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Everything you want to know about sodium polytungstate


General information: Solid, crystalline sodium polytungstate is not hygroscopic and has an unlimited shelf life at room temperature.

Please note the following when using aqueous polytungstate solutions:

  • Only use distilled or de-ionised water
  • Containers must be properly re-sealed after use
  • Only use glass, plastic or stainless steel containers
  • Do not allow the solution to come into contact with reducing agents (e.g. base metals). If this should occur however, the resulting blue discolouration will have no effect on the adjusted density or quality of the solution. The solution can be made colourless, and any blue discolouration prevented, by adding a few drops of hydrogen peroxide. Please use SPT-5 for metal samples or samples containing metal (copper, aluminium, etc.)
  • The sink-float substance should not contain ions which are soluble in water. In some instances, Pb2+-, Ag+-, Sn2+– und Ba2+ ions in particular may form largely insoluble precipitates spontaneously or after a period of time. If these soluble ions are present in samples, we recommend that you wash them beforehand with hot water.
  • If SPT-4 is used, stir the solution thoroughly before use.

More information regarding the composition, properties, production and use of sodium polytungstate can be found below in our FAQ:

What are the key product details for sodium polytungstate?

Product: SPT-0, SPT-1, SPT-2, SPT-3, SPT-5, SPT-R

Substance type/trade nameHexasodium tungstate hydrate
Main ingredient of the substanceHexasodium tungstate hydrate
Purity> 99%
Physical stateSolid or liquid (aqueous solution)
ColourWhite greenish solid or slightly yellow-greenish solution
Relevant identifiable usesAqueous solution acts as a heavy liquid (sink-float analysis)
SynonymsSodium metatungstate hydrate, sodium metatungstate, sodium polytungstate, SPT
Index no.074-001-00-X
EC no.412-770-9
CAS No.12141-67-2, 12333-13-0
REACH registration no.01-2120061128-61-0000

What are the safety-related and physical specifications of sodium polytungstate?

ph valueapprox.. 3 at 20°C
Melting point/freezing pointcannot be determined (decomposition)
Initial boiling point and boiling rangenot applicable (solid)
Flash pointnot applicable
Flammability (solid, gas)not applicable
Explosive limitsnot relevant
Vapour pressurenot relevant
Relative density5,47 g/cm³ at 20°C
Viscosityapprox. 60 mPa•s (solution with density 3.1 g/cm³)
Bulk densityapprox. 1570 kg/m³
Solubility in water> 1000 g/l at 20°C
Surface tension73,4 mN/m at 20°C and 1254 mg/l
Partition coefficient n-Octanol/H2O< -5,2 at 20°C
Auto-ignition temperaturenot applicable
Explosive propertiesnone
Oxidising propertiesnone

What are the analysis results for sodium polytungstate?

Solid sodium polytungstate contains at least 86% ± 1 % WO₃.
Bound water in the sodium polytungstate may vary very slightly.
Typical analysis results (no guaranteed values):

  • Al ≤ 0,0015 %
  • As ≤ 0,012 %
  • Cu ≤ 0,001 %
  • Fe ≤ 0,005 %
  • Mo ≤ 0,005 %
  • Si ≤ 0,005 %
  • Ti ≤ 0,001 %

What are the benefits of SPT (sodium polytungstate, sodium metatungstate)?

The benefits of this heavy liquid solution are clear to see when sodium polytungstate is compared with organic heavy liquids and aqueous LST:

PropertiesSodium polytungstate (SPT)LSTHalogenated hydrocarbons
PropertiesBaseSodium polytungstate (SPT)aqueousLSTaqueousHalogenated hydrocarbonsorganic
PropertiesSupply formSodium polytungstate (SPT)liquid & crystalLSTliquidHalogenated hydrocarbons
PropertiesReusabilitySodium polytungstate (SPT)yesLSTyesHalogenated hydrocarbons
PropertiesThermal stability (see also question below)Sodium polytungstate (SPT)>100°C (liquid and solid)LST>100°CHalogenated hydrocarbons
PropertiesToxicitySodium polytungstate (SPT)non-toxicLSTnon-toxicHalogenated hydrocarbonshighly toxic
PropertiesSolubility >20°CSodium polytungstate (SPT)very goodLSTvery goodHalogenated hydrocarbons
PropertiesSolubility <15°CSodium polytungstate (SPT)very goodLSTcrystallisationHalogenated hydrocarbons
PropertiesCrystallisation temperature at density of 2,8g/cm³Sodium polytungstate (SPT)< 0°CLST< 16-18°CHalogenated hydrocarbons
PropertiesMaximum density at 25°CSodium polytungstate (SPT)3,08g/cm³LST2,95g/cm³Halogenated hydrocarbons
PropertiesViscosity at density of 2.4 g/cm³Sodium polytungstate (SPT)4cPLST4cPHalogenated hydrocarbons
PropertiesViscosity at density of 2.8g/cm³Sodium polytungstate (SPT)19cP solution prepared in minutesLST10cP adjustment of high densities is time-consumingHalogenated hydrocarbons
PropertiesIn combination with tungsten carbide for high densitiesSodium polytungstate (SPT)very suitableLSTless suitableHalogenated hydrocarbons
PropertiesIn combination with organic matters or humic-acid soilsSodium polytungstate (SPT)partly inertLSTsome reactionHalogenated hydrocarbons

How do I adjust a density by dissolving a solid?

In order to produce a heavy liquid solution, the SPT (SPT-0, -1, -2) must be dissolved in de-ionised water (ultrasound bath optional). The higher the concentration, the higher the density (checked e.g. with a hydrometer). The maximum achievable density of an aqueous solution is 3.1 g/cm³ at room temperature. The density can be lowered by adding water, or increased by evaporating the solution in a beaker (preferably with a large surface) at 70°C in a drying chamber. The liquid must not be boiled! You can also add solid SPT to increase the density again.

Producing the solution in this way allows fine granules, particles or solids to be separated by density (sink-float process). A laboratory centrifuge accelerates the separation process.

Overview for production of an SPT solution (50 ml)

Density p in g/cm³SPT mass in gH2O mass in g
Density p in g/cm³1,08SPT mass in g5,00H2O mass in g48,80
Density p in g/cm³1,15SPT mass in g10,00H2O mass in g47,66
Density p in g/cm³1,23SPT mass in g15,08H2O mass in g46,63
Density p in g/cm³1,32SPT mass in g20,00H2O mass in g45,87
Density p in g/cm³1,40SPT mass in g25,02H2O mass in g44,98
Density p in g/cm³1,56SPT mass in g35,01H2O mass in g42,94
Density p in g/cm³1,73SPT mass in g45,15H2O mass in g41,10
Density p in g/cm³1,88SPT mass in g55,01H2O mass in g38,93
Density p in g/cm³2,04SPT mass in g65,17H2O mass in g36,99
Density p in g/cm³2,19SPT mass in g75,00H2O mass in g34,68
Density p in g/cm³2,35SPT mass in g85,00H2O mass in g32,54
Density p in g/cm³2,50SPT mass in g95,00H2O mass in g30,10
Density p in g/cm³2,67SPT mass in g105,01H2O mass in g28,42
Density p in g/cm³2,82SPT mass in g115,17H2O mass in g25,92
Density p in g/cm³2,98SPT mass in g125,12H2O mass in g23,72
Density p in g/cm³3,10SPT mass in g133,00H2O mass in g22,00

Density of aqueous sodium polytungstate solutions as a function of mass concentration

How do I adjust the density by adding water?

You can reduce the density of an SPT solution as required by adding de-ionised water.
This reduces the density as follows:

Initial solution: 500 ml solution SPT-3 with density of 3.00 g/cm³

Addition of H2O in mLDensity ACTUAL
Addition of H2O in mL0Density ACTUAL3,00
Addition of H2O in mL10Density ACTUAL2,95
Addition of H2O in mL20Density ACTUAL2,91
Addition of H2O in mL35Density ACTUAL2,86
Addition of H2O in mL52Density ACTUAL2,81
Addition of H2O in mL71Density ACTUAL2,75
Addition of H2O in mL91Density ACTUAL2,69
Addition of H2O in mL112Density ACTUAL2,63
Addition of H2O in mL132Density ACTUAL2,58
Addition of H2O in mL151Density ACTUAL2,54
Addition of H2O in mL168Density ACTUAL2,49
Addition of H2O in mL182Density ACTUAL2,46
Addition of H2O in mL200Density ACTUAL2,42
Addition of H2O in mL225Density ACTUAL2,37
Addition of H2O in mL252Density ACTUAL2,32
Addition of H2O in mL283Density ACTUAL2,26
Addition of H2O in mL314Density ACTUAL2,22
Addition of H2O in mL347Density ACTUAL2,17
Addition of H2O in mL383Density ACTUAL2,12
Addition of H2O in mL427Density ACTUAL2,07
Addition of H2O in mL476Density ACTUAL2,01
Addition of H2O in mL586Density ACTUAL1,91
Addition of H2O in mL726Density ACTUAL1,81
Addition of H2O in mL886Density ACTUAL1,71
Addition of H2O in mL1096Density ACTUAL1,62
Addition of H2O in mL1396Density ACTUAL1,52

Please notice that this table or better each cell is only valid for initial solution of 500 ml with density 3.0 g/cm³ because of an exponential dependence.
If you have reduced the density of the solution too much, you can increase it again by evaporating the solution in a beaker at 70°C in a drying chamber. Should the solution be brought to boiling note the following point.

May an SPT solution be brought to boiling (thermal stability)?

Yes, a diluted SPT solution can be brought to boiling to drive out excess water and thus increase the density. The boiling point of SPT solutions is, as water, about 100°C. However, it is important to note that crystallization should be avoided when concentrating (initial skin formation on the boiling surface). This leads to a slight turbidity of the solution, which has no influence on the quality of the solution (except the optical properties). To avoid this we recommend not to exceed 70°C. At this temperature a SPT solution can also be completely and reversibly converted to a solid.
In general we recommend the use of a rotary evaporator at moderate temperatures.

How is the viscosity linked to the density?

Viscosity is shown in the following table and diagram as a function of density at 25°C. As seen in the figure, the viscosity only increases very slightly to a density of 2.5 g/cm³. This also allows separation within the fine grain range. The use of laboratory centrifuges can also accelerate the separation process here. If you need a very low viscosity please ask for LVP-3.

Density ρ in g/cm³Viscosity η in mPa•s
Density ρ in g/cm³1,08Viscosity η in mPa•s0,99
Density ρ in g/cm³1,15Viscosity η in mPa•s1,05
Density ρ in g/cm³1,23Viscosity η in mPa•s1,13
Density ρ in g/cm³1,32Viscosity η in mPa•s1,19
Density ρ in g/cm³1,40Viscosity η in mPa•s1,29
Density ρ in g/cm³1,56Viscosity η in mPa•s1,51
Density ρ in g/cm³1,73Viscosity η in mPa•s1,83
Density ρ in g/cm³1,88Viscosity η in mPa•s2,27
Density ρ in g/cm³2,04Viscosity η in mPa•s2,90
Density ρ in g/cm³2,19Viscosity η in mPa•s3,93
Density ρ in g/cm³2,35Viscosity η in mPa•s5,15
Density ρ in g/cm³2,50Viscosity η in mPa•s8,19
Density ρ in g/cm³2,67Viscosity η in mPa•s12,18
Density ρ in g/cm³2,82Viscosity η in mPa•s20,55
Density ρ in g/cm³2,98Viscosity η in mPa•s36,65
Density ρ in g/cm³3,10Viscosity η in mPa•s59,52

Viscosity of aqueous sodium polytungstate solutions as a function of density at 25°C

What is the ideal pH value for sodium polytungstate?

The pH value of the solution depends on the density. A solution with a density of 3.0 g/cm³ typically has a pH between 2 and 3. The solution has long-term stability at this pH value (several years). Each SPT solution must therefore be stored at a pH between 2 and 3.

The pH value can short term (several hours) be changed e.g. to a pH of 7 by adding e.g. NaOH. After completion of the test at the increased pH value, the solution should generally be reduced again to a pH of 2 to 3 using e.g. a few drops of HCl. Please recheck next day.

Is an SPT solution conductive ?

Like all electrolyte solutions, an aqueous solution of sodium polytungstate is also conductive. There is a maximum at a density of approx. 2.2 g/cm³ with a conductivity of 84 mS/cm. The conductivity decreases at lower densities and at higher densities.

What is the structural composition of sodium polytungstate?

It is a 12-fold aggregated isopolytungstate with a molar mass of 2986.12 g/Mol.

According to the models, polytungstate has a octahedral structure, in which oxygen ions are found in the corners and tungsten ions in the centre of the octahedra. As the spherical model shows, the oxygen ions form a very dense spherical shell, in which the tungsten ions are found in the open spaces of the octahedra.

The structure indicates that this compound is a “true” metatungstate with the formula


In a true metatungstate, the two hydrogen atoms are present in the central cavity of the polyanion, and cannot penetrate the spherical shell.

What is SPT-3R?

Our product SPT-3R is a recycled sodium polytungstate solution that we have reprocessed in a complex cleaning process. Ideally, customers collect their used SPT solutions and return them to us free of charge. We produce reusable solutions using multi-stage purification steps and offer these solutions at a lower price. SPT-3R solutions are ideally suited for simple sink-float analyses or for preparing suspensions and do not differ physically from original solutions, e.g. SPT-3. You can work with the recycled version as usual. As we depend on the return of used SPT solutions, availability is always limited. We also recommend not to dry SPT-3R solutions until they are solid. Likewise, SPT-3R is not suitable for elemental analyses or e.g. radiocarbon dating.

What international registrations are in place for sodium polytungstate?

RegulationNZIoCCountryNew ZealandStatusyes
RegulationDSL / NDSLCountryCanadaStatusin process
RegulationKEIC / KECLCountryKoreaStatusin process

Benefits of SPT-0 to SPT-5



easy to handle



low viscosity

water can be used to set any densities from p = 1.1 – 3.1 g/cm³

sink-float substances can be easily cleaned with water

environmentally friendly