a blue color in alkaline solution and a red color in the presence of calcium. Accordingly, if' asolution of a calcium salt and murexide, said s0= dicator responds immediately by changing color.

to thewater sample is a measure of the total hardness present in the water sample. The above subject matter is claimed in my divisional, co-pending application, Serial. No. 195,473, filed November 13, 1950.

It has further been found that the concentra tion of the v'ario'us metals above enumerated can also be determined by titrating a solution containing the metal ion with a standard solution of an alkaline-reacting salt of one of the complex-forming compounds. Here no acid is liberated by the reaction between the metal ion and the complex-former as in the preceding exam:

ples. Instead, so long as all the metal ionshave not reacted'with' the polycarbox-ylic acid saltto forma complex, the pH value of the solution will. remain substantially unchanged. However,

' when. a. balance or equilibrium is, reached and" all or the ine'tal ions liave ben. iihited withfthe, j

lution having been rendered alkaline, is titrated with a standard solution of the complex-forming compound, the red color or the indicator plex-for'ming compound. Such an indicator functions on'an entirely different basis than indicators. The latter respond solely to pH changes while the 'former: undergo color change in solution in the presence of the metal or metals I to be detected". The change-in color occurs whenamazing-tor calcium by this Method since the" complex-forming compound tiesupall of the 1111- combin'ed calcium in solution and-finally extracts the calcium which had previously combined withthe murexide indicator. When this final extraction of the previously combined calcium occurs,- the indicator changescolor. Since the murexide does not-respond to magnesium ions, this method is suitable forthe determination of calcium in the presence'of magnesium. V

For these determinations. of: metals, there is advantageously selectedfrom the complex-forming compounds. mentioned a. compound having an equilibrium constant of formation of the cominlexlwith the. metal: to. be determined which, is. as large as; possible and' amounts. to at least; 10(1'.

Examples 1;. A natural, water, the calcium content oil which, according to the known standard method; hereinbeiore mentioned; amounted to 24.4. French. degrees, was to be analysed- A sample. measuring 200 C05} was, after the, addition. or methyl red, brought to a. value; of r 5 with the, addition of 0.021s. hydrochloric acid. "incidem, tally, thfnuinbr dice-required for thisv a by 2', ves the temporary .harenes new 9 estimat on. I

contained 20 gms. of uramildiacetic acid per litre, neutralised to a pH value of 5, the solution; which became acid, was titrated with 0.02N caustic soda solution until the color of the solution was the same shade of color asthat of the original water sample after addition of methyl red and the 0.02N hydrochloric acid. 24.3 cc." were, required, corresponding to a total hardness of 24.3 French degrees. Instead of methyl red, bromocresol purple can be employed with the same success. 9

' If ethylenediamine-tetracetic acid is employed instead of uramildiacetic acid, twice the amount to 0.02N caustic soda solution is required, so that the number of cc. of standard solution run in is to be divided by -2 to obtain the total hardness in French degrees. Thus, 200 cc. of the same water, after adjustment to a pH value of 5 and subsequent-addition or a solution of ethylenediamine-tetracetic acid, the pH value of which also amounted to 5, required 49.0 cc. of 0.02N caustic soda solution, corresponding to a hardness of 24.5 French degrees.

2. A sample of 200 cc. of the natural water employed in Example 1 was treated with cc. of normal caustic soda solution, a few drops of an aqueous solution of murexide were added and the wholewas titrated with a strongly alkaline solution of the complex-forming compound until a color change took place from redto blue-violet. The standard solution employed was one containing 5.84 gms. of ethylenediaminetetracetic acid. Only the calcium but not the magnesium, is determined by the change of color of the murexide, so that only the calcium hardness is obtained, because this standard solution causes the calcium ion to disappear before the magnesium ion.

3. For the purpose of determining the lead in a solution, produced by dissolving 6.96 gms. of the purest lead chloride per litre, cc. were treated with 1 gm. of sodium chloride and, after the addition of methyl red, were brought to a pH value of 5 with caustic soda solution. 40 cc..of a m./40 solution of ethylenediamine-tetracetic acid, which also had a pH value of 5, were then added and the solution was. titrated back with 0.05N caustic soda solution to a pH value of 5, for which purpose 19.9 cc. of the standard solution were necessary. The calculated quantity of lead was 0.103 gm., whilst 0.1025 gm. was found.

4. For the purpose of determining zinc, there were employed 20 cc. of a 0.1 m. solution of zinc chloride the content of which was determined gravimetrically. After adding bromothymol blue, the whole was titrated with a standard solution. of 0.1 m. of trisodium nitrilotriacetate until a very sharp change of colour took place. For this purpose, exactly 20 cc., i. e. exactly the theoretical quantity.--wererequired. The standard solution was prepared by dissolving 19.1 gms. ofthe purest nitrilotriacetic acid, dried in vacuo, in'300"cc.-of'-normal caustic soda solution and making up to one litre.

5. For the purposeof determining copper, 20 cc. .of a 0.1m. solution of cupric chloride andthe same standard solution as in Example 4 were; employed, but methyl red was employed as} indicator." The color change was from lilac to pure. blue. In order to reach this color change, 19.8 cc. of the standard solution were necessary, whilst theoretically 20 cc. should have been used.

The following are the formulae or the acids hereinbefore mentioned from which the complex-forming salts are produced:

20 C O OH--C 5 NIL-06 CHr-C 0 OH CHs-C 0 OH Imidodiacetic acid CHr-C 0 0H 0 Nitrilotriacetic acid COOH /CHr-G 005 N Anthranilie-acid diacetic acid COOK-43g, QH -COQH 4 N-CH|OHrN i CHr-G 0 OH Ethylenediamine-tetracetic acid NH-C 0 CHr-C 0 OBI o6 CH- CHr-C O OBI Uramildiacetic acid 0 0 0H CHI-C 0 on CHN CHr-C 0 0H Aminomalonic-acid (acetic acid What I claim is: 1. A method of volumetrically determining 1 metal cations in solution which comprises titrating a measured amount of the solution with a solution containing a known concentration of an amino polycarboxylic acid compound, said compound having in relation to one basic nitrogen atom more than one carbcxymethyl group directly attached thereto, until an end-point detectable by suitable indicating means is reached, said end-point being the point where all of the aforesaid metal cations are united with the anions of the amino polycarboxylic acid compound to form 45 a complex ion thereof, the amount of amino polycarboxylic acid compound being a direct measure I of the aforesaid metal cations present in solution. 2. A method of determining metal cations in solution which comprises titrating the solution with a solution containing a known concentration. of a compound of an amino polycarboxylic acid compound of the general formula CHzCOOH where R' is a member selected from the group consisting of hydrogen and the radicals until an end-point detectable by suitable indicata metal cations present in solution. a

3. A method of volumetric-ally determining assassinpound being a direct measure" of the aforesaid metal cations in solution which comprises titrating a measured amount of the-solution with a.

solution containing a known concentration of an alkaline-reacting saltof an amino polycarboxylic acid compound said compound having in relation to one basic nitrogen atom more than one carboxymethyl group directly attached thereto, until an end-point is reached, said end-point being the point'where-all ofthe aforesaid metal cations are united with the anions of the amino polycarboxylic acid compound to form a complex ion thereof, and being the point where there occurs an increase in pH value which is'detectable by suitable indicating means;- the amount of alkaline-reacting salt of amino polycarboxylic acid compound being adirect measure of the aforesaid metal cations present in solution.

4. A method according to claiin 3 in which the amino polycarboiiylic acid has the general formula emotion cfiiceoa where R is a member selected from the group eonsistingf or hydr gen and the radicals r I CHZC OOH 1 and 5. A method according to claim 8 in which-the" ami-no polycarboxylic acid compound is a salt" of metal cations in solution which comprises making the solution suitably alkaline, titrating a measured amount of the solution with a solution containing a known concentrationiof an amine polycarboxylic acid compound; said compound having in relation to one basic nitrogen atom more than one-'carboxy'methyl group directly attached thereto, said titratiori being carried out iii-the'presence 'of a suitable colorimetric indicator which is responsive to the presence and absence of the aforesaid metal cations in solution, until a visible c ange in the color of the indicator mugs. said change occurring when all the afar are metal cations are coiiibined with the" anions of the amino polycarboxvlicacid compound, there being substantially no change in the" pH valueer solution, the volume of amino polycarbo'iiyli a d coinnoiind being a'direct measure of tiieambcnt of the aforesaid metal cations present in theoriiriaisciuubn. v I V 9'. A method according to claiin 8'1 amino polycarboxylic acid has the g mind consisting of hydrogen and the radicals and salts thereof.

10. A method according to claim 8'in which the amino 'p'ol'ycarboxylic acid compound-is a salt of ethylene diamine tetra acetic acid.

11. A method according to claim 8 in which the amindpolycarboxylic acid compound is asalt ofuramildiacetic acid.

12 A method according to claini 8 in which the amino. polycarboxylic acid compound is asalt of nitrilo triacetic acid.

a 1 GEROLD SCHWARZE ACE-I.

. REFERENCES CITED The renew ng references are of fecord 1h me of this patenti UNITED STATES PATENTS OTHER REFERENCES" 7 'i a'yloi' and Baker o ganic Chemistry of itrogen," page 122 (1942); Oxford University Prssy. Chein; and Met. Eiig; fMeasureinent and-Control of pH, pages 553-560, August- 1940.

Claims (1)

1. A METHOD OF VOLUMETRICALLY DETERMINING METAL CATIONS IN SOLUTION WHICH COMPRISES TITRATING A MEASURED AMOUNT OF THE SOLUTION WITH A SOLUTION CONTAINING A KNOWN CONCENTRATION OF AN AMINO POLYCARBOXCYLIC ACID COMPOUND, SAID COMPOUND HAVING IN RELATION TO ONE BASIC NITROGEN ATOM MORE THAN ONE CARBOXYMETHYL GROUP DIRECTLY ATTACHED THERETO, UNTIL AND END-POINT DETECTABLE BY SUITABLE INDICATING MEANS IS REACHED, SAID END-POINT BEING THE POINT WHERE ALL OF THE AFORESAID METAL CATIONS ARE UNITED WITH THE ANIONS OF THE AMINO POLYCARBOXYLIC ACID COMPOUNDS TO FORM A COMPLEX ION THEREOF, THE AMOUNT OF AMINO POLYCARBOXYLIC ACID COMPOUND BEING A DIRECT MEASURE OF THE AFORESAID METAL CATIONS PRESENT IN SOLUTION