Bestaendigkeit Von Platin Englisch 24-09-09

7/31/2019 Bestaendigkeit Von Platin Englisch 24-09-09

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PLATINUM EQUIPMENTResistance and handling


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Content

Correct handling of platinum equipment 1Limits of applicability 3Treatment of platinum equipment 6Lifetime of platinum equipment 8Temperatures 8Physical data 9Resistance of Platinum group metals 10


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Correct handling of platinum equipment

Advantages of platinum equipment

Platinum is one of the most resistant metals.Above all other metals, it is suitable forchemical laboratory equipment because of itsresistance to chemical corrosion, its highmelting point (1768 C) and its low vapourpressure.Its acid resistance, particularly its completeresistance to hydrofluoric acid alone and inmixtures with sulphuric acid and/or nitric acid,together with its annealing resistance which ismanifest as constant weight even for strongly

annealed equipment and is extremely valuablein practice, is not achieved by any othermetallic material. Its unique catalytic efficiencyfacilitates the ashing of organic materials,especially filter substances.

Finally, a particularly valuable asset is thatplatinum can be processed with no problems inany respect so that there are hardly anyrestrictions on its forming. Another particularadvantage is that it can be homogeneouslywelded without any defects so that solderingwhich generally represents areas of low

chemical and thermal resistance can becompletely avoided.

Quality of platinum equipment

Quite understandably, the good properties ofplatinum equipment described above are onlyachieved for platinum which is completelyperfect in every respect. The degree of purityof the starting material and completehomogeneity are the most importantrequirements: but in order to producesatisfactory equipment in every respect theprocessing must be suited to the material, ascan only be ensured by dealing with platinummaterials over many decades.

Pure platinum is very soft and loses the workhardness achieved during its processing intoequipment when annealed at 700-1000 C,which is a disadvantage when using laboratoryequipment. It is therefore hardened by smalladmixtures of other precious metals such asiridium, rhodium or gold, with its resistance tochemical corrosion remaining the same or insome cases even being improved.

The materials listed in Table 1 are available for

various applications:


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Table 1: Precious metal materials for laboratory equipment having various functions

Material Equipment Applications

Pt/Ir 97/3% Crucibles, dishes

Normal analytical operations, e.g.: acid or alkalinedecompositions, vaporisation of HF, annealing offilter residues

Gooch or Neubauer filtercrucibles

Filtration of aggressive liquids followed by drying orannealing the residue

Pt/Ir 95/5%Pt/Ir 90/10%Pt/Ir 75/25%

Electrodescapillary tubes, spatulacapillary tubes

Electro-analytical determination of metals such asAg, Cu, Zn, Cd, Pb

Pt/Au 95/5 Crucibles and dishesNormal analytical operations, but especially cokingand melting of samples for XRF

Crucibles and solids ofrotation

Rotary viscosimeters for glass melts, also made ofPt/Rh 90/10% and Pt/Rh 80/20%

Pt/Rh 90/10% Crucibles

Decompositions at high temperatures in ceramics,

cement and metallurgical laboratories. If Rh2O3formed during alkaline decompositions has adisturbing effect, inner plating with Pt/Ir 97/3% maybe used

Pt/Rh 80/20% Dishes Decompositions of ferrophosphorus and ferrosilicon

Au/Pt 90/10% Dishes Flour ashing

Pt/Pd 80/20 Dishes Flour ashing

Rh or Ir Crucibles Growth of oxide single crystals


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Limits of applicability

The numerous good properties of the materialcan easily make us forget that in certain cases,extremely resistant platinum can be veryreactive. It is strongly corroded by a fewreagents and in some cases, completelydestroyed.

General remarks on substances havinga harmful effect

The substances harmful to platinum equipmentare given more or less completely in mosthandbooks of inorganic and particularly

analytical chemistry. The main ones are freechlorine, chlorine from hydrochloric acid - nitricacid mixtures (aqua regia), hydrochloric acidwith other oxidising agents, such as chromicacid, manganates or iron(III) salts, as well ascertain molten salts which chemically dissolveplatinum; also the low-melting metals such aslead, tin, zinc, silver, aluminium, mercury, alkalimetals, antimony, bismuth, either primary orsecondary as formed by reduction fromcompounds, elemental phosphorus, arsenic,boron and silicon which alloy with platinum,substantially reducing its melting point.

There is no need to indicate that theharmfulness of these various substancesexhibits considerable differences in degree. Insome cases a few hundredths of a milligramcan make an item of platinum equipmentcompletely unusable whereas other harmfulsubstances can come in contact with theequipment for long periods withoutsubstantially reducing its usability.However, little detailed information can befound on this topic which is all the moreunfortunate since the use of platinum

equipment offers major advantages whenworking with some inherently harmfulsubstances. Thus, in the following we shallgive an overview of the most importantsubstances having a harmful influence onplatinum, taking into account the severity of itsinfluence on platinum. We hope that ourdiscussions will contribute towards the

knowledge of platinum equipment which willnot only save the user of the equipment fromquite considerable trouble but will also bringabout a longer lifetime and, under certaincircumstances, extend the area of application.

Substances having a destructive effect

Apart from the purely chemical dissolvingeffect of atomic chlorine in liquids, such asfrom hydrochloric acid - nitric acid mixtures,

and from some molten salts, any markeddestruction of platinum equipment can mostprobably be attributed to alloying of platinumwith other metals or metalloids. These alloysmostly have a much lower melting point thanplatinum which is exceeded at the normalworking temperatures; holes or cracks are thenformed in the equipment as a result of localmelting.

Formation of alloys with metals

Of particular importance in relation toharmfulness are the low-melting metals suchas lead, tin, antimony or bismuth which even inlow concentrations, substantially lower themelting point of platinum, result in theformation of holes in equipment. In this case, itis not necessary for the metals as such to beintroduced into the equipment. Attempts toheat these metals in platinum equipment areprobably only made in very rare cases but it isoccasionally overlooked that some of theircompounds, namely their oxides, can veryeasily be reduced to the metal, for example by

filter charcoals. It occasionally also happensthat traces of light-melting metal,predominantly tiny soft solder particles, reachthe platinum equipment from outside, becomeattached to it and form holes when theequipment is next heated.


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The reasoning that has been put forward forthe light-melting metals (including mercury)and compounds also applies in principle to thehigher-melting metals if the heating is

extended into the vicinity of their melting point,which should basically be avoided in contactwith platinum equipment.

If during the formation of an alloy with a higher-melting metal, such as iron, the melting point isnot reduced to such an extent that it directlyresults in destruction of the equipment, thegeneral corrosion resistance of the equipmentis substantially reduced by this alloy formationand the material becomes embrittled so thatsecondary influences can bring about complete

destruction. A platinum-iron alloy is formed forexample if an item of platinum equipment isheated to temperatures above 1200 C in thepresence of iron oxide.

Apart from free carbon, where reference isagain made to the filter charcoal, possiblereducing agents for metal oxides include allorganic compounds, e.g. metal salts of organicacids. Reducing flame gases from the heatingsources have a similar effect. It should benoted in particular that hydrogen is firstoccluded from platinum starting at 400 C andat higher temperatures diffuses through theequipment walls. Contrary to incorrectinformation given in the literature, hydrogen byitself should not be considered as a harmfulsubstance, only its effect on reduciblecompounds in the annealed material ishazardous.

Formation of alloys with metalloids

Like metals, the metalloids also form low-melting alloys with platinum, which isfrequently not yet taken sufficiently intoaccount. Particular mention may be made ofsilicon, phosphorus and arsenic for whichinformation on their effect as harmfulsubstances is available in the more recentliterature and to which particular attentionshould be paid on account of theirtopochemically specific corrosion. Whereas theformation of a platinum-tin alloy extendsapproximately over the entire surface, theformation of alloys with phosphorus and

arsenic extends preferentially along the grainboundaries where it progresses rapidly andvery quickly results in cracking (Fig. 1).

Fig. 1: Topochemical differences between corrosion ofplatinuma) Surface corrosion (tin)b) Grain boundary corrosion (phosphorus)(from J. Fischer)

Silicon forms the silicide Pt3Si, drasticallyreducing the platinum melting point, which forits part forms a eutectic with platinum that

melts at 830 C.

The formation of alloys in the presence of freesilicon, if appropriately formed by reduction ofprimary silicon dioxide by carbon or water(in very small quantities), results in perforationof the crucible wall via the low melting eutecticPt-Pt3Si.

Phosphorus reacts in a similar way. Itcombines in fractions of a milligram with theplatinum to form the platinum phosphidePt20P7, which forms a particularly low-melting

(588 C) eutectic with platinum Pt-Pt20P7 Themelting point of the platinum infected withphosphorus immediately drops locally toaround 600 C (eutectic melt), with cracks andholes being unavoidable. This process isparticular hazardous during ashing of organicsubstances, such as flour, however small thespecific phosphorus content may be.In such cases, care must be taken to ensurethat the operations are carried out at thelowest possible temperature and withsufficient air intake.

a) Surface corrosion (tin)

b) Grain boundary corrosion (phosphorus)


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If a certain amount of phosphorus uptake bythe platinum cannot be avoided, it isrecommended that equipment made ofplatinum/rhodium alloys (e.g. containing20% Rh) should be used or equipment made

of pure rhodium whose alloys with phosphorusonly melt at substantially higher temperaturesthan the eutectic Pt-Pt20P7.

The alloy Au/Pt 90/10% has proved suitable forthe ashing of phosphorus-containing organicsubstances (cereals and flour). Since thisbegins to melt just above 1100 C, the usagetemperature of such equipment is restricted toa maximum of 950 C.

Arsenic and platinum also form a low-meltingeutectic (597 C), which, as has been known

for a long time, even made possible the firstmethod for producing workable platinum(Achard, 1779). In the same way as forphosphorus, platinum arsenides can be formedby annealing arsenic-containing compounds,e.g., Mg2As2O7, in the presence of reducingsubstances (flame gases or filter charcoal).The same measures as for phosphoruscorrosion are recommended to limit arseniccorrosion.

Harmful substances

Whereas the harmfulness of the substancesmentioned above is generally underestimated,the damage to platinum equipment by thefollowing substances is generally lower than iscommonly assumed.

For example, platinum dissolves only slowly inaqua regia in the cold and even at water bathtemperature a crucible takes a number ofhours to dissolve.

Alkali hydroxides, carbonates, sulphides,cyanide and thiocyanates certainly corrode the

platinum to some extent at highertemperatures (see Table 3) but frequently theuse of platinum equipment offers so manyadvantages that a certain loss of substance

can be accepted. During melting with causticalkalis or alkali carbonates the corrosion canbe limited substantially by adding alkali nitrate(for caustic alkalis twice the quantity of nitrate,for carbonates half the quantity of nitrate) and

adhering to an upper temperature limit of 700or 550 C. For sodium peroxide and cyanidemelts the critical temperature is500 C, for carbonates and neutral salts it isaround 800

oC. In all these cases it is

recommended that contrary to the operatingmethod during annealing or ashing, thecrucible should always be covered since lesscorrosion occurs when the air intake isreduced. Platinum equipment, like othercrucible materials, is generally more corrodedby potassium salts than by the correspondingsodium salts.

In the case of glasses and ceramics it shouldbe checked whether metallic components (e.g.lead, antimony, arsenic, iron and others) canbe formed by secondary reduction processesfrom heavy metal oxides. Platinum is notcorroded by magnesium silicates (asbestos) inan oxidising atmosphere up to 1400 Cwhereas in a weakly reducing atmospheremelting to form a slag mass of unknowncomposition occurs at red heat.

As a result of the action of carbon or reducing

flame gases (from luminous gas flames or fromthe interior cone of a gas flame) on emptyplatinum equipment, the surface becomesroughened. This roughening can only be partlyeliminated by strongly annealing the equipmentin an oxidising atmosphere. Thus, the originalsmoothness of the surface is not fully restored.This damage to the platinum by carbon andreducing flame gases is only minor and not ofa far-reaching nature as long as no reduciblecompounds of the platinum poisons mentionedabove e.g. metal oxides, are present.Otherwise the platinum forms alloys with the

elements released as a result of the reduction.Platinum equipment is only damaged bysulphur and sulphur-containing gases above1550 C.


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Treatment of platinum equipment

Heating

Annealing and ashing in platinum equipmentshould basically be carried out in an oxidisingatmosphere, i.e., in an open crucible withunhindered air intake, in order to avoid anyreduction of metals or metalloid compoundswhich may be present, e.g. by filter charcoal.It is advisable to initially preheat the material tobe annealed at a lower temperature until all theorganic substances have been burnt.Experience shows that it is only moreadvantageous to work with covered cruciblesto impede oxygen from the air from reaching

the crucible contents in the case ofdecomposition melts with alkali salt mixtures.

Platinum equipment is heated most gently inan electric crucible furnace or in a mufflefurnace. During annealing over a burner or ablowpipe the inner cone of the flame must notcome in contact with the crucible otherwisehydrogen can be absorbed which has areducing action on the crucible contents.

Unnecessarily long heating of platinumequipment to high temperatures should be

avoided since this can lead to coarsening ofthe crystal structure, embrittlement andcracking.

Red-hot platinum equipment should not comein contact with foreign metals. Duringannealing in electrical furnaces care should betaken to ensure that the ceramic material onwhich the crucible is placed is scrupulouslyclean; this also applies to the base plates onwhich the platinum equipment is left to coolafter the annealing has ended. W hen heatingover burners or blowpipes, the platinum

equipment must not be brought in directcontact with iron triangles or wire meshes buttriangles either having three spacer pointsmade of platinum-iridium or having tubes madeof a suitable ceramic material must be used.

Triangles made of heat-resistant steel wire andquartz tubes are very durable.

When working on water baths avoid usingmetal rings and use the most suitable porcelainrings. Tongs and pincers used to handle thered-hot equipment must not be made of iron orbrass. Tongs or pincers with platinum tips orshoes are the most suitable.

When several new items of platinumequipment are annealed at the same time in amuffle furnace, it should be borne in that thatpolished walls or crucible edges and lidscoming in contact may become welded

together as a result of self-diffusion even at redheat, i.e., they may stick together. Thistendency diminishes as the equipment isincreasingly used.

Cleaning and care

Platinum equipment naturally needs to becarefully looked after and cleaned after use.After boiling with suitable solvent in each case,it is recommended that equipment shouldoccasionally be rubbed with sea sand which

improves its appearance and lifetime quiteconsiderably. Abrasion using round grains ofsand not only removes alloys formed on thesurface but also polishes the surface at thesame time and makes it more resistant tocorrosion.

Vigorous cleaning is achieved by meltingpotassium hydrogen sulphate or potassiumdisulphate in the crucibles and dishes. It isincorrect to continue to burn off impurities byvigorous annealing from the platinum. Only in avery few cases does this method have the

desired effect but it is always better to removeany impurities mechanically or chemicallybefore annealing the platinum equipment.

Frequently used platinum equipment can easilybecome kinked, which soon results in the wallshaving a folded appearance.


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This folding not only makes it difficult to cleanthe crucible and remove any melt cake but alsoreduces the lifetime of the equipment.It is thus advisable to keep the crucible wallspermanently smooth. This can easily beachieved by using a wood or plastic mould ormore easily by rolling the crucible on a flatsurface whilst simultaneously pressing a roundwooden rod of approximately pencil thicknessonto the inner wall. It is much too easilyforgotten that in many cases mechanicaldeformations contribute towards prematurewear of the equipment, which far exceeds theslight chemical corrosion. Thus, from the very

beginning the wall thicknesses of the crucibleand dishes should not be chosen to be too thinin order to avoid any bending and twistingwhen held with the crucible tongs. Naturally the

jaws of the crucible tongs should be providedwith platinum shoes which are not too short.

Various tricks can be used to dissolve barelysoluble disulphate and sodium carbonate melts(decomposed substances) from platinumcrucible. It is especially convenient asproposed by H. and W. Biltz to allow a thick

platinum wire to "freeze in" the melt cakeshortly before solidification and suspend this inthe dissolving liquid by bending it twicetogether with the platinum crucible (see Fig. 2).The heavy salt solution formed in the cruciblesinks to the bottom and is automaticallyreplaced by fresh solvent. Fast quantitativedissolution is achieved by means of thiscirculating process with very careful treatmentof the crucible material.

It may be mentioned that the material Pt/Au95/5% among others has the feature that it is

poorly wetted by many glasses and moltensalts. In such cases, melt cakes can easily beremoved by simply slipping from thesecrucibles after cooling.

Figure 2: Dissolving out solidified melt from a platinumcrucible (from H. and W. Biltz (18))

Contrary to other information given in theliterature, metal deposits can be dissolved fromelectrodes after the end of electrolyticdeterminations using highly concentrated,analytically pure acids, provided that slightcorrosion is allowed (


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Lifetime of platinum equipment

Although premature destruction of platinumequipment can be reliably avoided by takinginto account the previous reasoning and beingattentive, an unlimited shelf life cannot beensured. Their lifetime is substantially longerthan that of most other laboratory equipmentbut highly stressed crucibles such as cokingcrucibles become brittle and unsightly after alarge number of determinations. For crucibles,dishes and laboratory electrodes an average

number of 1000 operations may be assumed.Reworking the equipment after suitableoperating times is to be recommended in anycase.

Should difficulties arise when dealing withplatinum equipment or special problems needto be solved, our research laboratories will bepleased to collaborate with you.

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The maximum permissible temperatures up towhich melts can be handled in platinumcrucibles you can see in the next chart.

A quantitative overview of the resistance ofplatinum group metals to commonly usedchemical reagents and the physical data arein the following charts.


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Bestaendigkeit Von Platin Englisch 24-09-09