Нитрит остался низким.

Остался is the masculine singular past tense of остаться.

• остаться = to remain, to stay, to be left
• остался = remained / stayed for a masculine subject

Here the subject is нитрит, which is masculine, so the verb appears as остался.

Other past-tense forms would be:

• осталась for a feminine noun
• осталось for a neuter noun
• остались for plural

Because after verbs like быть, стать, оставаться / остаться, Russian often uses the predicate noun or adjective in the instrumental case, especially with the meaning of becoming or remaining something.

So:

• Нитрит остался низким. = Nitrite remained low.

Here низким describes the state that the subject remained in.

This pattern is very common:

• Он стал врачом. = He became a doctor.
• Она осталась спокойной. = She remained calm.
• Уровень остался высоким. = The level remained high.

So низким is not the object of the verb; it is a predicate adjective in the instrumental.

Нитрит is in the nominative case because it is the subject of the sentence.

You can identify it as the thing that remained low:

• Нитрит = the nitrite
• остался = remained
• низким = low

So the basic structure is:

subject + verb + predicate adjective

Because нитрит is a masculine singular noun.

In Russian, both the past-tense verb and the adjective must agree with the noun they refer to.

So:

• нитрит — masculine singular
• остался — masculine singular past tense
• низким — masculine singular instrumental

Compare:

• Концентрация осталась низкой.
  концентрация is feminine, so both forms change.
• Значение осталось низким.
  значение is neuter.

Because низким is an adjective, and it describes the noun нитрит.

• низкий / низким = adjective, low
• низко = adverb, low / at a low level in an adverbial sense

In this sentence, Russian treats low as a property of the subject, so it uses an adjective:

• Нитрит остался низким.

An adverb like низко would not fit this structure.

Normally, no. With остаться in this kind of sentence, Russian usually wants the predicate adjective in the instrumental:

• Нитрит остался низким. ✓

Using низкий here would sound wrong or at least nonstandard to most learners’ ears.

So a very useful rule is:

• after стать / становиться → instrumental
• after остаться / оставаться → instrumental

Examples:

• Он стал сильным.
• Она осталась доброй.
• Показатель остался стабильным.

They are close, but not the same.

• Нитрит был низким. = Nitrite was low.
  This just states a condition in the past.
• Нитрит остался низким. = Nitrite remained low.
  This means it continued to be low and did not change.

So остался adds the idea of continuation or no change.

The dictionary form is низкий.

That is the masculine singular nominative form.
Here it changes to низким because it is in the instrumental masculine singular.

A quick pattern:

• nominative: низкий
• feminine nominative: низкая
• neuter nominative: низкое
• plural nominative: низкие
• masculine/neuter instrumental singular: низким
• feminine instrumental singular: низкой

The neutral word order is:

• Нитрит остался низким.

But Russian word order is flexible, so you can move things around for emphasis.

For example:

• Низким остался нитрит.
  This puts extra focus on низким.
• Остался низким нитрит.
  This can also work in the right context.

Still, for a learner, the original version is the safest and most neutral.

The given sentence is understandable, but in many scientific or technical contexts Russian often prefers to speak about the level or concentration rather than the substance itself.

So you may often see:

• Уровень нитрита остался низким.
• Концентрация нитрита осталась низкой.

These can sound more precise, depending on context.

But grammatically, Нитрит остался низким is still useful for learning because it clearly shows the pattern:

subject + остался + instrumental adjective