Lithium, quantitative analysis

Lithium occurs in nutrient solutions primarily as the lithium ion (Li+) . Li+ can increase the chlorophyll content of some plants (e.g., potato and pepper plants). A non-essential micronutrient.

There are different methods for determining lithium:

• Atomic absorption spectroscopy (AAS): High-precision method for determining lithium.
• Flame photometry: A simple and sensitive method for measuring lithium.
• Complexometric titration with EDTA: A less common method, but possible with selected indicators.

Detailed precipitation titration of lithium with ammonium tetraphenylborate

1. Principle of the method

Lithium ions (Li⁺) react with ammonium tetraphenylborate (NH₄BPh₄) and form a poorly soluble precipitate:

The end point of the titration is determined by turbidity or gravimetrically.

2. Chemicals

• 0.01 mol/L ammonium tetraphenylborate solution (NH₄BPh₄)
• Ethanol-water mixture as solvent
• Phenolphthalein as a turbidity indicator

3. Experimental setup

Required equipment:

• Burette (25 mL, division 0.1 mL)
• Erlenmeyer flask (250 mL)
• Pipette (10 mL)
• Magnetic stirrer

4. Implementation

1. Pour 10 mL of the nutrient solution into a 250 mL Erlenmeyer flask.
2. Add 20 mL of ethanol-water mixture.
3. Titrate with 0.01 mol/L ammonium tetraphenylborate solution until a permanent turbidity is visible .

5. Calculation of the lithium concentration

The concentration of lithium is calculated using the formula

:

6. Beispielrechnung:

• Ammoniumtetraphenylborat-Konzentration: 0,01 mol/L
• Verbrauchtes Volumen: 7,5 mL (0,0075 L)
• Probenvolumen: 50 mL (0,050 L)

Die Fällungstitration mit Ammoniumtetraphenylborat ist eine zuverlässige Methode zur quantitativen Bestimmung von Lithium in Nährstofflösungen.

Lithium analysis in hydroponic systems must account for concentration-dependent hormetic effects, where low concentrations may stimulate growth while higher concentrations become toxic. According to a comprehensive meta-analysis, lithium below 50 mg/L significantly influences plant germination and root biomass, whereas concentrations of 50–500 mg/L affect biochemical parameters. The EC50 values for lithium range from 24.6 to 196.7 ppm depending on exposure medium and plant species. Hydroponic exposure produces more negative responses compared to soil or foliar application, with lithium chloride more adversely affecting germination and root biomass than other lithium salts. Lithium nitrate shows stronger negative effects on shoot biomass. French bean studies indicate that toxicity thresholds and tissue concentrations require careful documentation. For analytical purposes, flame photometry or ICP-OES provides adequate sensitivity for most applications, with detection limits of 0.1 mg/L. Sample acidification with nitric acid prevents precipitation and container adsorption.

Quellen: Shahzad, B., et al. (2016). Lithium toxicity in plants: A meta-analysis of responses. Hawrylak-Nowak, B., et al. (2012). Lithium toxicity in French bean

: Shahzad, B., et al. (2016). Lithium toxicity in plants: A meta-analysis of responses. Hawrylak-Nowak, B., et al. (2012). Lithium toxicity in French bean.